Lesson video

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- Hi, everyone.

My name is Ms. Hummel, and together we'll be answering the question, "What are contact forces?" In this lesson, we will learn about contact forces.

We will discuss air and water resistance and examine what balanced and unbalanced forces are.

We will also investigate friction.

Based on our investigation, we will make conclusions about the causes and the characteristics of friction.

Our lesson will follow this structure.

We will first begin by recapping what contact forces are.

Then we will discuss air and water resistance.

We will investigate friction, and based on that, we will make those assumptions about what the causes and characteristics of friction are at the end of our lesson.

In this lesson, you will need an exercise book or paper, a pencil or pen, a coloured pencil or pen, and a ruler.

If you haven't got those things, pause the video now and go get them.

Here are our Star Words, which are the most important words of our lesson.

I'm going to say them and ask you to repeat them after me.

When I point at myself, it will be my turn, and when I point at you, it will be your turn.

Forces, contact, air resistance, water resistance, upthrust, friction, balanced, unbalanced, and that's it.

So, air resistance, water resistance, upthrust, and friction are all examples of contact forces.

We will discuss what we mean by balanced and unbalanced later on in this lesson.

There are lots of types of forces that can act on an object.

Sometimes these forces only take place if two objects are touching.

We call these contact forces.

Examples of contact forces include friction, air resistance and water resistance, and upthrust.

Contact forces only act when two surfaces are touching each other.

For example, with friction, it only acts when two surfaces are touching each other.

So friction acts when you rub your hands together, but it won't act if your hands are not rubbing together.

Air resistance will always act if an object like a car is moving in air, but it would not act if you were in space as there's no air in space, and so the car would not be touching air.

Water resistance will always act against something that tries to move through water.

And upthrust will only act if an object is pushing down on water, like a boat.

If the object is not touching the water, then upthrust will not act.

So let's have a look at what balanced and unbalanced forces mean in a little bit more detail.

Now, if forces are balanced, those forces which are pushing or pulling would be the same strength, and therefore those forces are considered to be balanced.

Now, this will cause an object to maintain its current state of motion.

Its current state of motion means that it could be still or it could be moving in a certain direction at a certain speed.

So if the forces are balanced, it's either going to stay still, if it was already still, or if it was already moving in a certain direction at a certain speed, it's going to maintain or it's going to keep moving in that same direction at that same speed.

However, if we have forces which are pushing or pulling an object and they are not the same strength, they are considered to be unbalanced, and that will cause the object to change its state of motion.

So for example, if an object was still and therefore another force acted upon that object, it may cause it to suddenly start moving.

Or if an object was moving at a steady pace towards a particular direction, maybe an unbalanced force could cause it to maybe change direction to another way, or it could cause it to accelerate, go quicker, or to decelerate, to go much slower.

So we're going to have a look at a few examples of what we mean by balanced and unbalanced forces, and hopefully it's going to help you understand it.

Now let's have a look at these two pictures.

Now first, I've got a person swimming.

You can see the person swimming right there.

And second, it's hard to tell, but I've got an object that is falling from the sky.

You can imagine something falling from the sky.

Now, in my first image and in my second image, there are some forces, remember a push or a pull, that is affecting those scenarios.

I want you to have a think.

Can you think of which forces could be affecting those scenarios? See if you can tell me.

Now, the forces you may have thought about for the person swimming are, first, the strength of the person swimming.

We're going to call that the manual force.

So the effort that that person is putting into swimming we're going to consider the manual force.

And then we've also got water resistance.

Remember, we discussed that in water, sometimes you're going to encounter some resistance when you're trying to move.

That means it's almost like it's pushing you back when you are trying to move through the water.

Now, if my forces were balanced, and it kind of looks like my two arrows here are balanced, that could mean two things.

Now, if the person wasn't moving, maybe the water resistance was really strong or the manual force wasn't very strong, if the person wasn't moving and the forces are balanced, then the person's going to stay not moving.

