Lesson video

Hello, welcome to this lesson on balanced and unbalanced forces.

This is from the forces topic.

My name's Mr. Norris.

This lesson is really getting to the nitty-gritty of how forces make things change.

And it's a bit more realistic than previous lessons we've looked at so far because we're looking at multiple forces acting at once and how they combine to produce changes in the world.

The outcome of this lesson is, hopefully, by the end of the lesson, you will be able to use the idea of the resultant force to explain whether a change of speed happens or not to an object.

Here are some key words that we'll be looking at this lesson: Balanced forces, unbalanced forces, and resultant force.

Now, each of those terms will be explained as they come up at the appropriate point in the lesson.

The next slide has an example of each word used in a sentence.

So once we get to the next slide, you might want to pause the video and have a bit of a read-through as soon as prepared as possible for the lesson.

So, you could pause the video now to have a read-through.

This lesson is split into three sections.

The first section is about, is introduces the idea of balanced forces and unbalanced forces.

The second section then introduces the term resultant force, for what we've just seen.

And then we'll put it all together in the third section and look at how resultant forces cause changes in speed.

So, let's get started with the first section on balanced and unbalanced forces.

So, we know that forces make objects change.

That's what forces do.

So, the driving force from the person is going to increase the speed of the bike.

That's the first way that a force can make things change.

Forces can change the speed of objects.

In this situation, we've got the force from the wind changing the direction of the golf ball, so forces can change the direction of objects.

And in this third example, we've got the force from the mass changing the shape of the spring, so forces can change the shape of objects.

And the bigger the force, the greater or the faster a change is.

So forces make things change.

But sometimes a force isn't making something change.

And that can only be because other forces are acting at the same time and cancelling each other out.

So, here is our cyclist again.

And there's the forward force from the person.

Pushes the bike forwards, the driving force.

But you'll have been cycling, or you might well have been cycling where you're pedalling, but you're going at steady speed.

So, why isn't the force changing the speed? Well, it's because there's another force acting, air resistance in this case, which must be completely cancelling out that first force.

So neither force ends up causing a change in speed because the two forces are cancelling each other out.

So, two forces of equal size acting on the same object in opposite directions, they're called balanced forces because they balance each other out, or cancel each other out.

So for each of these examples, say whether the forces shown are balanced or unbalanced.

I'm going to give you time to do this, so just look at each picture, and you need to write down, or say in your own head, are the forces balanced or unbalanced for each picture.

Off you go.

Okay, hopefully, that should have been enough time.

Let's go through them one by one.

So for that car, those forces are balanced because those forces are the same size, the force arrows are the same length, and they're in opposite directions, so those arrows will completely balance or cancel each other out.

So, that's balanced forces.

However, the forces on the aeroplane are unbalanced.

That forward force is bigger.

The forward force is going to win; that aeroplane is probably going to speed up.

What about the sailboats? There's a force from the wind on the sails and a force from the engine on the back of the boat pushing the boat forward.

Both of those forces push the boat forwards, but those forces are not cancelling each other out.

So, they're not balanced.

They're in the same direction.

So, they're adding up together.

So, there is no backward force to balance them.

So, those two forces are unbalanced.

They've got nothing, no backward force to balance against.

Well done if you got that one.

For the boat floating, those forces are balanced.

There's an upthrust force from the water upwards and a downward gravitational force, but they're balanced out, and that's why that boat's floating because those arrows are opposite directions, but the same length, so the same size force in opposite directions cancel each other out, or balance each other.

On this cyclist, the forward force is bigger, so that's unbalanced forces, and on the tug of war, the two forces look balanced to me because they're the same size, but acting in opposite directions, so that rope is not going to be pulled one way, or the other because the forces on it are balanced, so neither force kind of wins in effect.

Well done if you got most of those right.

Here's another quick check.

Forces are acting on this ball, so why does it not change speed? Because forces make things change.

Have a go at writing an answer to that.

