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 key words 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 this lesson comes in three parts.

The first part's about transferring energy by pushing or pulling, and we're going to think about what we mean by that.

And then in the second part, we're going to define this term doing work.

What is work and how does it relate to a transfer of energy? And then finally in the third section, we're going to calculate the amount of energy that you transfer when you do some work.

Okay, so let's make a start with the lesson.

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

As this crates 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 crates 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 that 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 of the friction and the rubbing has heated the surroundings a little bit as the crates being pushed up.

Now one thing to notice is that we haven't created energy.

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 their 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 creative correct? Is the tough amount of energy greater when the crates 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 decrease as the crate is pushed up the slope because of the friction? Pause the video while 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've 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 been 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 create 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 crater 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 is 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 are ready.

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

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 B, the amount of increase compared to the decrease in energy in the chemical store is different.

So there are wrong answers.

Energy's not been conserving either of those two.

So pulling that crate across 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.

Frictions 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 exact 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 while 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 dissipate.

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 store are shown for the create at the bottom of the ramp.

Your task is to fully describe the energy transfers that take place as the creative 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 are 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's 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, 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 that the energy has been conserved.

Okay, so let's move on to the second part of the lesson and 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 think 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, one and a half metres across the floor compared to pushing it just half 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 are 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 one and a half metres, it's going to need three times the amount of force.

So well done if you've 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 boot 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 once 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 to 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 to 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've 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.

So as we've seen earlier, the work done moving an object is directly proportional to both the size of the force and the distance the object is moved and we can represent this in shorthand by using this equation.

The work done is force times distance.

Let's just pause and think about that for a moment.

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

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

And if we double both the force and the distance, two times two is four, we get four times the amount of work done, which fits in with what we described earlier.

In symbols we use the capital E for work done because it's measuring the amount of energy transferred.

Force is capital F and distance lowercase d.

And as we've just mentioned, the work done is energy so that's measured in joules.

Force is measured in Newtons and the distance is measured in metres.

And it's worth just noting that the distance is measured in the direction that the force is pushing or pulling the object in.

Okay, so let's have a look at an example, Jacob pushes a supermarket trolley with a force of 18 Newtons over a distance of 61 metres.

How much work does he do? Giving your answer to two significant figures.

So as always, we start off by writing down the correct equation, work done is force times distance.

Looking back at the question, the force is 18 Newtons, the distance is 61 metres.

So we write those in, then we do the maths 4,880 and we've been asked to write this to two significant figures and 4,880 is to three.

So we need to round up to two significant figures, which gives us 4,900 and the units are joules.

So 4,900 joules is the correct answer, Okay, so here's an example, a similar example for you to have a go at.

Just pause the video whilst you do that and don't forget to give your answer to the correct number of significant figures and to show all of your working out.

Okay.

So how did you get on? Let's start by writing down the equation.

Work done is force times distance.

And then we can look back at the question to find the (indistinct) for force and distance.

Laura used the force of 260 Newtons and she pulled a bag of food seven metres up into a tree.

So pop those numbers in, do the maths and we get 1,820, but we need that to two significant figures.

So we need to round that down to 1,800 and the units are joules.

So 1,800 of work was done.

So well done if you've got the correct answer and have shown all the steps of your working out.

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 a thousand joules, just like a kilometres a thousand metres or a kilogramme is a thousand grammes.

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

So 23,000 joules is 23 kilojoules 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 2500 kilojoules.

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

Okay, how did you get on? We started from 2500 kilojoules or 2 500 000 joules.

So D is the right answer, 2500 thousands are 2 500 000 joules.

So to finish the lesson, I'd like to have a go at these five calculations and for each one to show you working out.

While you're doing those, just pause the video, then restart again once you're ready in order to check your answers.

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

So question one, Alex pushes a car with a force of 350 Newtons over a distance of seven metres.

So work done is force times distance and 350 Newtons times seven metres gives the answer of 2,450 joules of work done.

Question two, Aisha use a force of 0.

5 Newtons to lift a bar of chocolate 120 centimetres off the floor.

Now in this question we need the distance to be measured in metres.

So 120 centimetres is 1.

2 metres.

So our sum is 0.

5 Newtons times 1.

2 metres, which gives us 0.

6 joules.

Question three, Izzy watches a crane lift a pile of bricks 18.

6 metres.

The crane uses a force of 7.

2 kilo Newtons, how much work is done? So in this case we've got the force in kilo Newtons and we need to calculate it in Newtons.

So 7.

2 killer Newton is 7,200 Newtons times 18.

6 metres.

Gives us a total work done of 133,920 joules.

And question four is asking for that in kilojoules, which is 133.

92 kilojoules.

We don't need to include that last zero 'cause it doesn't actually mean anything.

And for question five Andeep does 28 joules of work lifting a bag of potatoes, a distance of 1.

4 metres, how much force does he use? Well if we put those into our equation, we've got 28 joules is the force times 1.

4 metres.

We divide both sides by 1.

4 metres and we get 28 divided by 1.

4 is equal to 20, which is equal to the force.

So the force is 20 Newtons.

So well done if you've got all of those correct.

This slide shows what hopefully you've understood from the lesson.

The law of conservation of energy states that energy cannot be created or destroyed.

Instead it's transferred.

A work done by a force that's pushing or pulling on an object is equal to the amount of energy transferred in doing so.

And this can be represented by the 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 in the surroundings by heating.

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