Lesson video

Hello there, I'm Mr. Forbes and welcome to this lesson from the forces make things change unit.

This lesson's all about resultant forces and how we find them and what the effect of those resultant forces are on objects.

The key words you'll need for this lesson are shown here.

First is resultant force, and that's the overall effect that a set of forces have on an object.

Then there's Newton's First Law of motion, and that's the law that describes what will happen to an object when there's no resultant force acting on it.

And the object will stay at rest or it will continue moving in a straight line at the same velocity.

Velocity is the rate of change of displacement, how fast an object is moving in a particular direction.

Acceleration is the change in speed or direction of an object, and deceleration is a decrease in the speed of an object.

You can return to the slide at any point during the lesson if you need to look at those definitions again.

The lessons in three parts, and in the first part we're gonna find out how to calculate a resultant force.

The resultant force describes the overall effect of all the forces acting on an object.

In the second one, we're gonna see what Newton's First Law of motion is and how that describes what happens when there is no resultant force on an object.

And in the third part, we're going to see what the overall effect of a resultant force might be on an object.

So when you're ready, we'll begin with resultant forces.

Forces can cause changes in movement of objects and I've got a couple of examples of that here.

So I've got a rocket taking off and this produces a massive thrust force downwards that acts on the rocket, giving it a force that pushes that rocket upwards.

It overcomes the downwards weight of the rocket.

And that rocket's very heavy, so it's thrust is enormous.

You can also use forces to slow down objects.

So if I've got a car moving very, very fast, like this sports car here, driving past a finish line, it can use parachutes and opening those parachutes produces a very large drag force and it slows down the car quite rapidly so it can stop safely.

If you have a tug of war, the forces of each team are acting in opposite directions on the rope.

So I've got the simplest tug of war here, just two people, and they're both pulling on the rope in opposite directions.

Now the team that produces the largest total force is going to be winning and pulling the person towards them and they're gonna have the victory in the end.

So if I draw some forces on here, we've got 500 newtons to the right and 350 newtons to the left, you should be able to work out that this guy's winning.

They're getting pulled towards the right there.

So the 500 newtons force is winning.

Or if I add another person on the other team and they produce a force as well, 250 newtons, well now we've got 500 newtons to the right and we've got 600 newtons to the left.

So the team pulling towards the left is now winning.

So a quick check of that for you here.

So which of these two teams is winning the tug of war? So pause the video, make your decision.

and then restart please.

Welcome back, well, in fact, the team of two is winning.

If we add up the forces going left and right, we've got on the left 900 newtons and on the right 950 newtons.

So the two people are beating the three people.

Well done if you spotted that.

Many objects have forces that act against each other.

So when you've got an object moving through the earth, for example, you can have forces pushing and pulling in opposite direction.

So the cyclist has got a driving force pushing them forward, generated as they pedal, there's gonna be some air resistance that's opposing their motion and there's gonna be some friction between the bicycle and the ground.

So you've got three forces acting in opposite direction, well, sorry, acting in two different directions there.

And we're gonna have something called a resultant force produced by those forces.

A resultant force is the overall effect of the real forces acting on the object.

So resultant force itself is not a real force, it's just a kind of summary of all the forces acting on the object that we can use to explain what's gonna happen to the motion.

So I'm gonna draw the resultant force here.

From those forces, the resultant force would be a force towards the right.

To find the size of that resultant force you follow the following process.

So I've got a simple object here.

I've got four forces acting on it, and I'll show you how to get the resultant force.

The first stage is we add all the forces acting in one direction together.

So we're gonna add these two forces together.

The forces acting left and it's five newtons and three newtons.

Adding those gives eight newtons.

And then I add the forces acting in the other direction together.

So the forces acting to the right.

Two newtons and four newtons added together is six newtons.

So now I've got the summary of the forces in each direction.

Next I subtract the smaller total from the larger total to give the size of the resultant force.

So I had eight newtons and six newtons, eight newtons minus six newtons is two newtons.

And the direction of that resultant force is going to be in the direction of the largest total.

In our case that was to the left.

So I've got two newtons to the left as my resultant force, and if I wanted to draw a diagram of that, I could draw like this.

So two newtons to left is my resultant force.

Let's try some examples of calculating the resultant force.

I've got one I'll do and then one you can do.

So I've got the forces here acting on space shuttle as it takes off.

I've got the two thrusts from the rocket engines, they're 12 mega newtons each upwards.

