GCSE Science podcasts - Forces

JAMES: Hello, and welcome to the BBC Bitesize Physics podcast.

ELLIE: The series designed to help you tackle your GCSE in physics and combined science.

JAMES: I'm James Stewart, I'm a climate science expert and TV presenter.

ELLIE: And I'm Ellie Hurer, a bioscience PhD researcher.

JAMES: We are going to be your guides. We're going to cover everything from forces to electricity, energy to gravity. We are going to explore some of the fun and complex parts of physics to help you revise.

ELLIE: And if you want to really get into it, be sure to grab a pen and paper, so you can make notes and try out equations throughout the whole episode.

JAMES: Absolutely, this is episode one of our eight-part series all about forces! So let's begin.

ELLIE: When it comes to physics, we're often measuring things we can and can't see, like weight, direction, and speed.

JAMES: And the types of things we measure can be split into two groups. So we have scalar quantities and vector quantities.

ELLIE: Okay, so what makes a quantity scalar or vector?

JAMES: Well, it's kind of in the name. Let's start with scalar quantities, which, as you might have guessed, sounds a bit like the word scale.

ELLIE: A scalar quantity is a physical quantity that only has a magnitude. The word magnitude means size, so examples of scalar quantities are mass and distance.

JAMES: Okay, so if you were to ask me how fast I drove my car here this morning, right, I would say 30 miles per hour. Or if you asked me how much pasta I was having for lunch later, I would say 50 grams of pasta. Is that right?

ELLIE: Exactly. A car's speed and the mass of pasta just have a magnitude. So they're scalar quantities because we just measure them with numbers.

JAMES: What's a vector quantity then?

ELLIE: Well, a vector quantity is something we measure with both magnitude and direction, like weight and displacement.

JAMES: Okay, so if I walked to the skate park and it was 2km to the east of my house, let's say, would that be a vector quantity?

ELLIE: Exactly, because there you're measuring displacement, so both the direction and distance.

JAMES: Okay, so another example would be diving, I don't know, five metres down into a swimming pool.

ELLIE: Yep, that's right, because you're talking about both a magnitude, five metres, and a direction downwards.

JAMES: Good. Okay, right, time to get your pen and paper out. Now, we want you to write down your own vector quantity, something that has both a direction and a magnitude. Now, remember that the word magnitude simply means size.

ELLIE: Like swimming 10 metres to the left or pushing 7 Newtons to the right or driving 20 miles per hour to the north.

JAMES: Ellie, this whole series is all about forces, so what is a force? Is that a scalar quantity or a vector quantity?

ELLIE: Drumroll, please? Force is a vector quantity because it has both a magnitude and direction. A force is a push or pull that can change the position, speed or state of an object. A force occurs due to an object interacting with another object.

JAMES: And we measure those forces in Newtons. The sign for that is an uppercase ‘N’.

ELLIE: Exactly. Forces are vector quantities that are measured in Newtons and have a direction. So if you pushed a shopping trolley, you could say you pushed it 4 Newtons to the left.

JAMES: Yeah, and there are two different types of forces, aren't there?

ELLIE: Yeah, so there's contact forces and non-contact forces. So let's dig into the differences between them.

JAMES: Contact forces are forces that occur when the objects physically touch. For example, friction, air resistance and tension. Now one key thing to know about contact force is that when an object at rest exerts a force on the surface it's placed on, there's a reaction force that acts at right angles to the surface.

This is what we call a normal contact force. For example, a book on a table exerts a force down on the table, and the table exerts a normal contact force of the same size back up on the book.

ELLIE: Whereas, non-contact forces are forces that act between objects that aren't physically touching. For example, electrostatic force, magnetic force and gravitational force, which we'll hear more about in future episodes.

JAMES: Yes, we will look forward to that. But before we go, we're not quite finished yet. Let's do a quick summary of where we've got to so far, what we've learned. I think that's a good time to do that. Number one, magnitude simply means size despite the complicated word.

JAMES: Number two, scalar quantities just measure magnitude, whereas vector quantities measure magnitude and direction. Three, forces are vector quantities. And four, finally, there are both contact and non-contact forces.

ELLIE: That's great, James. And thank you everyone for listening to Bitesize Physics. If you found this helpful, go back and listen again and make some notes so you can come back to them when you revise.

JAMES: Yeah, great idea. And in the next episode of Bitesize Physics, we are going to focus on one particular force. Gravity. Be sure to tune in then.

ELLIE: I can’t wait. And until then, may the force be with you.

JAMES: See you next time. Bye!

ELLIE: Hello and welcome to the BBC Bitesize Physics podcast.

JAMES: The series designed to help you tackle your GCSE in physics and combined science. I'm James Stewart, I'm a climate science expert and TV presenter.

ELLIE: And I'm Ellie Hurer, a bioscience PhD researcher.

ELLIE: We're covering lots of different aspects of forces in this series, so make sure to listen to the rest of the episodes too.

