Lesson video

Hello and welcome to this lesson on the different kinds of force.

This is from the year seven forces topic.

My name is Mr. Norris.

This is a really important lesson because as we've seen, forces are what make things change.

Everything in the universe that changes changes because a force acts.

So, understanding the different kinds of force can help you understand everything in the universe.

So let's get going with that.

The outcome of this lesson is that hopefully you'll be able to identify and describe different kinds of force and the effects that they have.

Here are some key keywords for today's lesson.

Contact force, friction, drag, and non-contact force.

Now, each of these words are gonna be explained as they come up in the lesson.

But on the next slide there is an example sentence of each keyword being used, which you could pause the video and have a look at to get as prepared as possible for the lesson.

So you could pause the video now to have a read through and then we'll be ready to start.

The lesson is split into three sections.

The first section talks about contact forces.

And the second section then talks about friction and drag forces, which are actually a kind of contact force, but there's enough to say on those that they get their own section.

And then the other kind of force is a non-contact force.

So all forces are either contact forces or non-contact forces.

So let's look at the contact forces first.

So, contact forces are the forces that only act when objects are touching.

So there are lots of different kinds of contact forces, but what links them all is they only act when objects are touching.

So for example, there's a force here, represented by the purple arrow, of a foot kicking a football.

So the foot contacts the ball, it touches it, that's when the foot actually kicks the ball.

And then the force of the foot on the ball.

What effect does it have? Well, it increases the speed of the ball 'cause forces make things change speed, direction or shape.

And in this case it's the speed of the ball which is probably being changed.

So now the ball's rolling along.

Contact with the foot has now been lost.

So there's now no force from the foot on the ball.

The force of the foot on the ball only acted when the foot was in contact with the ball.

So the football's not speeding up anymore.

'Cause there's no force acting on it because the force stopped acting on it when it left the foot when it lost contact with the foot.

So contact forces only act when objects are touching.

And as soon as contact is lost, the force stops acting and so the effect stops happening.

So that ball is no longer speeding up.

Here's a different kind of contact force.

This is a pulling force from a string, which is lifting the pencil case upwards.

So that purple arrow represents the force from the string pulling the pencil case upwards.

So pulling forces from strings or ropes are called tension forces.

So that's a really useful word, okay? A kind of contact force from a string or a rope is called a tension force.

We say the rope will string is under tension.

So we could label that force arrow as the tension force of the string on the pencil case.

So the string provides the force and the pencil case is what the force acts on.

So it's the tension force of the string on the pencil case.

Let's look at another kind of contact force now, driving forces.

So a driving force is any force that's provided to push something forwards.

Now there's lots of different ways that you could create a driving force.

The firework shows one way.

So, hot gases shoot out the back of the firework and that's what pushes the firework forward.

So a force acts on the firework because of the explosion, because of the hot gases shooting out of the back.

But the forward force on the bike comes from a kind of a completely different idea.

The rider pushes on the pedals and that moves the chain, and that makes the back wheel turn and that's what ends up pushing the bike forwards.

So driving forces can arise in different ways using different mechanisms. Now you might see the term thrust force used where I've used driving force.

Now, technically a thrust force is caused by pushing or releasing material away in the opposite direction.

So the force on the firework is a thrust force, but technically the force on the bicycle is not a thrust force 'cause it's not caused by pushing material away in the opposite direction.

With the firework, all the chemicals which burn get pushed out the back of the rocket.

And that's what causes the forwards force on the rocket.

However, be aware that you might see the term thrust force being used where I've used driving force.

We'll look at another kind of contact force now.

So on that hot air balloon, how does it stay in the sky? A hot air balloon stays in the sky because there's an upwards force acting on it.

And it's an upwards force actually from the surrounding air.

That's what could make it rise.

And the same with boats floating.

How do boats float? Well, there's an upwards force from the surrounding water on that boat.

That's what allows it to float.

So both of those are called upthrust forces.

So forces from surrounding air or surrounding gas or forces from a surrounding liquid are called upthrust forces that, and they always do push vertically upwards.

So let's do a check about some of the contact forces that we've just gone through.

I would like you please to match each force to the correct name.

So I'm gonna give you five seconds to have a go at doing that.

Pause the video if you need to.

Right, let's see how you got on.

The force from a rope pulling.

That's called tension force.

Well done if you've got that.

The forwards force on a vehicle, that's called a driving force.