And I've got the weight, the downward force, acting on the shuttle of 20 mega newtons and I'll find the original force.

So what I do is I add the forces acting upwards and that gives me 24 mega newtons, 24 million newtons.

And then I've got the forces going downward.

Well it's just one of those.

So downwards, it's 20 mega newtons and I subtract the smaller one from the larger one and that gives me a result of four mega newtons.

And that's in the direction of their largest total, which was upwards.

So I've got 4 million newtons upwards, four mega newtons upwards.

Now it's your turn.

What I'd like you to do is define the resultant force acting on the cyclist here.

So pause the video, find a resultant and then restart please.

Welcome back, you should have calculated it like this, towards the right there we've got 300 newtons.

So there's just that one force and towards the left we've got 45 newtons and 22 newtons.

So we add those together to get 67 newtons.

And then we subtract the smaller value from the larger value.

That gives me a total of 233 newtons and that must be towards the right in the direction of the largest total.

Well done if you've got that.

Now it's time for the first task of the lesson.

I've got two questions here.

First of all, I'd like you to find the resultant force for each of those force diagrams there, A, B, and C.

And then I'd like you to draw a free body diagram based upon that description there, a helium balloon with a weight of 0.

5 newtons produces an up thrust of 1.

5 newtons.

As it rises through the air, there's a downward drag force of 0.

5 newtons acting on it.

So pause the video, answer the two questions, and then restart please.

And welcome back.

And here's the resultant forces you should have found.

We've got 150 newtons to the right there for situation A.

For situation B, 250 newtons to the right and for situation C, 1,900 newtons upwards.

So well done have you've got all of those.

And here's the answer to the baloon question.

We've got the up thrust drawn on, I've got the weight drawn downwards, the dragged on drawn downwards, and when I calculate the resultant, it's not 0.

5 newtons upwards.

Well done if you've got that.

And now it's time for the second part of the lesson.

And we're going to look at Newton's First Law of motion.

And that describes what happens to an object when there's no resultant force acting on it.

Isaac Newton's First Law of motion describes what happens when there's no resultant force and it can be summarised quite simply.

If there's no resultant force acting on something, then the motion of an object won't change.

And so Newton's First Law of motion is stated like this originally.

"Every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force." And what that means in everyday language is when there's no resultant force on an object, a stationary object will stay stationary.

Its motion doesn't change.

So it stays stationary if it was stationary.

If it is moving, a moving object keeps moving at the same speed and in the same direction.

So again, there's no change in its motion unless there's a resultant force.

Let's check if you understand that concept.

What does Newton's First Law predict will happen when an astronaut moving through space with no resultant force acting on them? So what will happen to that astronaut? Is it A, they'll suddenly stop? B, they will slow down over time.

C, they will not change speed or D, they will speed up over time.

Pause the video, make your selection and restart please Welcome back.

You should have chosen option C.

The speed will not change and the direction of travel will not change either.

So well done if you selected that.

Newton's First Law of motion is easy to see when you've got stationary objects.

So I've got an example here.

I've got a book resting on a table and there's going to be a gravitational force pulling that book downwards.

So let's say that's 15 newtons, it's quite a large book and at the same time the table is gonna be pushing back up on the book.

So we've got a normal reaction force acting on the book.

And as you can see, those forces are equal in size.

So there's no resultant force acting on the book.

And as you'd expect that book's not gonna start moving.

It's just gonna sit there at rest on the table.

So the two balanced forces aren't gonna change the motion.

The book isn't gonna start moving, it's just gonna stay stationary as it was initially.

Okay, I've gotta check to see if you understand the consequences of Newton's First Law of motion.

I've got a box being pushed to the right with a force of 80 newtons and the box does not move.

Which of these statements must be correct? The box weighs 80 newtons.

A frictional force of 80 newtons is acting on the box.

A frictional force of less than 80 newtons is acting on the box or the box will speed up over time.

So pause the video, make a decision, and then restart please.

Welcome back, well the answer to that one is a frictional force of 80 newtons is acting on a box.

The box is stationary and it stays stationary.

So there must be balanced forces acting on it.

So there must be a force acting towards the left that's also 80 newtons.

So well done if you selected that, we've got that frictional force.

And there's the 80 newtons shown there.

And because the box isn't moving, there's no resultant force acting on it, well done.

So now let's imagine a situation where there's no forces at all acting on an object.

I've got a spaceship and it's in deep space.

It's far, far away from many stars or planets.

