Lesson video

Hello, welcome to this lesson about Work Done.

It's part of the physics unit, Energy of Moving Objects, and my name is Mr. Fairhurst.

By the end of this lesson, you should be able to describe how energy is transferred by a pushing or a pulling force, and you should also be able to calculate the amount of work that you've done when you've transferred that energy.

So these are the keywords that we're going to use during this lesson.

Conservation of energy is quite an important principle of physics, which essentially says that you cannot create or destroy energy, but what you can do is you can transfer it from one store to another.

And if you transfer energy to the surroundings, maybe by friction and causing heating, then that energy dissipates into the surroundings.

Work done is a measure of the amount of energy that you transfer when you push or pull something.

And joules and kilojoules are the different ways, the different units we can use for measuring the amount of energy that we've got.

So these are those definitions, and at any point during this lesson, you might like to come back to this slide, and just remind yourself of what those definitions are.

So the lesson's split into three parts.

The first part's about transferring energy, and actually understanding what we mean when we say we're transferring energy by pushing or pulling.

And then the second part, we're going to define what work is, what is it we're doing when we're doing work in a physics sense of the word.

And then, finally, we're going to calculate the amount of energy that you transfer when you do some work.

So let's make a start by looking at this first part, transferring energy by pushing or pulling.

We'll start by thinking about a crate being pushed up a slope.

As the crate's pushed up, energy is being transferred.

It's transferred from the person to the crate, but it's also being transferred to the surroundings.

And in this process, as we said in the definitions, we're not creating energy.

We're not using energy.

We're simply transferring it from one store to another store, and this is called the law of conservation of energy.

At the top of the slope, the crate has got more energy in its gravitational store, but there's also more energy in the thermal store of the surroundings because as that crate was pushed up the slope, there was a rubbing and friction between the crate and the ground.

That very slightly heated up the ground, and then that heat spread out into the surroundings, heating up the air particles a little bit.

And we can also say that there's less energy in the chemical store.

That person has used up some of the chemicals, some of the chemicals inside that person have reacted, and energy in the chemical store has been transferred into them doing the work, pushing on that crate up the hill.

And we can represent these changes using these bar charts.

So let's start with the crate at the bottom of the slope.

That's the bar chart on the left.

Now, there's a colour key at the bottom to show what each of the columns mean.

So each column represents the amount of energy in each energy store.

So the gravitational store is in blue, the thermal store is in a sandy colour, and the chemical store is in a green colour.

So when the crate's at the bottom of the hill, there's no energy in the gravitational store.

There is some energy in the thermal store because the surroundings have got a temperature, they're relatively warm, and there is energy in that thermal store.

And there's also energy in the chemical store because there's lots of chemicals inside the person that can react together and enable their muscles to work, and for them to push the crate up the hill.

Now, when the crate gets to the top of the hill, you can see that some of that energy in the chemical store has now been transferred to the gravitational store because the crate is higher up, and some energy has been transferred to the thermal store because the friction and the rubbing has heated the surroundings a little bit as the crate's being pushed up.

Now, one thing to notice is that we haven't created energy, and we haven't destroyed energy either.

If we look at the total amount of energy at the start and add up the thermal store of energy and the chemical store of energy, the amounts of energy in each of those, we get this amount of total energy at the start.

And then if we add up the total energy we have at the end, when the crate is at the top of the slope, we find we get different proportions in different stores, but the total amount is just the same as at the start.

We've transferred energy, but we haven't created or destroyed it, and that's really important to understand.

Now, here's a quick question to check your understanding.

Which statement about pushing that crate is correct? Is the total amount of energy greater when the crate's at the top of the slope, is the total amount of energy the same wherever the crate is on the slope, or is the total amount of energy, does it decrease as the crate is pushed up the slope because of the friction? Pause the video whilst you think of your answer, and then turn it back on to find out the correct answer in the moment.

Okay, how did you get on? The correct answer is B, the total amount of energy is the same wherever the crate is on the slope.