And the force that makes things float, that's called upthrust.

Very well done if you got all three.

There's another kind of contact force that I want us to look at now.

And this is the contact force provided by surfaces.

You might think of surfaces as not doing much, it's just you put stuff on surfaces.

But actually, surfaces push back on objects that push on them.

Okay? It's true.

So that box is pushing down on the table because it's a heavy box.

It weighs down on things that you put it on.

So that is a force of the box on the table, the force of the box on the surface of the table.

So what did we say that surfaces do? Surfaces push back.

So because the box is pushing on the table, the table pushes back up on the box.

And that's how surfaces support objects that are put on them.

If the table could or wasn't strong enough to push back, then that box would fall through the table, break through the table.

But the table is strong enough because the table provides an upwards force on the box to stop the box falling.

So that is called a normal contact force, okay? It's a contact force because the table is in contact with the box.

The force only acts when there's contact between two objects.

When one object pushes on another in contact, the second object pushes back on the first with a normal contact force.

What's the word normal doing there? Well, the word normal in maths and physics and mathematical sciences, the word normal can mean at 90 degrees.

So a normal contact force is a contact force that acts at 90 degrees to the surface.

It acts normal to the surface.

It acts at 90 degrees to the surface.

So that's why the force from a surface pushing back on objects that push on them, that's called a normal contact force.

Okay, so we've gone through lots of different kinds of contact force now and we wanna put it all together.

So we're ready to do a task in a moment.

So every single force, all forces can be described using that sentence which is on the screen now.

Any force that acts, you can write this sentence to describe that force.

You write the type force.

So you have to say what type of force it is, like the upthrust force or the driving force or the tension force or just the contact force would do or a push force or a pull force.

The type force of the object A on the object B, that's just like we did in lesson one of this topic.

All forces involve one object, object A, that provides the force, pushing on or pulling on a second object, object B.

So there's the object that provides the force, object A, and the object that the force acts on, that's object B.

So any force can be described as using the type of force it is, what object provides the force, what object the force acts on.

The type force of the A on the B, like the friction force of the ground on a sledge, for example.

Okay, so I'm gonna show you another work example.

So if you were asked to describe this force, you can use that exact sentence.

What type of force is it? What provides the force and what does the force act on? Well you need to look carefully at the diagram and work out what type of force is it first.

Now that force is pushing upwards, but it's an upwards force from the ground, or that's a surface.

So the force pushing of a surface pushing back as the sledge is pushing down on the ground on the surface of the ground.

So the surface of the ground pushes back.

So that's a normal contact force, okay? So it's the normal contact force of the ground on the sledge.

Simple as that.

Okay, now I'd like you to have a go at using that sentence structure using this force that acts on the same sledge.

So what type of force is it? What provides the force and what does the force act on? Have a go at filling in those three gaps now.

Pause the video if you need to.

Okay, so let's see how you got on.

So hopefully you had a good look at that diagram and you realise that that arrow's being positioned kind of between the rope and the sledge.

So this is the rope pulling the sledge.

So that's the last two gaps.

So what kind of force comes from a stretched rope? It's a tension force.

So, very well done if you spotted that this is tension force, 'cause it's a force from a rope.

So it's the tension force of the rope on the sledge.

Very well done if you've got that.

Hopefully you're now ready to do a task about contact forces.

I want you to use the exactly the sentence I just showed you to describe the four contact forces that are shown.

So two of those forces are acting on a helium balloon.

A helium balloon is one of those balloons which is gonna float up and up and up and up if you let go of it.

Okay, so you've gotta hold onto the string to stop it from flying away.

So there's force one.

Can you complete that sentence to describe the force? And there's force two.

Can you complete that sentence to describe the force? And then what about these forces on a climber? That's force three.

Could you describe that force? And then think about what that force is in number four, what those forces are, how would you describe those to complete those sentences? So you need to pause the video now and have a go at completing those four sentences to describe those contact forces.

So pause the video now, off you go.

Right, let's go through some answers and so you get some feedback.

Well done for your effort on that task.

So force one is pointing up, it's a helium balloon and it's gonna end up floating upwards.

So the force that makes things float is an upthrust force.

So it's an upthrust force.

And what provides an upthrust force? Hopefully you remembered it's the surrounding air or the surrounding gas.

So it's the upthrust force of the air or the surrounding air on the balloon, because that force is acting on the balloon.