So there's no gravitational forces and no frictional forces acting on it.

In that situation, the object is moving, it's travelling very fast, but there's no forces acting on it at all.

So there's no forces causing it to speed up and slow down.

That spaceship would travel at a steady speed in a straight line without having to use any rockets.

So once the spaceship's moving in a straight line at a speed, it will maintain that direction and that speed as long as there's no external forces acting on it.

The resultant force acting on it is zero and its motion doesn't change.

If there's any frictional forces acting on an object, then that object is going to slow down over time because there is a resultant force act.

If there are any frictional forces acting on an object, then that object is going to slow down over time 'cause there are resultant forces acting on it.

So if I can reduce that frictional force as close to zero as possible, the object continues at a fairly steady speed in a straight line for a very long period of time.

It takes a long time to slow down.

So we've almost got steady speed in that situation.

But there's always gonna be some so small resultant force acting on it, even if it is like going across ice or something like that.

So if I pushed an object across this ice rink, it wouldn't keep going at the same speed, it would gradually slow down as it moves in a straight line across it.

So it's very difficult to observe Newton's First Law of motion when there's frictional forces involved.

We can try and reduce the frictional forces further by doing something like making an object float.

So if you've played air hockey, you've got a table that's got a small puck that you smash from either side to the other and try and get it in the goal.

And on that table you've got small air holes on the on it and it causes the puck to hover over the surface and it can travel very quickly at steady speeds over the surface.

And that's because we've got friction that's almost zero.

By making the puck hover above the surface, it's not in contact, and the only real force you've got slowing it down there is air resistance or drag.

So we've got this bit of air being pushed upwards through the hole, making the puck float and with reduced friction to zero and we've just got a small drag force on it.

So that puck will only change speed and direction when it hits or touches the sides of the table or the opponent hits it as well.

So a resultant force is needed to cause a change in motion in that case.

And when it's not hitting the sides, there's no resultant force, so there's no change in motion.

It goes in a straight line at a steady speed.

Now I'd like you to answer this question about Newton's First Law.

I've got a car moving at steady speed along a flat road.

Which of the following statements is correct? Is it A, the forces pushing the car forwards are greater than the forces pushing it backwards? Is it B, the forces pushing the car forwards are less than the force pushing it backwards? Or is it C, the forces pushing the car forwards are equal to the forces pushing the car backwards.

So pause the video, make a selection and restart please.

Welcome back, hopefully you selected C, the forces pushing the car forwards equal to the forces pushing it backwards.

If it's at a steady speed in a straight line, there's no resultant force.

So the forward forces and the backward forces must be equal and opposite.

Well done if you selected that.

Okay, now it's time for the second task of the lesson.

I'd like you to answer these two questions please.

I'd like you to explain why an ice skater moving in a straight line will eventually come to a stop unless there's a driving force on them.

And then I'd like you to explain why a probe in deep space could continue forever in a straight line and what assumptions have to be made for that to happen.

So pause the video, answer those two questions and restart please.

Welcome back, well the answers that you should have come up with should be similar to this.

For the ice skater, there's a small frictional force acting on the skater as they go over the ice.

And that force, that resultant force will gradually slow them down unless they produce a driving force pushing them forwards.

So they could try and push themselves forwards, but if they just pushed off and then skated without trying to push forward again, they would gradually slow down and stop.

Well done if you got that one.

And for answer two, according to the Newton's First Law of motion, the probe will continue to move in a straight line at a constant speed as long as there's no resultant force.

The assumptions we need to make for it to keep on going in that straight line forever is there's no frictional forces, which they aren't generally in space.

So the probe doesn't slow down because of those and we have to assume that there's no nearby star or something like that where there'd be gravitational forces that could cause a change in speed or direction of travel.

Well done if you've got those answers.

And now we've reached the final part of the lesson.

We're gonna look at the effects that resultant forces can have on an object.

Any resultant force acting on an object will cause it to accelerate.

And what I mean by that is there'll be a change in velocity over a period of time.

So either the speed or the direction of the object will change.

So a change in velocity can mean a change in speed.

So we've got starting and stopping, speeding up and slowing down.

So again, if you imagine a motorcycle, it can start from rest, accelerate and speed up and then it can decelerate and slow down.

So a change in speed is a change in velocity there.

But also a change in velocity can mean a change in direction.

So as you turn around bend, your velocity is changing because your direction is changing.

And velocity is a vector, so its direction is very important.