So well done if you got that answer.

It's that idea that the total amount of energy is always the same.

We can't create or destroy energy.

We can only transfer it from one place, from one store, into another store.

So in that example, the crate was being pulled up a slope.

What happens when it's pulled along a level floor? Well, we still get that friction between the crate and the ground, that rubbing, that's causing the particles in each surface to vibrate more quickly, and we feel faster vibrating particles as warmth.

So energy as it's being dragged along because of the friction is being transferred into the thermal store.

But in this case, there's no energy being transferred into the gravitational store because the height of that crate is staying the same.

But then if we think about the energy in the chemical store, those chemicals in the person's body are still reacting in order to make their muscles work, so that they can pull the crate along.

So energy's being transferred out of the chemical store, and overall, the total amount of energy is still being conserved.

There's no energy created.

It's simply being transferred from one store into another.

So let's have a think about that.

Here's a check to see if you've understood that.

And as you can see, there's a load of bar charts here.

The top one shows the energy stores before the crate is starting to be pulled.

You can see some energy in the gravitational store.

There's some energy in the thermal store, and there's energy in the chemical store as well.

So which of those three diagrams, three bar charts, A, B and C, shows what happens to the energy in each store as that crate is being pushed along? Just pause the video whilst you make your decision, and then start again when you're ready.

Okay, how did you get on? The correct answer is B.

In this case, you can see, well, in all three cases, the gravitational store stays the same because the crate isn't raising upwards.

In C, the thermal store of the surroundings has increased, and the chemical store of the person has been reduced by the same amount as the chemical store was increased.

In A and C, the amount of increase compared to the decrease in energy in the chemical store is different, so they are wrong answers.

Energy's not been conserved in either of those two.

So pulling that crate across the floor doesn't change the total amount of energy that we've got.

We had exactly the same total amount of energy after we pulled the crate as we had before we pulled it.

And in this instance, we're not increasing the energy in the gravitational store.

Instead, we're increasing the energy in the thermal store.

Friction's causing rubbing between the bottom of the crate and the ground, which warms them up.

But in this case, those surfaces, in turn, will heat the surrounding air, and the warmth will then move from the ground and into the air, and the air particles will spread out.

So the whole of the surrounding air will heat up ever so slightly, and the energy will dissipate into the surroundings.

It will spread out.

But overall, even though we've not got the same amount of energy in the objects that we can see, we still have overall exactly the same amount of energy, and the energy has been conserved.

Okay, quick question for you.

Which statement about pulling this crate is correct? Does the friction cause the energy to disappear, to diffuse, or to dissipate? Pause the video whilst you think of the answer, and then start again when you're ready.

Okay, how did you get on? The correct answer is that friction causes the energy to dissipate.

It warms the surfaces, and then it warms the air in turn, and the energy spreads out or dissipates.

Okay, here's a task that I'd like you to have a go at.

In this example, a heavy box is pushed up a ramp into a van, and the amounts of energy in each door are shown for the crate at the bottom of the ramp.

Your task is to fully describe the energy transfers that take place as the crate is pushed up the ramp and into the van.

So what I'd like you to do is to pause the video once you have a go at that, and then when you've completed the task, start the video again, and we can check your answers.

Okay, so how did you get on? You were asked to describe the energy transfers when this heavy box is pushed up into the van.

It started with some energy in the thermal store and some energy in the chemical store.

So as it's pushed up, energy is transferred from the chemical store, and into the gravitational store as the box is lifted upwards.

Energy is transferred also from the chemical store of the person into the thermal store of the surroundings because of friction, causing the surfaces and then the air around the surfaces to warm up in turn.

But the total amount of energy overall does not change.

If you've decided to draw a bar chart, which might look something like this one, then well done, you.

In this example, the amount of energy in the thermal store's gone up a little bit, the amount of energy in the gravitational store's gone up, and the energy in the chemical store has been reduced by the same amount as the energy increase in the other stores.

