Lesson video

Hello, I'm Mr. Norris, and welcome to the first lesson in this topic on forces for year seven.

This lesson is all about what forces do.

This is part of the wider big question of how forces make things happen in the universe.

Everything that happens in the universe at the deepest level, any change happens, because a force has acted.

And that's why this is such a great, such an important topic to do at the start of year seven.

So let's get started.

The outcome of this lesson is that you're gonna be able to describe what forces are and what forces do.

Here are some keywords we're gonna be focusing on this lesson.

Force, exert, speed, and mass.

Now each word will be explained in detail when we get to it at a suitable point in the lesson.

The next slide will show each keyword being used in an example sentence.

So when the next slide comes up, you could pause the video and take a moment to carefully read through each keyword so you're as ready as possible for the lesson.

So you could pause the video now to have a read through if needed.

The lesson is divided into three sections.

The first section is about throwing, catching, and pushing.

The second section is about what forces are.

And the third section looks at what forces do, which is, well, forces make things change.

That's the title of the last section of the lesson.

So let's start with the first section of the lesson on throwing, catching, and pushing.

This will go over some hopefully really intuitive ideas about forces acting and what forces do.

So to throw a ball, you have to exert a force on the ball.

So look at the picture.

That purple arrow shows or represents the force exerted on the ball by the person throwing it.

And when you throw a ball faster or further or higher, then you'll need to exert a bigger force.

We could show that with a bigger arrow.

You would also need to exert a greater force to throw a heavier ball at the same speed.

So there's a bowling ball, that would be much heavier and you can see the arrow got bigger to represent a bigger force pushing it if it was being thrown at the same speed.

That's because with a heavier bull, there's more mass to get moving.

Okay, so the more mass, the bigger force needed to get to the same speed.

So quick check.

What are two factors that affect the size of the force needed to throw a ball? Have a think.

I'm gonna give you five seconds.

Make sure you come up with two factors you think affect the size of the force needed to throw a ball, and hopefully you came up with the speed you want to give the ball.

So the higher the speed you want to give it, the bigger the force needed.

And if you want the ball to go further or reach higher, then that, you have to give it more speed to go further or reach higher from your force, your throwing force.

And then the second factor would be the mass of the ball.

Okay, the greater the mass of a ball, then the greater the force needed, the greater the force needed to throw it.

So well done if you've got both of those.

What about catching? Well, to catch a ball, you have to exert a force on the ball to stop it.

So just imagine catching a ball for one moment.

The ball is going towards you, you put your hands out to catch it and the ball goes into your hands, okay? Now to stop the ball, you actually have to push on the ball to stop it moving with a force like that.

So to catch a ball, you have to exert a force on the ball to stop it moving.

So you have to kind of push against its movement to stop it.

That's why the force arrow is pointing that way for the person catching that ball.

And of course a faster ball would be more difficult to stop.

You'd need to exert a bigger force on it to stop it.

To stop a faster ball, you need to cause a greater change in speed and you'd have to exert a greater force.

A heavier ball would also be more difficult to stop.

That's a bowling ball being thrown, a bit unrealistic.

But you could see a greater force would be needed to stop that, because it's heavier, it's got more mass.

It's more mass to stop, more material to stop.

So you're gonna need a bigger force to do that.

Okay, let's do a check of what we just went through.

This check has got two parts.

I'm gonna show you part one first.

Which tennis ball is hardest to stop? Is it option A, ball one, three metres per second, option B, ball two, eight metres per second? They're both tennis balls.

So, or is it option C, are they both the same difficulty to stop? So make a decision for part one, A, B, or C.

Five seconds.

Okay, I'm not gonna give you the answer to part one straight away.

I'm gonna show you part two.

Part two asks you to explain your answer to part one for which tennis ball is hardest to stop.

So whichever you chose or are they both the same? Whichever you chose, is that because it's faster, it's slower, because they're the same size, because it has more force, or because they have less force? So make a decision for part two now.

Okay, I'm gonna go through the answers.

The correct answer for question one, the ball which is hardest to stop, which I'm sure you got, was ball two, which is option B.

And the correct reason why that's harder to stop is just because it's faster.

Faster objects are harder to stop.

If you chose option D, that is the common misconception that moving objects somehow have force.

They don't have their own force.

Forces act from other objects as we'll see later in the lesson.

Ball two is harder to stop just because it's faster.

It's nothing to do with force.

You would need to exert more force to stop it.

It would hit your hand with a greater force.

It would exert a greater force on your hand, but it doesn't have more force.

It needs a greater force to stop it.