So acceleration can mean a change in speed, a change in direction, or it can mean a change in both at the same.

You can change speed and direction.

So you can turn around a corner so you're changing direction and slow down at the same time.

So acceleration can mean those things.

Let's see if you understand the forces acting on a object and what that does to it.

I've got a ball here and it's kicked and it's moving forward and it's slowing down.

So what's the forward force on the ball at Y? So you can see it's being kicked, it's moved forward, it's got to point Y.

I'd like you to think about what the forward force on it is.

So is it A, the forwards force at Y is bigger than at point X, which was earlier in the motion.

The forward force at Y is the same size as it was at X.

The force at Y is smaller than X or there's no forward force at Y at all.

So pause the video, make your decision and restart please.

Welcome back, well the answer to that one was there's no forward force at Y.

It's no longer being kicked, there's nothing driving the ball forwards.

So there can be no forward force acting on it at all.

Well done if you selected that.

Now it's time to look at the consequences of a resultant force.

So if there's a resultant force acting on a stationary object, it will start moving in the direction of that resultant force.

So for example, if I've got a stationary football and I kick it, the ball will speed up in the direction you kick it.

The resultant force will make it accelerate in the direction of the force, it will start to move forward.

So if you kick it in that direction, the ball will start to move in that direction.

It will accelerate in that direction because there was a force in that direction.

If I've got a a stationary stone, I'll let go of it, it's gonna fall towards the ground.

Because it's going to be a resultant force acting on it.

The weight or the gravitational force putting it downwards and it will start to move downwards.

So it accelerates downwards because there's a resultant downwards force.

Okay, let's test your understanding of what's going to happen in a range of scenarios.

I've got a small stone attached to some helium balloons as you can see there.

And it's floating in a fixed position.

So it's just floating, it's stationary.

What's going to happen to the stone if one of the balloons pop? So is the stone gonna accelerate downwards? The stone accelerate upwards or the stone not going to accelerate at all? So pause the video, make your decision, and then restart please.

Welcome back, you should have selected the answer A, the stone will accelerate downwards.

There's going to be a resultant downward force now.

Initially the stone was balanced, there's no resultant force, but if one of the balloons pops, there's a smaller upwards force now, smaller up thrust from all those balloons pulling their stone upwards.

So there's a result in downwards force.

And so the stone will accelerate downwards and start to fall towards the ground.

Well done if you got that.

So I've got a similar scenario here.

Small stone attached to some helium balloons as shown and it floats in a fixed position.

So it's stationary again.

Then the string breaks.

So what's gonna happen to the balloons if the string breaks.

So pause the video, make your decision from the list there and then restart please.

Welcome back, well, if the string breaks, then the balloons are gonna accelerate upwards.

So well done if you selected that and they accelerate upwards because there's no longer a downwards force, the weight of the stone acting on them.

And so there's a result in upwards force and so the balloons will accelerate in the direction of that resultant force.

And so they accelerate upwards.

Well done if you spotted that.

So we've seen what happens to stationary objects when there's a resultant force, but similar things happen to objects that are already moving.

So we're gonna look at some scenarios involving moving objects and they'll also accelerate when a resultant force acts on them.

So imagine they've got a spaceship, a bit like this, travelling through space and it's travelling at high speed and I put a resultant force on it, I fire the thrusters and that produces a large forward force.

What's going to happen is that that spaceship is going to accelerate, its speed's going to increase because there's a resultant force acting on it in the direction it's moving, it's going to get faster.

Similarly, if you are on a bicycle and you're on a hill and then you start going down the hill, there's a resultant force acting on the freewheeling cyclist here, that resultant force is gonna cause acceleration of the cyclist and they're going to get faster.

They're going to speed up as they travel down the hill because of that resultant force.

Let's check if you understand that idea.

Again, I've got a balloons and a stone scenario.

We've got a small stone attached to some helium balloons, but in this example, the balloons are rising, this is already rising upwards at a steady speed.

What's going to happen to the stone if all of the balloons suddenly pop? So pause the video, make your selection and restart please.

Okay, well the balloon was travelling upwards, but there's now a resultant force acting on it, so it's going to accelerate downwards.

What's gonna happen is the balloon that was, sorry, the stone that was moving upwards is going to slow down, stop and fall.

It's accelerated downwards, it'll fall to the ground.

Well done if you've selected that one.

And a final balloon example for you to work through, I've got a small stone attached to some helium balloons.