However, if the person was moving at a steady pace and swimming at a steady pace, and those forces are balanced, they're going to continue to move at that steady pace.

Now let's have a look on the right.

On the right, remember, I had an object falling.

What did I have? I had an object falling from the sky.

And I can think of two different forces that could be acting on that object.

I could think of gravity.

Remember, gravity is pulling us towards the centre of the Earth.

And I can think of air resistance as well, which might be acting against gravity in this case.

Now, if they are balanced, like they look to be quite balanced in this case, then the object, if it's falling, we know it's actually moving, it's not just stationary, then the object will continue to move but at that steady pace and in that same direction.

However, you may realise that behind me, I've actually got two people who are falling through the sky, but they've actually got something else.

They have got a parachute.

Now, if I opened a parachute onto that object, suddenly the parachute is going to allow there to be a greater air resistance force because it's got a greater surface area.

And because of that greater air resistance force, now my forces are no longer balanced.

So now I was going, my object was going at a regular speed, and it was staying at that steady speed, but now that my forces are unbalanced, it's going to either cause me to accelerate or it's going to cause me to decelerate, to get slower, or it's going to cause me to suddenly stop moving, or it might even change direction.

So it could change all of those things.

An unbalanced force could change all of those things.

So, have a think.

In this case, what do you think would happen with the parachute? I think that the speed of the object would decelerate.

So if before it was going really quickly, I think with the parachute, it's still going to be moving downwards, but I think it's going to be going much slower.

So because it has changed its current state of motion, it's considered to be an unbalanced force.

Eventually, once it reaches a steady speed, it's going to be balanced again, okay? But at the moment, when it changes from moving really quickly to suddenly opening the parachute, getting more air resistance and then moving a bit slower because I've changed that speed, I had an unbalanced force in that moment.

Okay, so first, I would like you to pause this video to complete this task.

And I want you to think, hmm, what forces did we say were involved when a parachute was activated? And were those forces balanced or unbalanced, or were they both? And how do you know? See if you can explain it.

If you're confused, maybe go back in the video and rewatch my explanation.

You can resume once you are finished.

Now, if you wanted to check your work, remember that we said that with our parachute, if they were originally balanced forces of gravity and air resistance, that means that the object or the person was moving at a steady speed or velocity.

When I opened that parachute, suddenly I encountered a greater force of air resistance.

So my forces were no longer balanced, and what that meant was that I actually slowed down, so I changed my state of motion.

Eventually, over time, I might go back to a steady speed, and then my forces may be balanced again.

Now, hopefully you said the same thing.

All right, let's carry on with our lesson.

Now for this next part of the lesson, we're going to investigate friction.

We are going to do a small investigation.

If you would like to do the investigation, you don't need many things.

You need your hands and water.

If you would like to get a little bit of water to do this part of the investigation, you can pause the video and go and get that now.

Now, to investigate the force of friction with our hands, we are first going to gently rub our hands together.

Now, how can you tell that there is friction present when you are gently rubbing your hands together? As we're going through this investigation, it would be a good idea to write down your answers.

Otherwise, you're going to have to write them all down at the end.

I can tell there is some friction because as I'm rubbing my hands, they can't go as smoothly as if my hands were not rubbing.

Next, you should push your hands together, but more firmly this time.

So with a bit more force, we're rubbing our hands, like that.

Now, was that easier, or was it harder? And what happened to the amount of friction? Next, I would like you to think, when you were rubbing your hands harder, what did you notice about the temperature of your hands? Was it the same the whole time? Did it change? Now, if you're not sure, try again with your hands like I'm doing now.

Finally, put a small amount of water on your hands and repeat the experiment.

What do you notice? Now, when I put some water, oh, that's too much! When I put some water into my hands, and I'm rubbing my hands, I'm certainly noticing a change.

Let's see if you notice a change as well.

Well, now that my table's wet, you can pause the video to complete your questions and write down your answers.

Finally, I would like us to write a conclusion about generalisations that we can make by writing down the answers to those questions.

You can pause the video now to complete your conclusion.