The reason it needs a greater force to stop it is simply 'cause it's faster.

So that's the correct answer.

Very well done if you've got option B for question one and option A for question two.

Great.

Right, we just need to say one more thing about the effect of mass.

We need to really spell out that the more mass an object has, so the heavier it is or the more material it's made up from, that's also gonna make it more difficult to change its direction.

So look at these two trolleys.

The full trolley, I'm sure you'll have had this experience if you've pushed trolleys around a supermarket.

The fuller the trolley gets, the harder it is to push it round the corner, because to go round the corner, you have to change the direction from going straightforward to going in a different direction round the corner.

That's because it's got more mass, okay? Needs a greater force to turn, because there's more mass, there's more material that you've got to change the direction of.

So that's why something with more mass is harder to push around a corner.

So what do you think about trolley one and trolley two? Okay, so this is called a confidence grid.

for each statement A, B, and C, you have to choose one of the boxes to tick.

If you're sure it's right, if you think it's right, so depending on how confident you are or if you think it's wrong or if you're sure it's wrong.

So for each statement, tick, choose one box that you would tick.

So for statement A, trolley two is harder to get moving.

How confident you are that, is that right or is it wrong? And how confident are you? Then statement B, trolley two is harder to stop.

Is that right? Is that wrong? How confident are you? Choose a box.

And statement three, trolley two is harder to change the direction of.

So is that right, is that wrong? And how confident are you? So make a decision on those now.

I'll give you five seconds just to finalise your choices for A, B, and C.

Okay, are you ready? So trolley two is harder to get moving in the first place.

Yeah, that's definitely right.

Okay, 'cause it's got more mass, there's more mass to get moving, they would need a greater force to get it moving than trolley one, which is empty.

But what about if they were already moving? Is trolley two then harder to stop than trolley one would be if they were already moving? Well, it would be 'cause it's got more mass.

The more mass something has, the bigger the force needed to stop it.

And trolley two is harder to change direction.

Well, we just said that in the previous explanation.

That's right as well.

So well done if you thought all of those were right and very well done if you were sure each of those were right 'cause they all were, well done.

Okay, so we're now gonna do a task.

You're gonna do this yourself.

So the question is, which bowling ball would be most difficult to stop? Ball A, that's eight kilogrammes going at three metres per second.

Ball B, that's going, that's eight kilogrammes, but going at 10 metres per second.

Ball C, that's 16 kilogrammes going at three metres per second.

Or ball D, that's 16 kilogrammes going at 10 metres per second.

So which bowling ball would be most difficult to stop? So you need to choose an option, A, B, C, or D.

And you need to write an explanation using what's the words we just talked about if you can, about why is it most, why that choice would be the one that's most difficult to stop.

So you should pause the video now to give that a go.

Okay, you should now have completed that task.

You should have chosen a letter and written the explanation of why that bowling ball is most difficult to stop.

So here's the feedback.

Well, the correct option is ball D.

Okay, ball D is most difficult to stop and here is an example where you could have explained why.

Well, that's actually two reasons combined.

You kind of need to say both really.

Why is it the most difficult to stop out of all of those four? Well, it's 'cause it's got the highest mass and it's also got the highest speed.

Okay, so it's got the highest mass and the highest speed and both of those factors together means it needs the greatest force to stop it.

So well done if you've got the word force into your answer, it's the hardest to stop, 'cause it needs the greatest force to stop it.

Might be a good idea to pause the video and make any improvements to your answer.

Very well done if you got, wrote an answer that was on the right track there.

So the next section of the lesson is about what forces are.

We've mentioned forces in passing quite a bit to do with throwing a ball.

You have to exert a force on it, catching a ball, stopping a ball or changing a trolley's direction, you have to exert forces on those objects.

So what are these things called forces that we've mentioned? So we've said throwing a ball involves pushing on the ball to set it in motion and the arrow represents the push.

That's the force.

Catching a ball also involves pushing on the ball, but the push is against the motion to stop the ball moving.

So in both cases, the hands exert a force on the ball and we use that word exert for the action of a force.

A force is exerted by one object on another.

That word exert.

It's a really nice, it's the perfect word for describing where forces come from.

Forces get exerted or forces are exerted on an object.

The pushes and pulls that the objects exert on each other are called forces.

So that's what forces are.

Forces are the pushes and pulls that objects exert on each other.

And forces, the pushes and pulls, that is how all objects interact.

How do objects affect each other? How can one object affect another object? The answer is by pushing or pulling it through forces.