JAMES: Yeah, and they're really good. Okay, let's get started because today, I thought so, because today we're talking all about the force that keeps our feet on the ground, gravity.

ELLIE: While we often think about space and astronauts when we talk about gravity, gravity actually acts all around us every single day. Because the definition of gravity is a force of attraction between two objects.

JAMES: The gravitational field is the area around an object where another object will feel a force of gravitational attraction from it.

Gravitational field strength is measured in newtons per kilogram, written out as ‘n’ forward slash ‘kg’.

ELLIE: And the size of the gravitational field strength affects the force of gravity acting on an object in that gravitational field. The other thing that affects the size of gravity is the object's mass. The bigger the mass, the greater the force of gravity.

JAMES: So one key thing to know that a lot of people misunderstand is that weight and mass are actually two different things.

ELLIE: Yeah, so when we say, oh, this loaf of bread weighs 400 grams, we're actually saying that the mass of the loaf of bread is 400 grams.

JAMES: Because mass is about the amount of matter, whereas weight is a force and is the heaviness due to gravity.

ELLIE: Exactly. So let me tell you about the equation you need to calculate the force of weight of an object.

JAMES: Yeah, I'm gonna get my pen and paper out for this one, so if you're listening, please feel free to do the same thing and write along as we go through this.

ELLIE: So, weight equals mass multiplied by the gravitational field strength.

JAMES: Weight is measured in newtons. Mass is measured in kilograms and gravitational field strength is measured in newtons per kilogram.

ELLIE: So to calculate the weight of an object in newtons, you multiply its mass in kilograms by the strength of the gravitational field in newtons per kilogram.

JAMES: That was a lot. Don't panic. Let's just hear that again.

ELLIE: So weight equals mass multiplied by the gravitational field strength.

JAMES: Weight is measured in newtons, mass is measured in kilograms, and gravitational field strength is measured in newtons per kilogram.

ELLIE: So, to calculate the weight of an object in Newtons, you multiply its mass in kilograms by the strength of the gravitational field in Newtons per kilogram.

JAMES: Right, let's try out some examples then. And if you don't have your pen and paper just yet, now would be the perfect time to grab them and you can write down these calculations with us as we go along.

ELLIE: Let's say we want to find out the force of gravity, their weight, acting on your physics teacher as they stand at the front of the classroom.

JAMES: Good image. Now first, you would need to find out their mass. Now let's say it's 80 kilograms, then you need to know the gravitational field strength of the planet they're standing on, which for the planet of Earth is 9.8 newtons per kilogram.

ELLIE: So to measure the force of weight acting on them, you would write down their mass of 80 kilograms and then multiply it by the Earth's gravitational field strength of 9.8 newtons per kilogram to get the answer 784.

JAMES: And because weight is measured in newtons, their weight would be 784 newtons downward. We always have to include those units. And because weight is a force, which is a vector quantity (more about that in episode one), we also have to say the direction it is in, which in this case is downwards.

ELLIE: In those instances, the weight of an object and its mass are directly proportional. So let's say if something had a bigger mass, its weight would be higher. And if something had a smaller mass, its weight would be lower.

JAMES: Exactly. And when we're measuring weight in terms of gravity, we don't use regular kitchen scales. We use something called a newton meter, also known as a calibrated spring balance.

ELLIE: And when we do that, we say that the weight of an object, or in this case, person, acts at a single point. The object or person's centre of mass. The force of gravity, weight, always acts from the middle of an object, straight down.

JAMES: Okay, that was a lot, but I hope that helped you understand gravity a little bit more.

ELLIE: So, let's recap the three main points.

Firstly, gravity is a force of attraction between two objects. The next point is, mass is the amount of matter in an object. However, weight is the force of gravity acting from the middle of the object straight down.

And finally, the equation to find out an object's weight is mass multiplied by gravitational field strength equals weight.

ELLIE: There's your key points about gravity. In the next episode of Bitesize Physics, we're going to dig into work done and energy transfer, and I cannot wait.

JAMES: I believe you. Thank you for listening to BBC Physics. If you found this helpful, and hopefully you did, please do go back and listen, make some notes, so you can come back here and always have this as your point to revise from.

JAMES: Thank you, bye! Bye!

JAMES: Hello and welcome to the BBC Bitesize physics podcast.

ELLIE: The series designed to help you tackle your GCSE in physics and combined science.

JAMES: I'm James Stewart, I'm a climate science expert and TV presenter.

ELLIE: And I'm Ellie Hurer, a Bioscience PhD researcher.

JAMES: And just a quick reminder that whilst you're here in the BBC Sounds app, there's also the Bitesize Study Support podcast, which is jam packed full of tips to help you stay focused during revision and get the best out of your exam day.

ELLIE: Okay, let's get started. Today we're going to be talking about energy transfer and work done.