It's an upthrust force of the surrounding air on the balloon.

Very well done if you got that.

Number two.

Now what stops the balloon from floating upwards and upwards and upwards into the sky? Well, it's the string.

So, what's the name of a force provided by a string? It's a tension force.

It's the tension force of the string pulling down.

Well done if you've got that gap.

Force number three is another tension force.

'Cause it's a force from the rope that's holding the climber upwards.

That's what's stops the climber from falling.

So it's the tension force of the rope on the climber.

Well done if you got that.

And then what about number four? Now the climate in that position, their feet are gonna be pushing on the surface of the rock cliff.

So when something pushes on the surface, the surface pushes back.

And that's called a normal contact force.

So each foot pushes on the wall, that's why there's two arrows.

And therefore the wall, the rock wall pushes back on their feet.

So they are the normal contact forces of the cliff on the climber's feet.

So very, very well done if you've got lots of those right.

Right, that's the first section of the lesson completed.

So we've gone through lots of different contact forces, but there are two that we've not mentioned yet.

Friction forces and drag forces.

So these are more contact forces, but there's a lot to say on them.

So they get their own section.

So let's look at, we'll start with looking at friction forces.

So friction is the force of one surface gripping or rubbing on another.

So looking at that diagram, there would be, as the person is dragging the sledge across the ground, there's gonna be friction.

So that would be the friction force of the ground on the sledge.

Ccause the sledge is kind of, the ground is gripping on the sledge as the sledge tries to slide over it.

The sledge kind of like rubs across, rubs across the ground causing that kind of gripping friction force.

So friction is a contact force, because if the sledge wasn't touching on the ground, then the ground couldn't grip it or rub it.

So there has to be contact for friction to act.

And another really key point about friction is friction forces always act in the opposite direction to motion.

So they cause moving objects to slow down.

Friction has a slowing effect on moving objects.

But not only that, friction forces can also prevent movement if a pushing or pulling force is too small.

So here's a person trying to pull a crate, trying to drag it along the ground.

So that's the force of their hands on the crate.

But they might not pull hard enough.

The pulling force might be too small, so it might not be able to overcome the friction force from the floor on the crate.

So friction doesn't just slow things down.

Friction can also prevent movement as well.

But friction forces only act when there's movement or when there's attempted movement from another force trying to produce movement.

So there's no friction force on that box because it's not moving and there's no movement and there's also no attempted movement, so there's no grip, there's no friction force.

You only get a friction force when there's movement or when there's attempted movement from another force that's trying to produce movement.

Let's do a quick check about friction.

So a child in socks runs and then slides on a smooth, wooden floor.

You might have even done that as a smaller child.

Describe the forces shown and their effect.

So where are these forces, those forces? So please can you fill in the gaps? You should be able to do that fairly quickly.

Pause the video if you need to.

Right, let's see how you got on.

Can you fill it in the gaps to describe the forces shown? So those forces shown are the friction forces.

Okay, this is how we describe forces.

We say what type of force it is first, and then we're gonna say what provides the force and then what the force acts on, and then the effect of that force.

So these are the friction forces of the floor on the child's socks.

And what effect is that gonna have? Well, friction forces in this case cause moving objects to slow down.

So the friction forces of the floor on the child's socks cause the child to gradually slow down until they stop.

Well done if you had something along those lines for the effect of the force.

So that's friction forces.

We're now gonna talk about drag forces.

So think about stirring a cake mix.

It can be really difficult if the mixture's really thick, can't it? If you imagine kind of stirring it around, the mixture can be quite stiff and that can make it difficult to stir.

That's because cake mixture like pushes back.

As you try and move the spoon through the cake mixture, the mixture pushes back on the spoon making it difficult.

The mixture pushes back against the spoon's movement.

The force of the mixture on the spoon, that's called a drag force.

Or you could just say drag acts on the spoon, or a drag force acts on the spoon.

So drag is also a contact force because drag acts because there's contact between something moving through a liquid and the liquid itself.

If there wasn't contact, there would be no drag force.

And just like friction, drag forces always oppose motion.

So drag forces always gonna cause moving objects to slow down.

So, drag forces occur when any object moves through a liquid or gas.

If the gas something's moving through is air, then a drag force could be called air resistance.

So you've probably heard of air resistance before.

That's the force of the air pushing back on a moving object when the object is trying to move through air.