You could start your answer by saying the ball does not change speed because.

And if you're still thinking about that, I'm gonna give you a little clue in just a moment.

Here's a little clue if you need it.

Pause the video if you're happy to get started.

The ball does not change speed because.

Now, here's a zoomed-in picture of the forces acting.

So, this is the little clue I was giving you.

What can you say about those two forces that explain why it does not change speed? Right, pause the video now if you've not done so already and have a go at writing an answer.

The ball does not change speed because.

Okay, pause the video if you need to.

Here's the answer.

The ball does not change speed because the forces are balanced.

That's the word we were looking for.

The forces are balanced.

Or you could have said because the forces cancel each other out completely.

Well done if you wrote an answer along those lines.

Okay, so we've seen that balanced forces fully cancel out.

So I've redrawn the forwards force on the car again there, just below the car, so we've got a bit more space to do it.

And then here's the backward force on the car, and you can see that the backward arrow is the same length, so there's just as much forward as there is backward, so those forces kind of cancel out.

So overall, they have no effect because the effects are cancelled out.

So, neither force can cause anything no effect because the effects are cancelled out.

So, neither force can cause anything to change if the forces are balanced.

Now, let's look at unbalanced forces.

Unbalanced forces do not fully cancel out because one's a bit bigger.

So here's the forwards force on that car.

Here's the backward force.

And they do not fully cancel out because we've got this bit left over.

So that bit of force left over that's not cancelled out, we could draw it like that.

So, it's that force left over that causes a change in speed.

So, the car on the right can speed up because the forwards force is bigger because there's a bit of force left over, which can make the car change speed.

Make the car change speed, make it speed up.

So unbalanced forces will make a moving object speed up or slow down.

So, have a look at the top car.

That car is going to speed up because the forwards force is bigger.

But that second car, the backward force is bigger, so it slows down.

The mistake pupils make is they think, oh the backward force is bigger, that means it's going backward.

That's not right, okay.

Forces make things change.

So, the direction of the force shows the direction of the change, not the direction of the motion.

So, the backward force shows that the change that happens to that car is backward because the backward force is bigger.

So it moves forward, but the change is backward, so that slows the car down.

So if the forwards force is bigger, something speeds up.

If the backward force is bigger, something slows down.

Let's look at the last example, the pink car.

So that pink car is moving to the left, but its backward force is bigger, so it slows down.

The top car was moving to the right.

Those forces made the top car speed up.

But that bottom car is moving to the left, so forces made that car slow down.

Just to make this really, really clear, balanced forces then mean speed cannot change.

Speed can only change when there's unbalanced forces.

Balanced forces mean speed cannot change.

So, let's look at these examples.

Both of these cars are moving at the same speed.

But for both of them, the forces are balanced.

So they're both going at 50 miles per hour, and they're both staying at 50 miles per hour.

The size of the forces doesn't matter if the forces are balanced.

So for the first car, it's going at 50 miles an hour with small, balanced forces, so it stays at 50 miles an hour.

But for that second car, it's going at 50 miles an hour with bigger balanced forces, but it does exactly the same.

Because balanced forces just mean the speed isn't changing.

So that second car also stays at 50 miles an hour, just like the first car.

The size of the force doesn't matter if the forces are balanced.

Balanced forces just mean speed stays the same.

Speed cannot change.

Here's another couple of examples to look at.

Both of the cars are now moving at different speeds.

So the first car is now at 20 miles an hour, and the forces are balanced.

So, it's now staying at 20 miles an hour.

And the second car has also slowed down to 30 miles an hour.

So the forces are different, but they're balanced again at their new different values.

So, that second car is now staying at 30 miles an hour.

So, the important thing is here is the size of the force doesn't matter if they're balanced.

If the forces are balanced, speed cannot change.

So, let's do a quick check that we've got the absolute basics of that.

This should be a really easy match-up task.

Match each situation.

We've got balanced forces and unbalanced forces to the correct consequence for the object's speed.

So, match those up.