So what is a force? Is it option A, a push or a pull on an object is a force, what an object has when it moves, is a force the energy an object has, is a force the power an object has? Only one of these is a good physics answer.

So choose which one you think please.

Okay, the correct answer is A.

A force is just a push or a pull on an object.

If you said B, that's the mistake that students often make that a force is something an object has 'cause it's moving.

That's not what a force is.

A force is one object pushing or pulling on another object and a force is not energy and a force is not power.

They have different ideas.

A force is a push or a pull on an object.

That's really important to be clear about.

There has to be another object pushing or another object pulling for there to be a force on a second object.

So that leads us really nicely to this idea that every force always involves two objects.

One object to provide the force and another object that the force then acts on.

So for example, that arrow represents the push of the hands on the ball.

The hands provide the force, but the force acts on the ball.

This represents, that arrow represents a pull from the magnet acting on that iron nail.

The iron nail gets attracted to the magnet.

So we could label the force as the force of the magnet on the iron nail.

The magnet provides the force and the iron nail is the object the force acts on.

So any force that acts always involves two objects in this way.

Any force that acts can be described as the force of object A on object B, the force of one object on another object or one object pulling another or one object pushing another.

So for the rest of this lesson, we're gonna be doing lots of practise at using that structure of sentence to describe forces fully.

Because if we can describe the force fully and if we know what object is pulling or pushing on what other objects, then that helps us to understand forces much more clearly.

So let's do a first practise at describing a push or pull, a force.

So this is the force used to open a door by pushing it.

So there's gonna be three questions that come up or two questions and then a sentence to fill in which describes the force.

So firstly, think about which object provides that force.

Secondly, which object does this force act on? And that will give you the two objects to complete the sentence at the bottom to describe the force.

So please have a go at that now.

Okay, I'll go through what you should have come up with.

The hand or the person provides that force.

That force acts on the door, 'cause the hand is pushing the door and that gives you the last sentence.

This is the force of the hand on the door.

Okay, so it's as simple as that to describe forces.

You just need to look at what's pushing on what.

Let's do another example.

So this is a tennis racket hitting a ball back.

So that is the force we want to describe.

So there are two questions here.

First question, which object provides the force? Second question, which object does the force act on? And then the third part is then to put it all together and describe what that force is represented by that arrow.

Have a go at doing that now please, should be quite quick.

Okay, I'll go through the answers.

Which object provides this force? It's the racket.

Which object does the force act on? The ball.

So this is the force of the tennis racket, on the ball.

Very well done if you've got that.

Okay, so time for a test now where you can practise labelling forces yourself using that structure.

So describe one force in each example of the force of object A on object B.

So you've got to look at each picture and work out, well, what is pushing on what or what is pulling on what.

And there should be one force, one obvious force to label in each example.

But beware there is one example where there is no force acting, where there's no objects pushing on other objects.

So look out for that one, 'cause there's nothing to write for one of these, but you do have to describe the force in the other five.

So you need to pause the video now and have a go at that task.

Off you go.

Right, I'm going to give some feedback on that task now.

So make sure you've given it a good go.

So I'm going to identify the most obvious force acting in each example, the most obvious push or pull that's happening.

So in the first example, we've got a foot kicking a football.

So the foot exerts a force on the football.

That force arrow, which you didn't have to draw, you just have to describe the force.

That is the force of the foot on the football.

The foot provides the force and the force acts on the football.

The football gets kicked.

Okay, what about example two? Pushing a crate that slides.

Well, there's an obvious push there.

There's the force of the hands on the crate.

So you just had to do the writing.

I've just included an arrow on the diagram as well.

So you can see where that force is acting.

That's the force of the hands on the crate.

What about number three? Well, the person is pulling that crate.

So we've got the force of the hands on the crate again, or you could have said that the force of the person on the crate, that would be fine.

Four, there's obvious pushes.

You could have said the force of person one pushing on the ball.

So that's the force of person one on the ball.

But I'll give another option as well.

You could have said, well, the force of person two on the ball.

So they're both pushing on the ball.

So you could have had either, the force of person one on the ball or the force of person two on the ball.

That's an obvious push in that example.

It's number five.

That's the one with no forces acting, nothing is pushing on that rock moving through space.

It's spinning and moving through space just with the speed it's already had.

But nothing is pushing it to speed it up.

Nothing's pushing it to slow it down or stop it moving.

Nothing's pushing it to change its direction, it's moving through space at steady speed in that straight line in the picture.

So there's nothing pushing or pulling it that we can see in that picture anyway.

So that was the option where there were no forces.

Very well done if you spotted that one.