But air resistance is just a specific name for drag when something is moving through air.

So here are examples of air resistance forces.

The aeroplane moves through air.

So an air resistance force or drag force acts back on the aeroplane.

And the same for the car, it's moving through air.

So that's an air resistance force or a drag force.

And the same for the parachuter.

They're falling through air, so that is an air resistance force which slows them, which could also be called a drag force.

In water, drag forces can be called water resistance.

So there's a boat moving through water, and you could call that drag force.

The drag from the water could be called water resistance.

It's important to say that drag forces act on every part of a moving object's surface.

So that's kind of a representation of how drag forces really act on something that's moving through a gas, air in this case.

However, we don't draw that 'cause diagrams would just become so complicated you couldn't see what was behind the force arrows.

We just use a single force arrow to represent the drag forces that really act on every part of an object's surface.

Okay, let's do a quick check about drag forces.

Find one mistake in each sentence and correct it.

Have a go at doing that now.

You'll probably need to pause the video for this one.

Okay, let's see how you got on with finding those mistakes and correcting them.

So sentence one, drag forces only act on stationary objects.

Stationary means not moving.

That's not true.

Drag forces only act on moving objects.

Objects have to be moving for drag forces to act.

Sentence number two, drag forces act in the same direction to motion.

No, 'cause drag forces slow things down, so they act in the opposite direction to motion.

Drag forces always push back against the motion, not in the same direction as motion.

Well done if you got those.

Sentence number three, drag forces cause moving objects to speed up.

Nope, definitely slow down.

Drag forces cause moving objects to slow down 'cause the object is dragged back by trying to push through the air or push through a liquid.

And sentence number four, drag can be called gas resistance if the fluid is air.

We just need to change gas to air.

Drag can be called air resistance if the fluid is air.

Well done if you've got most of those right.

Right, it's time for a quick task on this learning cycle.

Everything we've just gone through in this section of the lesson.

So have a go at describing, please, the two forces that oppose the motion of the snowboarder.

There's this one.

Please fill in the gaps to describe that force.

Make sure identify what it is correctly, what type of force is that one? And there's this force that acts there, that second arrow that just appeared.

Please could you fill in those blanks to describe that force? So pause the video now and have a go at filling in those blanks to describe those two forces that oppose the motion of the snowboarder.

Pause the video now.

Okay, let's go through some answers now.

So the first force is a drag force or air resistance force.

It acts at every point on the surface, but we only draw one arrow to represent the drag force or the air resistance force.

So that's the force of the air pushing back on the snowboarder.

Or you could have just said pushing back on the person.

So the drag force or the air resistance force of the air on the snowboarder or person.

So what about the second force? Well that's a friction force 'cause it's caused by rubbing or gripping.

The bottom of the snowboard rubs and grips on the snow a little bit as they slide over it.

So that's the friction force of the snow on the bottom of the snowboard.

So, very well done if you've got most of that right.

Okay, so we've covered contact forces in a lot of detail.

Now we need to look at the non-contact forces and talk about how they're different to the contact forces we've looked at before.

So, non-contact forces.

Well, the clue is in the name, non-contact.

They don't need contact.

They're the forces that can act with or without objects touching.

And there's only three kinds of non-contact force.

The first kind is magnetic forces.

I'm sure you've come across magnetic forces and magnets and played with 'em before.

So magnetic forces act between magnets and magnetic materials.

So that blue arrow represents the force of the magnet on the iron nail.

And the magnet doesn't need to touch the nail to exert the force on it.

That's why magnetism, magnetic forces are non-contact forces because objects do not need to touch for the force to act.

So that's the first kind of non-contact force.

The second kind of non-contact force is called an electrostatic force.

This is the kind of force that acts between objects that have charge.

So here, a balloon's being charged up, perhaps by rubbing it on a jumper, and that's deflected the water that's flowing outta the tap.

That's a trick that you might have tried yourself.

So that represents the electrostatic force of the charged balloon on the water.

The water has changed direction because of that force.

Okay, and then we've got the third and final kind of non-contact force, which is called a gravitational force.

Gravitational forces pull objects towards large objects like planets, moons, and stars.

So that blue arrow there represents the gravitational force of earth on a parachuter.

This is why objects fall, because of gravitational forces.

Now, gravitational forces always point towards the centre of the nearest planet, moon or star, or the planet, moon, or star that you're on.