I'm gonna go through the correct matchings.

They were, in fact, already matched up.

For balanced forces, the object will not change speed.

For unbalanced forces, the object will change speed.

That's really, really important.

Well done if you got that right.

Right, time now to do a task on what we've just gone through on balanced and unbalanced forces.

You've got five examples here.

For each example, you need to do both instructions.

So for each example, state whether the forces are balanced or unbalanced.

We've done lots of practise of that.

You should be good at that.

And then the second thing to do for each example is describe the effect of the forces on the object's speed.

Remember what happens if the forces are balanced and what happens if the forces are unbalanced.

What could happen? Okay, so you should pause the video now and have a go at following those instructions for each example.

Off you go.

Well done for your effort on that task.

I'm going to go through the answers now, so make sure you follow the instructions for all five examples.

So, here are all the answers.

For the parachuter, the forces are balanced, there's no change in speed.

For that car, the forces are unbalanced, and the forward force is bigger, so that car is going to speed up.

For the tug of war, the forces are unbalanced, and the rope is going to speed up to the left.

For that cyclist, it looks like the forces are balanced.

There's no change in speed.

And for that ice hockey puck that's gliding along the surface of the ice, there is no forwards force.

It's not being pushed forwards anymore.

It's already been hit, and it's just moving.

The only force is backward.

There's no forwards force to balance it.

So, the forces are unbalanced because there's just one force with no force in the opposite direction to balance it.

So that is a backward force, so that puck is going to slow down.

So the forces are unbalanced, and the ice hockey puck slows down.

Very well done for your effort on that task, and hopefully, you got lots of that right.

So it's time to move on to the second section of the lesson, where we're gonna introduce the idea of the resultant force.

So, let's have a look at this model sailboat.

If there's low winds, this model sailboat must be quite a fancy one.

It's got sails, but it's also got a built-in motor for if there's not enough wind.

So there's a force from the wind on the sails.

Let's say that's four newtons on this model boat.

And here's a driving force from the motor.

We could say that's perhaps 10 newtons.

So, that means the sum of the forces is 14 newtons because there's four newtons of force from the wind, and 10 newtons of force from the motor, giving a total of 14 newtons of force acting on the boat, pushing the boat forwards.

But what if the wind changed direction? So, the boat was sailing into the wind, okay.

So now, there's a four newton force from the wind on the sails, but that's backward.

The driving force from the motor might still be 10 newtons forwards.

Now, the sum of these forces, it's not 14 newtons anymore because you've got to take the direction into account.

You've got 10 newtons forwards, 4 newtons backward.

What's left over? Well, only 6 newtons forwards is left over because 4 newtons backward cancel out some of the 10 newtons forwards, only effectively leaving you with the 6 newton forwards overall.

So, when you add forces, the directions need to be taken into account.

That's really important.

So the sum of all the forces acting on an object, taking into account their direction, that's called the resultant force.

So in the first diagram, the resultant force was 14 newtons forwards.

But in the second diagram, the resultant force was only 6 newtons forwards.

So resultant force kind of means overall force, or the combined effect of all the forces, or the sum of all the forces acting on the object, taking into account their direction.

The resultant force on the object.

So, what's the size of the resultant force on this car? There's a force of 15 newtons forwards and 4 newtons backward.

Please choose A, B, C, or D.

What's the resultant force on this car? Five seconds to decide.

Okay, which did you choose? Hopefully, you chose option B.

The resultant force is 11 newtons forwards.

And the next slide is really going to spell out why.

Well, it's because resultant force is the force left over when unbalanced forces do not fully cancel out.

So, here's the same diagram of the same car.

You've got 15 newtons forwards and 4 newtons backward.

There's the force of 15 newtons forwards, and there's the force of 4 newtons backward.

So the resultant force is the force left over because the 4 newtons backward cancels out some of the 15 newtons forwards.

So, the force left over is this bit of the forwards force that's left over.