So one of the most important things you need to know about energy is that it's never created nor destroyed, it's just transferred through different objects and forms.

JAMES: So when you kick a football, or you push a suitcase, you use a force to move an object through a distance and displace it from one place to another, which is called work done.

ELLIE: If you've got a pen and paper, let me give you a simple definition to write down.

Work done is when a force causes an object to move through a distance. When a force transfers energy from one store to another.

JAMES: So when you kick a ball, you transfer energy from the chemical energy store in your muscles and your foot, to the kinetic energy store in the ball. Now remember that any moving object has a store of kinetic energy.

ELLIE: When you light a match, you transfer energy from the kinetic energy store in the moving match into the thermal and light energy stores of a fire. Thermal energy simply means heat.

JAMES: So as with a lot of the topics we've talked about so far, we need to use an equation, of course it's physics, to calculate work done. So if you don't already have it, again, good time to grab that pen and paper, and here we go.

ELLIE: So, the equation to calculate work done is: work done equals force multiplied by distance.

JAMES: Work done measures the transfer of energy. Force is measured in newtons and distance is measured in metres, which means work done is simply measured in newton metres.

ELLIE: But when we talk about work done, we measure it in joules. One newton metre is the same as one joule. So work done should be measured in joules. Or simply the letter ‘J’.

JAMES: In case you didn't get a chance to write that down, don't worry, I will repeat it again for you.

Force, measured in newtons, multiplied by distance, measured in metres, equals work done, measured in joules. There we go, it's simple when you sell it, isn't it? Let's say you push a shopping trolley down the aisle with the force of 2 newtons. And that pushes it along by a distance of 5 metres. How, Ellie, would you write that down as an equation?

ELLIE: So, let's write this out together. Okay, so, two newtons of force, multiplied by five metres of distance, equals ten joules of work done.

JAMES: Hey, perfect. And one other thing to know, is how energy is transferred when it comes to friction. Friction, of course, is the force that opposes motion.

Let's say it's your best mate's birthday party, and you want to strike a match, so you can light up the candles on their cake. You're a very good friend. So to do that, you use kinetic energy, movement, to swipe the match against the side of the matchbox.

ELLIE: And that kinetic energy creates friction.

Friction causes the temperature to rise on an object, and once that match has been swiped across, it lights a fire.

JAMES: This means that lighting the match transferred energy from the kinetic energy store into the thermal energy store through friction.

ELLIE: And that your best friend can finally blow out the candles and celebrate their birthday in style.

JAMES: They probably melted by the time I figured that equation out.

So I hope those explanations were super helpful. Let's recap the three main things that we learned. Number one, when a force causes an object to move through a distance, work is done on the object. Secondly, the equation to calculate work done is work done equals force multiplied by distance.

And finally, work done against the frictional forces acting on an object causes a rise in the temperature of the object, transferring kinetic energy into thermal energy,

ELLIE: Right. So those are some of the key facts about energy transfer and work done. In the next episode of this series, we're going to be talking all about forces and elasticity.

JAMES: Thank you for listening to Bitesize Physics. If you found this helpful, go back, listen again and make some notes so you can come back to this anytime you want and revise away.

BOTH: Bye!

JAMES: We did it in time that time!

ELLIE: I sort of watch you, and I'm like “Bye!”

ELLIE: Hello and welcome to the BBC Bitesize Physics podcast.

JAMES: The series designed to help you tackle your GCSE in physics and combined science. I'm James Stewart, I'm a climate science expert and TV presenter.

ELLIE: I'm Ellie Hurer, a bioscience PhD researcher.

Before you listen, just a reminder that you can listen to the whole series or find an episode that you want to focus on. Whatever works for you.

JAMES: Okay, let's get started. Today we are going to be talking about forces and elasticity.

So, when you apply more than one force to a stationary object, it can either compress, stretch, or bend. And when something is stretched, compressed, or bent, there is always more than one force acting on it.

ELLIE: Imagine a spring. If you push both ends, it compresses. If you pull it from both sides, it stretches. And if you try to get the ends to meet, it bends.

JAMES: Exactly! And when you bend, stretch, or compress an object, you cause it to deform, which means changing its original form. And there's more than one type of deformity.

ELLIE: Right, so the first type is called elastic deformity. That's when it returns to its original shape once the force has been removed from it. For example, a spring usually bounces back after I push it down.

JAMES: Yep, and the other type is called inelastic deformity. That's when an object stays deformed even after you stop applying force to it.

ELLIE: For example, if you pull the ends of a spring really far apart so it breaks, then it won't go back to its original form.

JAMES: Wait Ellie, I've got a joke for you. What did the worker at the rubber band factory say when he was fired?

ELLIE: I have no idea.

JAMES: Oh snap. I think we're there. Are we there?

ELLIE: Dad joke! Okay, well, whether it's elastic or inelastic deformity, when you apply a force to an object, you can extend it. So let's move on to the next topic, extension.