However, we don't often draw the whole Earth like in that diagram.

More often, we just draw something falling vertically downwards like that parachuter.

So we should draw gravitational forces also pointing vertical downwards in that representation as well as if that force arrow is pointing back down towards the centre of the Earth.

So gravitational forces should always point in the direction of the centre of the nearest planet, moon or star.

Like in those two examples.

Gravitational forces can also be called weight forces, or sometimes just weight.

And gravitational forces should never be labelled just as gravity, 'cause Earth's gravity surrounds the entire Earth.

Whereas this is the specific gravitational force that's acting on that skydiver because it's in Earth's gravitational field.

So never label these forces just as gravity, 'cause that's the word for what the Earth's got that causes this specific gravitational force.

So you can label these forces as gravitational force or weight force, or weight, but never gravity.

Time for a quick check on that.

Please could you describe this non-contact force and its effect? So this force is acting on the tennis ball.

The tennis ball's just been dropped and it's falling back to Earth's surface.

So what kind of force is that? Try and fill in all of the gaps, please.

Pause the video if you need to.

Okay, I'll give you some feedback now.

So this is a downwards force.

It's a gravitational force.

The force that makes things fall downwards is called a gravitational force.

It's the gravitational force of Earth.

'Cause that's what provides the force on the ball, the gravitational force of Earth on the ball.

But what's the effect of this force? Well, gravitational forces, that gravitational force, they have the effect of causing the ball to change speed.

That's what forces do.

Forces make things change speed, direction or shape.

And gravitational forces will tend to make objects speed up as they fall downwards.

Very well done if you've got that.

Right, so let's now do a task about non-contact forces or focusing mainly on non-contact forces.

So part one of the task, I'd like you to write the names of the three forces which are acting on this floating paperclip.

So have a look at the picture, make sure you can work out what three forces would act on that paperclip.

Then part two of the task, I then want you to describe each force using the proper sentence structure that I've taught you this lesson.

So for each of the three forces, you need to write one sentence that describes that force as the type force of something on the something.

And then finally, part three of this task, explain which of those three forces are non-contact forces.

So it says explain, you've got to say why each one that is a non-contact force, why is it a non-contact force? So the task has three parts.

You should pause the video to give each part a good go.

Have a go now.

Right, well done for your effort on the three parts of that task, I'm gonna give you some feedback now.

Okay, so for part one, three forces acting on the paperclip.

Was there tension force? That's the force from the string.

Now that's a contact force.

There's the magnetic force from the magnets.

That's why the paperclip is not falling.

But there's also a gravitational force that is trying to make the paperclip fall.

So well done if you identified if those three.

There's a tension force, magnetic force, and a gravitational force acting on the paperclip.

Those three forces, they're all cancelling each other out.

So they're not actually causing a change in this situation.

The paperclip's still where it is in that picture.

Right, part two of the task was to describe each force using the structure that I've taught you, the type force of what on the paperclip.

'Cause all of these three forces are acting on the paperclip.

So we should have, hopefully, the tension force of the string on the paperclip, the magnetic force of the magnet on the paperclip, and the gravitational force of Earth on the paperclip.

Well done if you got those right.

And now finally, part three, explain which of those three forces are non-contact forces.

And we're looking to give an explanation.

So the magnetic force is a non-contact force because the force acts without the magnet having to touch the paperclip.

That's why magnetic force is a non-contact force.

Contact isn't necessary.

They don't have to touch, the magnets and the paperclip.

Same for the weight force or the gravitational force.

That's a non-contact force because the force acts without Earth having to touch the paperclip.

The paperclip is floating well above Earth's surface.

But the gravitational force of Earth still acts on it.

That's why it's a non-contact force.

But of course the tension force is a contact force 'cause the string has to be attached to the paperclip for the force to act.

If you've got some scissors and cut that string, then the tension force would no longer act and the paperclip would probably fall down 'cause the forces wouldn't be in balance anymore.

So, very well done for completing this lesson on the different kinds of force.

Here's a quick summary of the lesson.

So, contact forces only act when objects are touching, whereas non-contact forces can act with or without the objects touching.

We looked in detail at friction and drag.

Friction is a contact force that slows or prevents movement.

And drag is a contact force that pushes against objects that are moving through liquids or gases.

And I've also given you a little table there of the three non-contact forces and the different contact forces that we've studied this lesson, too.