Well, how do we work out what that is? We already know it's going to be 11 newtons.

But the principle for working out what is the resultant force is this.

When you've got forces acting in opposite directions, then the resultant force is always the difference between the two forces.

You have to do the bigger force take away the smaller force.

So that's really easy to remember.

Resultant force is just the bigger force, take away the smaller force because you're finding the difference between them.

So in this case, the bigger force is 15 newtons, the smaller force is 4 newtons.

That gives you 11 newtons for the resultant force on this car.

So if you look back at the arrows, that 11 newton resultant force, basically the 15 newtons and the four newtons, partly cancel out to just leave you with the 11 newton resultant force.

So effectively, those forces acting on the car, the 15 newtons forwards and the four newtons backwards, have exactly the same effect as the resultant force acting on the car.

So, the resultant force is the result of the actual forces acting.

That's why it's got that name.

And in situations like this, where the forces are acting just in opposite directions to each other, then the resultant force is in the direction of the bigger force.

So, I just want to really spell out something I did just mention.

The 15 newtons forwards and the 4 newtons backwards, they are the forces acting.

The resultant force is 11 newtons forwards.

But it's really important to remember the resultant force is not an extra force that's acting.

It combines all of the individual force into a single force arrow.

So that's a mistake that pupils sometimes make.

They think, oh, you've got the two forces, the forwards force, 50 newtons, the backward force, four newtons.

Oh, and there's also an 11 newton force forwards because there's a resultant force.

That's not right.

The resultant force is not an extra force acting.

It's the combined effect of all the actual forces into a single force arrow.

So, the forces acting on the car are still the 50 newtons force and the 4 newtons force.

The resultant force is the force that's left over that then changes the speed of the car.

Time to do a quick check of what we've just done on resultant force.

So in this check, I'd like you to match each diagram to the correct resultant force and the correct direction.

So the resultant forces to choose between, I've given you six.

There's 120 newtons, 100 newtons, 80 newtons, 40 newtons, 20 newtons and zero newtons.

And the direction of the resultant force is either gonna be forwards, backwards or it's not gonna have a direction.

So can you choose from those, choose one resultant force value and one direction for A, for B, for C, and for D? Some of them could be used more than once.

Pause the video now and have a go at that.

Let's check what you've got.

So for A, 100 newtons forwards, 20 newtons backwards.

That is gonna be 80 newtons forwards.

For B, 100 newtons forwards and 20 newtons also forwards is gonna be 120 newtons forwards.

That's that one.

For C, 20 newtons forwards and 100 newtons backwards, that's gonna be 80 newtons backwards.

And then for D, 20 newtons forwards, 20 newtons backwards.

That is zero newtons because there's no resultant force.

There's no direction because there's nothing to have a direction.

Well done if you got those correct.

Let's do another check of your ideas about resultant force.

Is the resultant force on the rope greater in tug of war A or tug of war B? So, have a look at those and think about what your instinct is.

In which of those tugs of war is the resultant force on the rope greater? Tug of war A or tug of war B? Now, I'm going to put some numbers on the forces to help you decide.

So, the values of the forces might help you decide.

So, I'm going to give you five seconds now to make a final decision.

Is the resultant force on the rope greater in A or in B? Right, I'm going to tell you now the answer, so make sure you've chosen A or B.

The resultant force on the rope is greater in B.

Well done if you chose that.

And that's because, remember, resultant force is the force left over.

So the values of the forces in themselves don't matter.

It doesn't matter that 195 are both bigger than 70 and 10.

It's the force left over that we're interested in for the resultant force.

And the force left over in A is only 5 newtons because those forces are almost balanced.

100 newtons forwards, 95 newtons backwards.

That only leaves a resultant of 5 newtons forwards.

That's a very small resultant force.

Those forces are almost balanced.

Whereas in B, the forces are much more unbalanced.

You've got 70 newtons forwards and 10 newtons backwards, leaving you with a resultant force of 60 newtons.

So, that's a much larger resultant force in B.