JAMES: Extension is the way the length of an elastic object changes when you stretch or compress it.

ELLIE: The extension of a spring is directly proportional to the force you apply to it. Force is proportional to extension. That means that when you double the force, you double the extension. And if you half the force, you half the extension.

Another term for proportional you might hear is linear relationship, they mean the same thing. The extension has a linear relationship with the force.

JAMES: So force is proportional to extension until the object reaches its limit of proportionality, which is the maximum amount of force that can be applied to an object before it changes shape permanently.

ELLIE: When an object is elastically deformed and returns to its original form, there's a linear relationship between force and extension. That means, as force increases, extension also increases. If you pull the spring, so a pull force, the spring extends in length.

JAMES: Beyond this, when an object is inelastically deformed, there's a non-linear relationship between force and extension.

This basically means that as the force increases, the extension still increases. But a little bit slower, not at a proportional rate. And it might eventually stop increasing entirely if it has gotten as long as it can be. Now the equation we use to calculate that force is: force equals spring constant multiplied by extension.

The spring constant is a measure of the stiffness of a spring. Think of it like how much force has to be applied to make it stretch by a certain amount. And the units of spring constant are newtons per metre, or capital ‘N’ slash lowercase ‘m’.

ELLIE: So the higher the spring constant, the stiffer the object is because it needs more force to be applied to it in order to stretch.

JAMES: Yeah, we talked about work done in the last episode actually, which you can always go back and listen to. Ellie, how does that apply to elasticity?

ELLIE: When a force compresses or extends a spring, it does work and stores elastic potential energy in the spring. And if the spring hasn't been inelastically deformed, the work done on the spring will equal the amount of energy transferred into its elastic potential energy store.

JAMES: That was a mouthful, well done.

So how then do you calculate the work done on an elastic object? I'm going to keep testing you. How do you figure out how much elastic potential energy is stored in an object when you stretch or compress it?

ELLIE: With our final equation of the day, grab that pen and paper again, and let's write it out.

Elastic potential energy equals 0.5 multiplied by the spring constant, multiplied by extension squared. So let me repeat that again. Elastic potential energy equals 0.5 multiplied by the spring constant, multiplied by extension squared. Oh, that was a mouthful.

JAMES: You’ve earned a day off after that. Shall we do that as a real-world example? This might make it a little bit easier. So, again, pen and paper if you want to write this one out.

If a spring had the spring constant of three newtons per metre, and it was stretched until extended by 0.4 metres, you would square the extension of 0.4, then multiply this by the spring constant of 3. And then multiply this by 0.5 to get an answer of 0.24 joules.

ELLIE: Wow, that's definitely something that needs to be written down.

JAMES: Thank you, yep. Put that on a t shirt.

ELLIE: So, I hope this helped everyone listening to better understand elasticity.

JAMES: And I think you should have a prize for saying elasticity 17,000 times.

ELLIE: Thank you.

JAMES: Yep. Let's recap the three points. The main things that we covered in today's episode. There are two types of deformation. Elastic and inelastic deformation.

Elastic deformation is when the object goes back to its original shape when the force is removed. And inelastic deformation is when it changes shape permanently. Now, the equation to calculate force in a spring is force equals spring constant multiplied by extension. And finally, the equation to calculate elastic potential energy, is elastic potential energy equals 0.5 multiplied by spring constant, multiplied by extension squared.

ELLIE: Smashed it, James.

That's the key points you need to know about elasticity. In the next episode of BBC Bitesize, we're going to dig into displacement, distance and speed.

JAMES: Oh, they're hard words, just keep on coming, don't they? Thank you for listening to Bitesize Physics. If you found this helpful, and I hope you did, please do go back and listen again, and make some notes, and always come back here as many times as you want to help you revise.

ELLIE: There's also lots more resources available on the BBC Bitesize website, so be sure to check it out.

JAMES: One, two, three.

BOTH: Bye!

JAMES: Hello and welcome to the BBC Bitesize Physics podcast.

ELLIE: The series designed to help you tackle your GCSE in Physics and combined science.

JAMES: I’m James Stewart, I’m a climate science expert and TV presenter.

ELLIE: And I’m Ellie Hurer, a bioscience PhD researcher.

JAMES: Today we're going to be talking about displacement, distance and speed. Shall we?

ELLIE: Let's begin. So, James, do you remember how we talked about scalar and vector quantities in episode one?

JAMES: Oh yeah, I do. Scalar quantities just have a magnitude, whereas vector quantities have a magnitude and a direction.

ELLIE: And we can apply that to today's topics too, as we talk about distance, displacement, speed and velocity.

JAMES: Yeah, let's start off with some definitions. We love a definition. So distance is how far an object moves, for example, getting into a car and driving 10 miles.

Distance is a scalar quantity because it only includes magnitude. In this case, that's miles.

ELLIE: Right, and a similar but different term is displacement.