If you were feeling stuck with your conclusion, here are some sentence stems to help us structure that conclusion.

It says, "When you rub your hands together, the force of mm resists the movement of your hands.

When you press your hands together more firmly, it becomes more mm to move your hands past each other.

This means the force of friction is mm.

Your hands also feel mm because friction produces mm.

When you add a small amount of mm to your hands, it becomes mm to move your hands past each other.

This tells us the amount of friction has become mm." Hopefully you can now complete a conclusion by looking at those sentence stems. Now I'm going to read this out but with the answers, and you can use that to help you annotate your conclusion and make it better with a different coloured pen.

When you rub your hands together, the force of friction resists the movement of your hands.

When you press your hands together more firmly, it becomes more difficult to move your hands past each other.

This means the force of friction is stronger.

Your hands also feel warm because friction produces heat.

When you add a small amount of water to your hands, it becomes easier to move your hands past each other.

This tells us that the amount of friction has become smaller.

For the final part of our lesson, we're going to discuss the causes and characteristics of friction.

- [Instructor] Friction is the force in action between two surfaces.

It can be used to control speed and to change direction.

Smoother surfaces like the ramps have less friction so you can go faster.

Rougher surfaces like the road or the grass create more friction, so slow you down.

Friction can be a help as well as a hindrance.

Without friction, the bike wheels wouldn't grip the road, your feet wouldn't grip the pedals, and your brakes wouldn't help slow the bike down.

- Now, that character may have helped us understand friction in a little bit more detail, but let's take a closer look.

Although it's difficult for us to see, all surfaces of any material have a lot of very small bumps across them.

This picture shows what it might look like if we could zoom in on the surfaces of two materials.

In this case, the two materials are two people shaking their hands.

As you can see, most surfaces are not completely smooth, and they have some bumps and ridges.

This is why friction is present and why it could be difficult to move two surfaces past each other.

Now, you may be thinking, "Well, I know I have ridges on my hands.

I can see them, because you can see the lines on your hands, However, even with my table, I can't really see any ridges." But if I was to use a really expensive camera to zoom in, I could definitely see the ridges on my table, which is a flat surface.

Now, if the material is very rough, the bumps on the surfaces are large.

If the material is smooth, the bumps are much smaller, and you will therefore experience less friction.

Now, can you point on your screen to the picture that is showing a rough surface? Now, can you think of anything in your house which could match the bumps that you can see on the picture for a rough surface? I can see a few things in mine.

I would now like you to pause the video to answer this question.

What can you say about the size of the bumps on rough and small surfaces? On rough surfaces, the bumps.

And on smooth surfaces, the bumps.

You can resume once you've finished.

In our experiment, we did ask you to put some water on your hands.

In my case, I made a huge mess.

But I also learned something, and I learned that it was easier for my hands to rub together when I had some water on them.

Now, we can make it easier for two surfaces to move past each other by putting liquid like oil or water in between those surfaces.

This makes the friction much smaller because the water's filling in the little gaps in between the bumps and ridges.

Now, to end the lesson, we do have an optional investigation for you to complete so that you can be your own scientist as well.

You can complete this if you've got a carpet at home.

If you don't have one, check that you haven't got another rough surface that you could use instead.

Maybe you've got a cushion that's got a bit of a rough surface, or you might have a sofa or a chair that you feel has one as well.

Now, you will need a smooth object, maybe something made from plastic, like a toy, and a rough object, something with a varied texture.

In general, most things that have a textured pattern tend to be rougher.

Now, for the instructions, you are going to move the smooth object across the carpet and try to notice how difficult or how easy it is to do that.

Then you're going to move the rough object across the carpet.

Now, was that easier, or was that harder? And then based on that, can you make any conclusions about rough and soft objects and how it affects friction? We have now finished our lesson, and it is time for you to complete your exit quiz.

You're going to need to exit the video and complete the quiz to test your knowledge and understanding of this lesson.

I hope you enjoyed this lesson, and I hope you did not make a mess with the water on your table, like I did.