So, well done if you picked that option.

Right, we're ready to do a task about the idea of resultant force.

So for each example, calculate the resultant force and give the direction of the resultant force.

So either forwards, backwards or no direction.

So do that for each of these five, please.

Pause the video now, have a go.

Well done for your effort on that task.

You should have done all five.

I'm gonna go through the answers now.

So for number one, the resultant force is 10 newtons.

So you're going to do the largest, take the smallest.

50 take 40 gives you 10 newtons.

So the resultant force is gonna be 10 newtons to the left.

So for number two, the resultant force is 30 newtons.

That explains why.

So the resultant force is 30 newtons to the right, or 30 newtons forwards.

For number three, the correct answer is 1.

1 newtons downwards.

You do the bigger force, take the smaller force to find the force left over.

The resultant force is 1.

1 newtons down.

For number four, 19.

3 forwards, 19.

3 backwards.

No resultant force.

Don't need to put a direction because there's nothing that has a direction.

And number five, resultant force, is biggest.

Take smaller force, which is 75 newtons in this case.

And the resultant force is in the direction of the bigger force.

So the resultant force is 75 newtons upwards.

That submarine would be speeding up on the way up.

Well done for your effort on those five questions in that task.

So we're now ready to put it all together and talk about how a resultant force causes changes in speed.

So this brings together everything we've already talked about, but just using the language of resultant force.

So we know that forces make things change, and we know that if forces are balanced, they fully cancel each other out.

And that means there's no resultant force.

Those forces disappear because they've effectively cancelled each other out, even though the forces are still there.

There's no resultant force, so the speed of that car doesn't change when there's no resultant force, when forces are balanced.

But if you've got unbalanced forces, they do not fully cancel out.

A bit of that forwards force is going to cancel out, but there'll be a bit of the forwards force left over.

Here it is.

That's the bit which is left over.

So that's the resultant force, the force left over when forwards and backwards forces cancel out.

The force left over is called the resultant force, and that car will change speed.

In this case, that car is going to speed up.

So, the resultant force on this car is forwards.

The forwards force is bigger than the backwards force.

There's the force left over.

The forces acting on it are going to speed it up.

The resultant force is forwards, causing a increase in speed.

But on this car, the resultant force is backwards.

There's the backwards force and the forwards force.

You can see some of the backwards force gets cancelled out, but some of the backwards force is left over.

Here it is.

So, the resultant force on this car is backwards, so it's going to continue to move forwards but slow down.

Because remember, the direction of the force shows the direction of the change, not the direction something's moving.

So, this car's moving forwards but the direction of the change is backwards.

So, that's against the forwards motion.

Therefore, the change is going to slow the car down.

It acts against the motion, slowing the car down.

So the resultant force in both cases causes a change in speed.

In the first car, the resultant force is forwards, causing an increase in speed.

And on the second car, the resultant force was backwards, causing a decrease in speed.

Okay, let's check your understanding of that.

This picture shows the forces on a parachutist.

The moment after their parachute opens, what happens next? Is it A, B, or C? The parachutist is falling, and they've just opened their parachute.

What happens next because those forces are acting as they fall? Is it A, B, or C? Five seconds to decide.

Very well done if you chose the correct answer, which is B.

The correct answer is not A because forces, although the upwards force is bigger, forces don't show the direction something's moving.

Forces show the direction of the change that happens.

When you open the parachute, it's going to slow down the parachutists.

So they're still moving downwards, but they're going to be slowed because the upwards force is bigger.

So, that shows the direction of the change.

Well done if you got that.

So for a given object, the greater the resultant force, the greater the change of speed is, or the faster the change of speed happens.

So let's look at some examples.

So here's a car with 15 newtons forwards, and 2 newtons backwards.

So biggest takes smallest, gives the resultant force of 13 newtons forwards.

So that car could be changing speed quite quickly.

It's certainly going to speed up because the resultant force is forwards.