Displacement is a vector quantity. Displacement includes both the distance an object moves, say 10 miles again, but also the direction, for example, north. And when we measure displacement, we measure it from the start point to the finish point in the direction of a straight line.

JAMES: So if we were describing the displacement of a car, we'd say something like, I drove the car ten miles north. Which, you're right, makes it a vector quantity because of measuring both distance and direction.

ELLIE: Okay, yep, that makes sense. Distance is just distance, and displacement is both distance and direction.

JAMES: But a moving object can have a different value for its distance and its displacement.

For example, a car going along a winding road might travel a distance of half a mile. But have a displacement of a quarter of a mile to the east. Now that could be because the displacement is measured as a straight line from the start to the end point.

ELLIE: Another interesting example is someone running around a 400 metre track. They would have a distance of 400 metres, but a displacement of zero.

So, for the next topic, we need to head to the racing track, because we're going to explore… speed.

JAMES: Can you hear how quickly those cars are going in here? It's like being trackside at the Formula 1.

ELLIE: It is! Did you know that the fastest recorded Formula One car drove at just over 397 kilometres per hour.

JAMES: So you're probably already familiar with speed. It's how fast or slowly an object moves. Speed is a scalar quantity because we only measure magnitude. And if you want a quick way to remember that, speed and scalar, well they both start with an ‘s’.

ELLIE: But how do we calculate speed? Well, grab your pen and paper, because you're going to want to write this down. Right, let's start with the equation. Distance equals speed multiplied by time taken. But you can rearrange that equation to calculate speed.

So speed equals distance divided by time taken. Distance is measured in metres, time is measured in seconds, and speed is measured in metres per second.

JAMES: So let's look at that as a practical example. Again, grab your pen and paper if you want to write this out, and follow along with the calculation.

So let's imagine a race car drives 100 metres in the space of 10 seconds. Speed equals distance divided by time taken. So the answer to the calculation would be that the race car drives at a speed of 10 metres per second.

ELLIE: So let's flip that equation around to measure distance, too. If you're measuring the distance travelled by a moving object in a specific time, distance travelled equals speed multiplied by time.

JAMES: Okay, now it's your turn to try and work it out. I'm gonna give you an example. So if a car travelled at a speed of 15 metres per second, and it travelled for 20 seconds, how would you calculate its distance? We'll give you a few moments to work that one out.

Using that equation, we could calculate its distance by writing out that 15 metres per second multiplied by 20 seconds equals a distance of 300 metres.

ELLIE: Well done, James.

JAMES: Thank you.

ELLIE: So, let's maybe rewind that equation and make sure you have it down. If you're measuring the distance travelled by a moving object in a specific time, distance travelled equals speed multiplied by time.

JAMES: By the way, good time to mention the standard units of measurement, you know how important that is in physics, in these equations are metres, seconds, and metres per second. However, some questions might use different units, so whatever's being used in the question, best to use that in the answer.

ELLIE: Let's say, for example, if the question asked you to calculate the speed of a race car that drove 100 kilometres over the course of 10 minutes, you would identify the time is in minutes, and the distance is in kilometres, therefore the speed would be measured in kilometres per minute instead.

But in the exam, if you're asked to give your answer in a particular unit, then you might have to convert that answer, for example from minutes to seconds.

JAMES: Okay, now we've learned about speed, how about velocity?

ELLIE: Well, well, well. Velocity is the speed of an object in a given direction. For example, a race car driving 80 miles per hour south.

Because we're measuring both speed and direction, velocity is a vector quantity. To memorise that, just remember that vector and velocity both start with a ‘V’.

JAMES: Although speed and velocity are different things, they are calculated using the same equation. So instead of saying that speed equals distance divided by time, you would say that velocity equals displacement divided by time.

ELLIE: But the key difference to know when you're doing that calculation is that when you calculate velocity, you must also include a direction in your answer. So you can't just say that a race car is driving at a hundred and fifty miles per hour. You would say that race car is driving at one hundred and fifty miles per hour east.

JAMES: Before we go, let me give you a quick recap of what we've learned today then. So firstly, speed and distance are scalar quantities. Because we just measure their magnitude.

Secondly, displacement and velocity are vector quantities, because we measure both magnitude and direction. Remember, v and v.

And finally, the equation to calculate distance travelled is distance travelled equals speed multiplied by time.

ELLIE: Fabulous.

So that's all you need to know for now about displacement, distance and speed. But be sure to listen to the next episode where we're going back to the racetrack to learn the essentials of velocity and acceleration.

JAMES: Oh yes, back to the racetrack, finally. Thank you for listening to Bitesize Physics. If you found this helpful, please do go back and listen again, make some notes and come back here as many times as you want to help you revise.

ELLIE: There's also lots more resources available on the BBC Bitesize website, so be sure to check that out.

BOTH: Bye!

ELLIE: Hello and welcome to the BBC Bitesize Physics Podcast.