Whereas six, well, I thought the most obvious force in this one is when you put a heavy object like a crate down on a table, it's obviously gonna push down on that surface, because it's pushing into it.

Okay, so I would call that the contact force of the crate on the table, because the crate only pushes down on the table if it's touching it.

So that's what a contact force means.

Now I will just mention for question six that you might have thought of the force of gravity on the crate.

So the gravitational force of the Earth on the crate or the downwards weight force that's acting on the crate.

Now it's important to mention that if you did think of that one, that would actually be a different force to the one that I've described.

You've got the gravitational force of Earth on the crate.

So the crate gets pulled down towards Earth's surface, but the table's in the way, so the crate then pushes on the table.

So I've, I thought in this, in that picture, the force of the crates pushing down on the table was more obvious to label than the gravitational force pulling the crate down towards Earth.

But I will just mention that both would be a valid answer to question six.

Very well done for your effort on this task.

It's time to do the last section of the lesson.

And this covers what forces do.

And the answer, of course, to what forces do is forces make things change.

So let's look at that in detail now.

So where I'll start is the idea that forces can change the speed of an object.

So the force of the explosion on the firework is gonna make that firework speed up upwards out of the ground and the force of the air on this parachute is gonna make the parachute slow down.

So forces in both of those situations, forces are causing a change of speed.

That's really important.

Forces don't give things speed, forces change speed.

We should also take a moment just to look at how each sentence was constructed.

Each sentence there started with a description of the force and then gave the effect of that force.

So for the firework, we've got the force of the explosion on the firework, that's the description.

And then the effect of the force of the explosion on the firework is to make the firework speed up.

So describe the force and give the effect.

That's the same with the parachute.

The force of the air on the parachute, that's the force, makes the parachutist slow down.

That's the effect.

So describe the force and state the effect.

The force of the A on the B makes the B change speed.

So we're gonna have a go at practising using that sentence structure now.

So this football was stationary, the speed was zero, and then the football was kicked.

So complete the sentence to describe the kicking force and the effect, just like on the previous slide.

The kicking force of the something on the something causes a change in the ball's something.

See if you can fill in those gaps.

Pause the video if you need to.

Okay, I'll go through it.

The kicking force of the foot on the football causes a change in the ball's speed, 'cause forces can change the speed of objects.

So hopefully that was fairly straightforward and very well done if you got it.

It's important to mention at this point that if a force is not causing something to change, that can only be because other forces are acting too, cancelling out the effect.

So Ann and Beth are both pushing on that ball.

Here's the force of Ann on the ball.

And if there was only that force acting on the ball, that ball would speed up to the right, but the ball's not speeding up to the right and that's because Beth is pushing on it too.

There's the force of Beth on the ball.

So the two forces are cancelling each other out, so neither can cause a change.

So forces do make things change unless forces are cancelling each other out.

It's the same with this bicycle.

That bike's going at steady speed.

So the speed's not changing.

Yet there is a forward force on it.

There's the force from the person that pushes the bike forwards.

So why is the bike not speeding up? Well, the answer is there must be another force acting, cancelling out that force so that it's not causing a change.

And it's the force of air resistance, okay? If you try and cycle through the air, the air pushes back on you.

That's the force of air resistance, the force of air back on the bike and the person.

So that cancels out the force from the person that pushes the bike forwards.

So neither force ends up causing a change, because they're being cancelled out.

But forces do make things change.

That's what forces do.

So here's another change that forces can cause.

Forces can change the direction an object is moving in.

So the first thing, forces could change with speed.

And the second thing is direction.

Forces can change the direction an object is moving in.

So imagine a golfer taking a golf shot and they try to hit the ball in a straight line towards the flag, but the force of the wind on the ball is gonna change its direction.

So a force can change the direction of a golf ball force from the wind.

And in this example, when the cue ball collides with the cushion barrier of the snooker table, a force from the cushion on the ball changes the ball's direction, 'cause when the ball collides into the cushion, it bounces off in a different direction.

That's because there was a force from the cushion on the ball that changed the direction of the ball.

So forces can change the direction something's moving in.

There's just a zoomed in picture of the force from the cushion on the ball.

And then off the ball goes in a different direction.

Finally, a third thing that forces can change is that forces can change the shape of an object.

Okay, so we've got forces can change speed, direction, something's moving or shape.

Here's some examples of that.

The force of the hand on the balloon, that's the description of the force.

That force changes the shape of the balloon.

That's the effect.

The force of the hand on the balloon changes the shape of the balloon.

So that's the description of the force and its effect.

And the balloon gets squished like that.