At the second car, the forwards force is still 15, but the backwards force is now 6.

So a bit more of the forwards force gets cancelled out.

The resultant force is still- is now only 9 newtons.

So, they're still going to be speeding up because the resultant force is forwards, but the resultant force is smaller, so it's going to speed up a bit more slowly than the first car was speeding up.

And the same thing happens with this next example.

15 newtons forwards, but the backward force is 14 newtons.

So, the resultant force is now only 1 newton forwards.

So, the car's still gonna be speeding up, but just very slowly.

So, you can see the size of the resultant force affects how quickly the change happens.

And if there's no resultant force, then there's no change happening at all.

In that last example, the car is just continuing at whatever speed it's already got.

So, let's do a check on that.

Which of these will change speed? You just need to choose the letters of the examples that will change speed.

Have a go at doing that.

Pause the video if you need to.

Five seconds.

Off you go.

So the cars that will change speed are cars A, C, and D.

Car A changes speed because there's a one-newton resultant force.

Car B does not change speed because the forces are balanced.

Car C changes speed.

It's going to speed up because there's a four-newton resultant force forwards.

Car D will slow down because the backward force is bigger by two newtons.

So, the resultant force is backward.

And Car E, the forces are balanced, so does not change speed.

So, the ones that will change speed are A, C, and D.

Well done if you got that.

I'm going to ask you one more question on these, actually, which is this.

Out of A, C, and D, which one would change speed the most in maybe the next couple of seconds? So you need to look at the forces acting in A, C, and D, and decide which one would change speed the most, or at the fastest rate, in the next couple of seconds.

Each car, of course, is identical.

So five seconds to decide between A, C, and D, which one would change speed the most in the next couple of seconds.

And the answer to this is car C.

And that's because car C has the greatest resultant force.

The resultant force on car C is 4 newtons, whereas the resultant force on car A was only 1 newton, and the resultant force on car D was only 2 newtons.

So, the greatest resultant force causing the greatest change in speed in the next amount of time is car C.

So, well done if you got that.

It's time to do the final task of the lesson.

So for each diagram, we've got four diagrams on the right-hand side of the slide.

For each diagram, calculate the resultant force and state the direction.

We've done lots of practise at that.

And also, for each diagram, describe what happens next to the object.

Use one statement from list one and one statement from list two to do that.

So you should pause the video now, and for each diagram, have a go at following those instructions.

So pause the video now, off we go.

Well done for your effort on that task, which should bring together everything we've been talking about this lesson.

So, I'll go through the answers now.

So for the first one, the resultant force is six newtons forwards.

You do the bigger force, take the smaller force, you get six newtons forwards.

So that bicycle is going to continue moving forward, and it's going to speed up because the resultant force is forwards.

For number two, to get the resultant force, you do the bigger force, take the smaller force.

So that's 9 newtons take 3.

5 newtons gives you 5.

5 newtons, but the backward force is bigger, so the resultant force is backward, it's 5.

5 newtons backward.

So that bicycle is going to continue to move forwards, that's really important.

The direction of motion is the same, and the resultant force shows the direction of the change.

They're going to move forward and slow down because the resultant force is backward against their motion as they move forwards.

Very well done if you got that for number two.

For number three, the resultant force was zero because those forces are balanced.

So, the resultant force is zero.

That boat is going to stay still, so the speed is zero, and there's not gonna be any change to that speed.

And in number four, the same, the forces are balanced, so the resultant force is zero, they're moving forwards already, so they're gonna continue to move forwards at a steady speed, there's no change because there's no resultant force.

So well done for your effort on that task, and hopefully you got lots of those right.

Here's a summary of the lesson on balanced and unbalanced forces.

Two forces of equal size acting in opposite directions on an object are balanced.

They fully cancel each other out, so there's no change in speed.

Whereas unbalanced forces do not fully cancel out.

The force left over is called the resultant force, and resultant forces cause changes in speed.

And the greater the resultant force, the greater or faster the change in speed.