JAMES: The series designed to help you tackle your GCSE in physics and combined science. I'm James Stewart, I'm a climate science expert and TV presenter.

ELLIE: And I’m Ellie Hurer, a bioscience PhD researcher.

JAMES: And what we're going to do is break down all those quite complicated looking equations. The great thing about this being a podcast, Ellie, is people can rewind it. I know when you do your PhD you like to write loads of notes.

ELLIE: You know, it can take time to understand these concepts, but trust in yourself and you will get there in the end.

JAMES: We're going to guide you through all of these, to help you ace that exam.

ELLIE: You've got this.

JAMES: Today we're gonna be talking about acceleration.

ELLIE: In the last episode, we talked about the fundamentals of distance and displacement, then speed and velocity. So if you haven't listened to that, be sure to go back as this episode will make a lot more sense after listening to the previous episode.

Okay, let's go back to the racetrack and begin.

JAMES: So, when we talk about acceleration, we're talking about the change in velocity of an object within a specific time. Acceleration is a vector quantity too. So when we talk about it, we talk about both magnitude and direction.

ELLIE: All right, if you haven't already, grab a pen and paper because it's time for the equation to calculate acceleration.

JAMES: Straight into the equations. Okay, acceleration equals change in velocity divided by time taken. Okay, now it's your turn to write your calculations out and work out the answer to this question.

If a car starts off driving at 10 metres per second north and then reaches 30 metres per second in 4 seconds, how would you calculate its acceleration?

ELLIE: First, you would calculate the change in velocity, which is the final velocity minus the initial velocity. In this case, that is 30 metres per second, minus 10 metres per second. So the change of velocity is 20 metres per second.

Next, you would calculate the acceleration, which equals that change in velocity of 20 metres per second, divided by the 4 seconds it takes. So, in this case, acceleration equals 5 metres per second squared north.

JAMES: Yeah, and you can also apply this equation to something slowing down as well. In which case you end up with a negative acceleration. Which is commonly known as a deceleration.

ELLIE: Right, but what if the race car is constantly accelerating? Like, if its speed changes by 10 metres per second in the first second, and then by 10 metres per second in the second second, and so on.

JAMES: Well then, in that case, it's called a uniform acceleration. When a moving object's speed changes at the same amount each second. If you wanted to calculate uniform acceleration, you could use a different equation.

The equation for uniform acceleration is: final velocity squared, minus initial velocity squared, equals two multiplied by acceleration, multiplied by distance.

ELLIE: Sorry to ask James, but can you repeat that?

JAMES: I knew you were gonna say that. Yes, of course for you.

The equation for uniform acceleration is: final velocity squared, minus initial velocity squared, equals two, multiplied by acceleration, multiplied by distance, and you'd see that written as V squared minus U squared equals two times by A times by S.

ELLIE: Wow, James, that deserves a round of applause.

JAMES: Oh yeah, you actually did as well, thank you.

ELLIE: Okay, so when we're near the surface of the Earth, all free falling objects have an acceleration of 9.8 metres per second squared. So, let's say I went skydiving,

I would fall at an acceleration of 9.8 metres per second squared. Now that sounds kind of scary.

JAMES: It does, yeah. Would you go skydiving?

ELLIE: No.

JAMES: That acceleration would cause you to speed up. As you speed up, the friction force from the air will increase until it's equal to your weight. That makes the resultant force on your body zero, so it no longer accelerates. Instead, you would travel at a constant velocity, which is known as terminal velocity. Which is exactly, handily, what we're going to cover in the next episode.

ELLIE: Ooh, okay, I love a cliff-hanger, James. But before we go, let me give you a quick recap of what we've learned today. Firstly, acceleration is a vector quantity. It's the change in velocity of an object in a given time. The formula to work it out is acceleration equals change in velocity, divided by time taken.

Next, we learned that any object free falling near the surface of the Earth has an acceleration of 9.8 metres per second squared. And finally, the equation for uniform acceleration is: final velocity squared, minus initial velocity squared, equals two multiplied by acceleration, multiplied by distance.

Okay, so now you know more about acceleration. In the next episode of Bitesize Physics, we're going to find out what a resultant force is and get into Newton's three laws

JAMES: We certainly shall. Thank you for listening to Bitesize Physics. If you found this helpful, you might need to go back for this one, listen to those equations, make some notes and then come back anytime you need to revise.

BOTH: Bye!

JAMES: Hello, and welcome to the BBC Bitesize Physics podcast.

ELLIE: The series designed to help you tackle your GCSE in Physics and combined science.

JAMES: I’m James Stewart and I’m a climate science expert and TV presenter.

ELLIE: And I’m Ellie Hurer, a bioscience PhD researcher.

JAMES: And the great thing about doing it this way is you can rewind this podcast whenever you want. So, if there's an equation, then you're like, what on earth are they talking about? Go back, listen again, make some notes, listen on your dog walk on the commute, before you go to bed, whatever works for you, we are happy to be your guides.