It's moving downwards at a steady speed.

What will happen to the balloons if the string breaks? So this is moving again, and I'd like you to know what will happen when that string attaching the balloons to the stone breaks.

So what will happen to the balloons? Pause the video, make your selection and restart please.

Welcome back, and you should have like the balloons will accelerate upwards.

There's a resultant force on the balloons upwards now because there's no weight pulling them down.

So they're very buoyant.

They'll rise upwards, they'll accelerate upwards.

So well done if you selected that.

If you've got a moving object and there's resultant forces on it, it can speed up.

But if that resultant forces in the opposite direction to the movement, what's gonna happen is the object is going to slow down and that's known as deceleration.

So if we go back to the spaceship example, I've got a spaceship that was moving towards the right here, but I fire the thrusters and cause a force acting towards the left.

What's going to happen is that that force is acting in the opposite direction to the movement and that's going to slow down the spaceship.

It's going to decelerate it.

Similarly, if you've got a car here and it was travelling quite fast along the road and it's engine fails, it stops.

There's no forward force acting on it anymore.

I've still got frictional forces and drag forces acting on it.

Those forces which act in the opposite direction to movement are going to cause the car to slow down.

It's going to decelerate.

So resultant forces, if they act in the opposite direction to movement, will cause deceleration of slowing down, a decrease in speed.

Okay, let's see if you understand what I mean by decelerating and what causes it.

I've got four scenarios here.

I've got identical vans and I've got forces acting on them.

And what I'd like you to do is to decide which of the vans are decelerating.

So it's important to look very carefully to see the direction of movement of the van and find the direction of the resultant force.

So pause the video, work out which of these vans are slowing down, and then restart please.

Welcome back, well, in the first scenario, I've got a resultant force in the direction of movement.

So that is gonna speed up, that van's going to be speeding up.

In the second scenario, I've got a larger backwards force and forward force.

So that one is going to slow down.

The resultant force is backwards opposite to the movement, so that one slows down.

In the third scenario very carefully I've seen that the movement's in the other direction this time, and I've got a van gradually slowing down, so that one's decelerating as well.

So that one's correct.

And in the final van, well there's no resultant force on that one.

So that one's actually going to move at a steady speed.

So well done if you selected those options.

Resultant forces acting on objects don't just change the speed.

They can also change the direction of motion.

And that's also a type of acceleration.

So if you kick a football that was moving and it still goes off at the same speed, but it going in a different direction, that's accelerated.

The football has accelerated due to the force acting on it.

Similarly, when the earth is orbiting the sun and it orbits at a fairly steady speed, but it's constantly accelerating because the direction it's travelling is changing.

So acceleration also involves any change in direction of travel.

Let's check if you understand that about acceleration.

I've got a spaceship travelling through space.

As you can see there, it's moving towards the right and it uses a side thruster to produce a force.

And it, as you can see, I've drawn that on as a red arrow pointing sort of downwards there in the diagram.

What will happen to the movement of the spaceship is A, it will continue to move in the same direction.

B, it will come to a stop, or C, the direction of travel will change.

Pause the video, make your selection and then restart please.

Welcome back, while there's a resultant force acting on a spaceship.

So it's going to accelerate and its direction of travel will change, it will start bending so upwards, sorry, bending towards the direction of that thrust force.

So well done if you've got that.

Okay, it's time for the final task of the lesson.

And I've got four statements by our students here.

For each of those statements about the forces acting on a freewheeling cyclist is incorrect.

I'd like you to write out what you would say to each of those pupils separately to help them understand about what's going on with the free wheeling cyclist.

So read the statements carefully and decide what you'd say to them in order to correct what they've said.

So pause the video, do that, and restart please.

Welcome back and here's some examples of the things you could have said to the students.

So well done if you made corrections, something like this.

And now we've reached the end of the lesson, so let's have a look at everything we've learned throughout it.

To find a resultant force acting in an object, you summed the forces acting in one direction and subtract those acting in the opposite direction.

And you can see my example there, I've summed the forces to the left, and the right subtracted them, and I've got a resultant force of two newtons to the left there.

Newton's First Law of motion states that when there's no resultant force, a stationary object stays stationary, it doesn't start moving, and a moving object keeps moving at the same speed, in the same direction.

A resultant force causes acceleration and that can produce a change in velocity, which could be a change in speed, direction, or both.

Well done for reaching the end of the lesson.

I'll see you in the next one.