So overall, that bar chart shows they took, that the energy has been conserved.

Let's move on to the second part of the lesson and have a think about doing work.

Pushing the crate up the hill, this person is doing work, and we define the amount of work that they're doing as the amount of energy that they transfer as they push the box.

So work done is the amount of energy transferred, and because it's an amount of energy, we measure it in the same units that we measure energy in.

We measure it in joules.

And by thinking of different situations, we can work out how we might calculate the amount of work that we've done.

In this situation, the person on the right is pushing the crate twice the distance.

So by pushing it twice the distance, if you think about it, they're doing twice the amount of work.

They're pushing the same box with the same force, but twice as far.

So overall, they do two times the amount of work.

And if they were to push it four times as far, they'll do four times the amount of work, and so on.

In other words, the work done pushing the crate is directly proportional to the distance it's moved.

Have a think of this question to see if you've understood that.

How much work is done pushing the same bed 1 1/2 metres across the floor compared to pushing it just 1/2 a metre? Is it a third as much, the same amount, or three times as much? Just pause the video whilst you think about it, and start again when you're ready.

Okay, so we're pushing the same bed with the same size force, we can assume.

So if we push it three times the distance at 1 1/2 metres, it's going to need three times the amount of force.

So well done if you got that answer.

Now, in this situation, the person's pushing really hard on the car.

It feels like they're doing a load of work, but the car's not actually moving.

So strictly speaking, in this case, if the work done is directly proportional to the distance it's moved, if you move it no distance at all, you are not doing any work, and that's just one instance that you need to be aware of.

The other thing that's going to affect the amount of work that you do pushing something or pulling something is the amount of force you need to push with.

Now, in this example, the person who's pushing the blue car on the right has to push with twice the size force than the person on the left.

So if they both push the cars the same distance, it's clear that the person pushing the blue car, who has to push twice as hard, has to do two times the amount of work.

In other words, the work done pushing a car is directly proportional to the force needed to push it.

If you have to push three times as hard the same distance, you do three times the amount of work, and so on.

So there we go.

The person pushing the red car does that amount of work.

The person pushing the blue car with twice the force the same distance does twice the amount of work, but only, if you remember, when the car is being moved.

If it's not moving, you're not doing any work at all.

Okay, so have a go at this question.

How much more work is done pushing a chair two metres with a force of 200 newtons compared to pushing a bed the same distance with a force of 600 newtons? Is it a third as much, the same amount, or three times the amount? Just pause the video whilst you think about that, and start again when you're ready.

Okay, how did you get on? The chair was pushed with a force that's three times smaller than pushing the bed, so the amount of work needed to be done in pushing the chair was a third as much as pushing the bed.

Well done if you got that one right.

We'll finish this section of the lesson with a task.

How much more work is done moving two identical crates twice as far and pushing with twice the force? Work that out, and then explain your answer.

And whilst you're doing that, just pause the video, and then start it again when you've completed the task to check your answers.

Okay, so the correct answer is four times the amount of work.

We've doubled the distance we're pushing the crates, so we've doubled the amount of work we've done by pushing them twice as far.

We've also doubled the size of the force we need to push the crates with because we've got two identical crates this time.

So again, we have to double the amount of work a second time and that will increase the amount of work by four times.

So well done if you got the correct answer with the correct explanation.

In the final part of this lesson, we're going to combine those ideas, and use them to calculate the amount of work done.

Now, we've seen that the amount of work moving an object is directly proportional to both the size of the force and the distance the object moves.

We can use this equation as shorthand to say the same thing.

Work done is force times distance.

And let's think about this for a moment.

If we double the size of the force, we double the amount of work done.

If we double the distance, we double the amount of work done.

If we double the size of the force and the distance, so they're both twice as big, we get two times two, which is four.

We get four times the amount of work done.

This fits with what we've thought about earlier on.

In symbols, work done is represented by a capital W, force with a capital F, and distance with a lowercase s.