And whilst you're here in the BBC Sounds app, have a listen to our other podcasts, where we cover loads more topics you need to understand for your physics GCSE, as well as lots of other subjects too. We've got you covered.

ELLIE: So James, if I say the name Isaac Newton, what's the first thing that comes to your head?

JAMES: Apples. I think, apples. He's the guy that discovered gravity when he started thinking about the apples that were falling from the trees right above his house. Is that right?

ELLIE: Yep, that's right. And if you want to learn more about gravity, be sure to go back and listen to episode 2 of this podcast, which is all about that force that keeps our feet on the ground.

JAMES: But today we're going to talk about something else. Sorry, Isaac. Newton's three laws of motion. Let's go to his garden and begin.

ELLIE: Newton's first law of motion is that a resultant force is required to change the motion of an object.

JAMES: What is a resultant force?

ELLIE: I'm so glad you asked. A resultant force is the combined effect of all of the forces acting on an object. For example, if an apple was falling downward off a tree, the resultant force is downwards. Hitting it with a cricket bat would change its motion.

JAMES: I did wonder why you brought a cricket bat with you. So the act of hitting the apple with that cricket bat causes a change in the resultant force on the apple, which causes a change in its motion?

ELLIE: Yeah, exactly. Newton's first law states that if an object is stationary and the resultant force starts to act on it, that object will start moving.

ELLIE: But, if the resultant force acting on an object is zero, and the object is stationary, the object remains stationary.

JAMES: So, if an apple is on the ground, and no other forces act on it, it stays on the ground. For example, if the force of weight acting from the centre of the apple straight down is 70 Newtons, and the force of the normal contact force, or reaction force, of the ground pushing up on the apple is 70 Newtons, then the resultant force is zero newtons. So the apple just stays still.

ELLIE: Yes, and then if an object is moving, Newton's first law states that the moving object will keep moving with the same velocity if the resultant force is zero. This means its speed and direction will stay the same if the resultant force is zero.

JAMES: So a falling apple will keep falling at the same velocity if the forces are balanced.

ELLIE: That's right, and we can apply it to a car too. When a car is travelling at a steady speed, the resistive forces, like air resistance, balance out the forces driving the car forward. And this is important to remember because people get this wrong all the time. If the resultant forces are zero and the object is already moving, it will keep moving with the same velocity.

JAMES: So according to Newton's first law, the velocity of an object will only change if a non-zero resultant force acts on that object.

ELLIE: Here's another example for you. If someone stopped pressing the accelerator pedal in the car, then the push force from the car would become less than the air resistance and friction.

JAMES: Then there is a resultant force in a direction opposing movement, so the car would begin to slow down.

ELLIE: And that tendency of an object to stay stationary or in uniform motion unless a resultant force acts on an object is called inertia.

JAMES: Okay, let's move on to Newton's second law. Get out of this apple orchard, it's raining.

ELLIE: So, Newton's second law states that the larger the force acting on an object, the more it accelerates. And the greater the mass of an object, the less it will accelerate.

JAMES: What's really important to know about this law is that the acceleration of an object is proportional to the resultant force. So let's imagine you're pushing a trolley down a supermarket aisle.

In this case, the resultant force is the way the trolley is being pushed. So if you double the size of the push forwards, the acceleration also doubles.

ELLIE: And if you were to put more groceries into the trolley, let's say two boxes of cereal, things would change a little.

JAMES: Yeah, exactly, because acceleration is also inversely proportional to the mass of an object. So even if the push force was the same, you'd actually accelerate less, because the mass in the trolley has increased. For example, if the mass in the trolley doubles, you put loads of groceries in there, the acceleration would halve. Okay, it's time to try and make that an equation. So grab your pen, grab your paper. Let's write this down.

ELLIE: The equation to calculate resultant force is: resultant force equals mass, multiplied by acceleration, which is written as uppercase ‘F’ equals lowercase ‘m’ multiplied by lowercase ‘a’. Force is measured in newtons, mass is measured in kilograms, and acceleration is measured in metres per second squared.

JAMES: Let's move on to Newton's third law. Newton's third law is that whenever two objects interact, the forces they exert on each other are equal and opposite. Every action has an equal and opposite reaction.

ELLIE: Right, so let me give you an example.

Let's say you're at a theme park and decide to get into the dodgem cars. The force of weight would act down from the car onto the ground. This is equal and opposite to the force of the ground pushing back up from the ground onto the car.

JAMES: And according to Newton's law, the two forces must be acting on different objects and be the same type and size.

ELLIE: But what would happen if your friend spotted you in your car and sped straight over to you? And then you drove straight over to them until you bumped into each other?

JAMES: Ellie, spoilers, spoilers, hang on. That's where we need to start getting into the topic of momentum. We'll do that in the next episode, I promise.