And in this case, the force of the mass on the spring changes the shape of the spring.

So the force of the mass on the spring, that's the description of the force, changes the shape of the spring.

That's the effect of that force.

So let's do a quick check.

Have you been paying attention? Name the three key properties of objects that forces might change.

See if you can do that please.

I'll give you five seconds to have a go.

Well done if you came up with forces can change the speed of an object, the direction of movement, or the shape.

Well done if you've, very well done if you've got all three.

Let's do another check of your understanding.

A moving tennis ball is hit back harder with the racket.

So it's hit back harder than it was going towards you.

What are the effects of the force of the racket on the ball? And this is another confidence grid.

So you need to tick for each statement, A, B, and C.

You need to decide if you think the statement is right or wrong and then say how sure you are.

So if you're sure it's right, tick the first box.

If you think it's right, but you're not sure, tick the second box.

If you think it's wrong, tick the third box, and if you're absolutely sure it's wrong, tick the fourth box.

So statement A, the force of the racket on the ball changes the ball's speed if it's hit back harder.

Which box do you think? Statement B, the force changes the ball's direction.

Is that an effect of the force? What do you think? And statement C, the force changes the ball's shape when it's being hit.

What do you think? Okay, I'll give you five more seconds to finalise your answers for A, B, and C.

Okay, let's see how you got on.

The force of the racket on the ball does change the ball's speed, because it's being hit back harder with the racket.

So it's gonna have a different speed.

The force changes the ball direction, yes, 'cause it's being hit back.

It goes towards the racket and now the force has changed the direction.

So the ball ends up moving away from the racket, 'cause it's hit back.

And finally the force changes the ball's shape.

Well, it does because the effect of the force of the racket on the ball is gonna squish the ball a little bit when it's being hit.

So actually this is an example where the force causes all three possible effects at once.

Very well done if you've got that.

Right, we're ready to do the final task of the lesson.

In this task, I would like you to add a description to each example, describing the force, the force of object A on object B.

So you need to look for what is pushing on what or what is pulling on what.

And just write that.

What you need to do next is then also state what the force changes about object B in that statement.

So here are the examples that you need to do that for.

You've got throwing a ball.

I want you to describe the force on the ball.

Here is an ice hockey puck sliding along ice.

I want you to describe that force on the puck.

So write a description and state what the force changes about the puck.

Here is a person landing on a trampoline.

I'd like you to describe the force on the trampoline and say what the force changes about the trampoline.

And the same example, but you're gonna focus this time on the force that's on the person when you land on a trampoline.

I'd like you to describe the force on the person and state what the force changes about the person.

Okay, so you should probably pause the video now and get a pen, because you need to write a description of each force and write down what the force changes in each example.

So have a go at that now.

Off we go.

Right, so I'm gonna go through each one in turn, you should have done all four.

Let's have a look.

So in number one, that is the force of the hands on the ball.

It's gonna cause the ball to speed up, because the ball is not going very fast before it's thrown.

And then just after the ball's been thrown, it's going faster.

So that is the effect of the force of the hands on the ball.

It causes the ball to speed up.

Great, if you've got that one, well done.

Number two, the friction force, 'cause that's a friction force, force of friction.

You didn't have to say that, but well done if you knew that.

The force of the ice on the puck, that's the description of the force.

It's a force of the ice on the puck.

What effect is it gonna have? It's gonna cause that puck to slow down as it slides.

Well done if you've got that.

Here's number three, the force of the person on the trampoline.

That's the description of the force.

The force of the person on the trampoline is gonna cause the trampoline to stretch.

And that's a change of shape, cause the trampoline to change shape.

Well done if you've got that one.

Now let's look at it from the other way round.

At the same time, when a person lands on the trampoline, they get slowed down and they change direction, 'cause they're falling down and they might end up bouncing back up again.

So the force of the trampoline on the person, that's the force, makes the person slow down and perhaps eventually change direction as well.

So very well done if you've got those.

Great, you've made it to the end of this lesson.

Well done.

Let's look at a summary.

Okay, so forces are the pushes and pulls that objects exert on each other.

Any force can be described as the force of one object, object A, pushing on another object, object B, or pulling.

For example, the force of the foot on the football.

Forces are what cause objects to change speed, direction, or shape.

So anytime anything changes speed, changes direction, or changes shape, that's because a force acted.

The faster an object is, the more difficult it is to stop, the bigger the force needed.

And objects with a greater mass also need a greater force to start the moving, stop the moving, also change the direction of their motion if they've got more mass.

Well done for completing this lesson.