And as we said earlier, work done is measured in joules, and the force in newtons, and the distance in metres.

One thing to note is that the distance is measured in the direction in which the force is pushing or pulling the object.

Okay, so let's have a look at this example.

Jacob pushes a supermarket trolley with a force of 50 newtons over a distance of 20 metres.

How much work does he do? Well, as always, we'll start by writing down the equation.

Work done is force times distance, and then look into the question.

We can find there's a force.

He's pushing with 50 newtons, times by a distance of 20 metres.

So put those values in.

Do the maths.

50 times 20 gives us 1,000.

And then not forgetting work done is measured in joules, so we add in the units at the end.

So that's an example showing the working out.

What I'd like you to do is have a go at this example.

Just pause the video whilst you do that, and then start again once you're ready, and don't forget to show all of your working out.

Okay, how did you get on? Let's have a look at the example.

Laura uses a force of 250 newtons to pull a bag of food four metres up into a tree house using a rope.

Well, let's start with the equation.

Work done is force times distance, and then looking at the example, we can find that she uses a force of 250 newtons, and she pulls the bag of food up four metres.

So we'll put those values in.

Do the maths.

250 times 4 gives us 1,000, and the units are joules.

So the correct answer is work done is 1,000 joules.

So well done if you got the correct answer with all the steps of the working out as well.

When there's a lot of work done, it's often simpler to calculate in kilojoules rather than joules, and that's simply because it gives you smaller numbers to deal with.

A kilojoule is the same as 1,000 joules.

Just like a kilometer's 1,000 metres or a kilogramme is 1,000 grammes.

So the word kilo is simply replacing the word a thousand.

So 23,000 joules is 23 kilo joules, and 147,500 joules is 147.

5 kilojoules.

When you're doing conversions, it's always useful to remember that the number of kilojoules you've got is going to be smaller than the number of joules.

So I'd like to try this conversion, and work out how many joules in 2,500 kilojoules.

Just pause the video whilst you work that out, and start again to check your answer once you're ready.

Okay, how did you get on? We started from 2,500 kilojoules or 2,500,000 joules.

So D is the right answer.

2,500,000 are 2,500,000 joules.

Now, to finish the lesson, I'd like you to try these five calculations.

I want you to show all of your working out, and just pause your video whilst you do that, and then turn it back on again to check your answers once you are happy with all of those answers.

Okay, how did you get on? Let's look at some answers.

So first of all, work done is force times distance.

So question one, Sam used the force of 500 newtons multiplied by three metres of distance.

She pulled the sand, 500 times 3 is 1,500 joules.

Question two, Alex pushes a car with a force of 350 newtons over 6 metres.

So how much work does he do? It's 350 newtons times 6 metres, or 2,100 joules.

Question three, Aisha uses a force of half a newton, 0.

5 newtons, to pick a bar of chocolate off the floor.

How much work does she do to lift it 1.

2 metres? So work done is force, 0.

5 newtons, times distance, 1.

2 metres, and that gives us 0.

6 joules of work done.

Question four, Izzy uses a small crane to lift a pile of bricks 1.

8 metres, and the crane uses a force of 3,500 newtons.

How much work is done? So it's one point.

I'll start again.

The force is 3,500 newtons times a distance of 1.

8 metres, and that gives 6,300 joules.

And question five is asking for that amount of joules in kilojoules.

So 6,300 is 6.

3 kilojoules.

So well done if you got all of those right, or most of them right.

So this slide describes what hopefully you've understood from the lesson.

The law of conservation of energy states that energy cannot be created or destroyed, but it can be transferred.

Work done by a force, which is pushing or pulling, is equal to the amount of energy transferred when it does so, and this can be written as an equation.

Work done is force times distance, where work done is measured in joules, force is measured in newtons, and distance is measured in metres.

And the other thing we learned was that friction can cause energy to dissipate or spread out into the surroundings by heating.

So I hope you've enjoyed the lesson, and I look forward to seeing you in another one.

Bye-bye.