But for now, all you need to be able to do is apply Newton's third law to objects that are at equilibrium. Which means the forces acting on them are balanced.

ELLIE: Let's sum up today's episode. Newton's first law is that a resultant force is required to change the motion of a moving or stationary object. Newton's second law is that the resultant force on an object is proportional to the acceleration of the object. And last but not least, Newton's third law is that whenever two objects interact, they exert equal and opposite forces onto each other.

So now you know all about Newton's laws. In the next episode, we're going to be exploring, drumroll please… momentum!

JAMES: Thank you for listening to Bitesize Physics. If you found this helpful, go back and listen again and make some notes so you can come back here whenever you want and revise away.

BOTH: Bye!

ELLIE: Hello, and welcome to the BBC Bitesize Physics podcast.

JAMES: The series designed to help you tackle your GCSE in physics and combined science. I'm James Stewart, I'm a climate science expert and TV presenter.

ELLIE: And I’m Ellie Hurer, a bioscience PhD researcher. This is the eighth and final episode of our series on forces.

JAMES: Let's kick off our final episode, I’m emotional, where we'll be talking about momentum.

ELLIE: Momentum is mass multiplied by velocity, which is written out as ‘p’ for momentum equals ‘m’ for mass and ‘v’ for velocity.

JAMES: Momentum is measured in kilogram metres per second, mass is measured in kilograms, and velocity is measured in metres per second.

ELLIE: So James, let me test your memory. Do you still remember what we talked about in episode one?

JAMES: Gosh, a long time ago. Yeah, scalar and vector quantities. And because we measure momentum by both magnitude and direction, it is a vector quantity.

ELLIE: So James, I think it's time for an example. Okay, let's get on the motorway, not literally, and measure the momentum of a lorry. So look at that lorry over there. Its mass is 40,000 kilograms and it's driving up north to Edinburgh at a velocity of 20 metres per second.

JAMES: Speedy lorry. So to calculate its momentum, we would multiply its mass, that's 40,000 kilograms, by its velocity. 20 metres per second north. To get the answer, 800,000 kilogram metres per second north.

So Ellie, do you remember how we talked about dodgems in episode 7?

ELLIE: Yeah, the dodgem cars at the theme park.

JAMES: Yeah, we're gonna use those as the example again to describe our next topic, conservation of momentum.

ELLIE: Imagine there were just two dodgem cars in a rink that's completely closed off to the rest of the world. No other cars can come in, and those two cars can't leave.

JAMES: That would be a closed system, because it cannot be affected or affect anything outside of it.

ELLIE: Right, so when two objects collide in a closed system, the total momentum before an event is equal to the total momentum after an event. This is called conservation of momentum.

JAMES: Let's zoom in to an example. Okay, so let's say you and I we’re in the dodgems. I'll take the green one.

ELLIE: Well, I'll take the blue one.

JAMES: Good. If I was driving straight towards you with a momentum of 60 kgm per second north, and you were driving straight towards me with a momentum of 50 kgm per second south, and then we collided, what would our combined momentum be?

I'll give you a second to use the equation to work it out. As a recap, remember the total momentum before an event is equal to the total momentum after an event.

Okay, so the answer is: our combined momentum before crashing will be 10 kgm per second north.

ELLIE: And our combined momentum after crashing would still be 10 kgm per second north. This is because energy can't move in or out of a closed system. Therefore, our momentum would be the same before and after we collide.

JAMES: So in this example, our cars would move together to the north. My car would push your car backwards.

ELLIE: Rude. However, we definitely wouldn't recommend driving straight into another car in a game of dodgems.

JAMES: Even for scientific research?

ELLIE: Not even for scientific research.

JAMES: We could also look at how this works when a total momentum of zero is conserved. Let's look at a party popper, for example. Didn't think I was going to say that, did you?

ELLIE: So, before you pull the string, the momentum is zero. So the total momentum after pulling it must be zero too.

JAMES: That means if you were to add up all the momentums of all the little bits of paper that came out the end of the party popper, they would all actually cancel each other out to make zero overall. A momentum of zero has been conserved. Zero fun was had at that party.

ELLIE: So let's summarise what we've learnt. Firstly, momentum equals mass multiplied by velocity.

Our second point is, in a closed system, the total momentum before an event is equal to the total momentum after the event. This is called conservation of momentum.

And finally, momentum is a vector quantity, which means both its magnitude and its direction must be given.

And sadly, with that comes the end of our eight-part series all about forces.

JAMES: We hope you found it helpful, and if you didn't get the chance to listen to all the episodes, please do go back. Make sure you can listen again and really get stuck into them. Thank you for listening to Bitesize Physics. If you're preparing for your GCSEs, firstly, good luck, and secondly, why not also check out our Bitesize Biology podcast, or our range of Bitesize English literature series.

ELLIE: There’s also the Bitesize Study Support podcast, which is full of tips to help you stay focused during revision and get the best out of your exam day.

BOTH: Bye!