Lesson video

Hello and welcome to this lesson on using force arrows.

This is from the Forces topic.

My name's Mr. Norris.

This is a really useful lesson because in any science topic where you mention forces, you probably are gonna want to be able to represent that force properly.

This lesson covers how you do that using force arrows.

The outcome of this lesson is by the end of the lesson, hopefully you'll be able to draw and label force arrows to accurately represent forces acting.

Here are the key words we're gonna be focusing on in today's lesson, force arrow, size and direction.

Now lots of those words are fairly straightforward, however, on the next slide, I'm still going to put an example of each word being used correctly in a sentence.

So in a moment when the next slide comes up, you could pause the video and just carefully read through each word being used just so you are as ready for the lesson as possible.

So pause the video now if you'd like to do that.

And we'll start in a moment.

So this lesson on using force arrows is divided into three parts.

Firstly, we're gonna focus on the length of force arrows.

Then we're gonna think about their placement and think really carefully about where force arrows should be placed to communicate, what force is acting really carefully.

And then finally we'll put that all together.

And when we draw and label forces acting using force arrows.

So let's get started on the first section.

So we know that forces are pushes and pulls that objects exert on each other.

So there is a force arrow representing the force of the foot on the football.

So we use force arrows to represent forces acting.

And every force can be described as the force of the A on the B.

So here we've got the force of the foot on the football and we can place our force arrows really carefully on making sure arrows are placed on the object that the force acts on because that helps force arrows much more clearly represent the force that are really acting.

So the first thing we need to say about force arrows is that the size of a force in newtons is represented by the length of the arrow.

So we can see we've got three different kicking forces here and the middle one is bigger than the first one and the final kicking force is bigger than both of the previous ones.

In fact, the length of the arrow is proportional to the size of the force.

So a force that's twice the size, well, it needs an arrow that's twice as long because the length directly represents the size of the force to scale.

And a force that's three times the size needs an arrow three times as long.

So that kicking force is three times the size of the kicking force in the first picture.

So it needs an arrow three times as long.

So as we said, the length of a force arrow is proportional to the size of the force acting.

Let's do a check on that.

This kicking force is 10 newtons.

Which letter force represents five newtons? That's question one.

And question two, which letter force would represent 20 newtons? Remembering that force arrows are to scale.

Choose your answers now for both of those questions.

Pause the video if you need to.

Right.

I'll go through the answers.

If the top force is 10 newtons, then a force of five newtons should be half the length.

So that's force A.

So well done If you've got that.

And a force of 20 newtons should be double the length.

So that could only be force E.

'Cause force D isn't long enough to be double the length and force F is too long for double the length.

So force E is double the length.

Well done if you've got both of those.

Let's do another check.

Now in this check there is one force arrow for each of the five forces.

So here are the five forces, a force of 30 newtons, force of 14 newtons, force of 15 newtons, a force of 10 newtons.

And the force to lift my bag, which hasn't got a number yet.

You need to try and match each force to the correct arrow.

So we've got five forces, five arrows, A, B, C, D, and E.

Have a go at doing that now.

Pause the video if you need to.

Okay, well done for giving that check a go.

Hopefully you spotted that you've got two forces in the list.

One of them is double the other.

We've got 30 newtons and 15 newtons.

And if you look at the force arrows, force E is double the length of force C.

So E might be 30 and C might be 15.

And if that was true then we'd need a force arrow to represent 14, just slightly shorter.

And of course that can be force arrow B.

So it looks like the force of 13 newtons could be arrow E, the force of 14 newtons would be B.

And the force of 15 newtons would be C because it's half the length of E.

And of course B, 14 newtons is gonna be just very slightly shorter than arrow C, which we think is 15 newtons.

The force of 10 newtons then fits with that 'cause it could be arrow A, which looks like it's about two thirds of the 15 newton force for C.

So well done if you've got those.

And that only leaves force D for the force to lift my bag, which didn't have a letter on it.

So you had to leave that one till last and see which one you have left.

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

That's very impressive.

You really understood the idea that the length of a force error is proportional to the size of the force.

Therefore you are looking for different proportions that that you could apply.

So final check.

Now we know that the force to lift my bag is arrow D.

What would you say that force is roughly in newtons if you compare it to the other arrows? So come up with a number now for what goes in that blank for the strength of the force in newtons to lift my bag.

Five seconds to do that, come up with a number.

Okay, I would accept anything in the range between 18 to 21 newtons, it's gonna be in the right ballpark.

'Cause we haven't got an exact scale to read here.

So it's definitely got to be bigger than 15 newtons, but not very close to 30 newtons, much closer to 20 newtons.

So somewhere around 18, 19, 20 or 21 newtons looks about right for the length of that force arrow.

D, well done if you've got in that range.

So because the length of force arrows matters, you should use a ruler to draw the arrows accurately.

So use one centimetre to represent one newton of force.

Let's keep the scale really simple this lesson, because this isn't a lesson on scale, this is a lesson on force arrows.

So there's other things to think about then, we're not gonna do difficult scales this lesson.

So a five newton force, you need to get your ruler, draw a five centimetre line for a five newton force using one centimetre for one newton, pop an arrowhead on the end.

And there you go.

That's your five newton force drawn to that scale.

And then a 10.

5 newton force, you'd get your ruler, you draw your 10.

5 centimetre line for a 10.

5 newton force, pop it, turn it into an arrow, pop an arrowhead on the end and there is a 10.

5 newton force.

So that is what you.

That's the process you need to go through.

Okay, so time for a task where you do that for some forces.

Draw force arrows for these forces please.

They all act horizontally to the right, these forces.

So we don't need to worry about direction, they're all pointing to the right.

And this task is really designed to be done on the accompanying worksheet that goes with this task which has the images and the dots on.

So start each arrow from the centre of each dot and use one centimetre for one newton.

So you've got four forces to draw.

Start each arrow from the centre of each dot, like that.

And use one centimetre for one newton for each force.

So pause the video now.

Get your pencil and your ruler and have a go at drawing four force arrows for those four forces.

Off you go.

Okay, well done for your effort in that task in drawing force arrows the correct length to a scale one centimetre for one newton.

It's a bit difficult to give feedback on this, but I'll show you the process you should have gone through.

So for that first force, a 10 newton force, you needed to draw it 10 centimetres long, starting from the dot and pointing to the right like the task instructions said.

So ruler down, a 10 centimetre line for a 10 newton force.

Drawn with a ruler, that would be great.

Okay, and then force number two, a four newton force from the hand on pushing the door should be four centimetres long to the right, starting from the dot, which is where the force is acting.

Force three, a 14 newton push force from the hands on this crate.

Get your ruler, 14 centimetres for 14 newtons.

And then force number four, a 7.

5 newton pull force from the hands on this crate star from the dot.

7.

5 centimetres for 7.

5 newtons.

Okay, so we just looked at how long to draw force arrows and in the next section of the lesson, we're going to look at where to put them.

So the first thing to say is that force arrows should always point in the direction that the force acts.

So I'm gonna add some force arrows to each of these examples.

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

That is the direction of the force of the racket on the tennis ball.

And that's the direction of the gravitational force of earth on the parachutist.

So all of those arrows, I've got the angle just right to kind of show the direction of the push or the pull of the force.

And it's important to remember that forces don't always act in the direction of movement because forces make things change.

Forces cause changes.

Forces don't provide motion, forces cause motion to change.

Can change its speed or change the direction.

So the force acts in the direction of the change, not the direction of movement.

So let's look at some examples.

Here is an example of a force from the wind on a golf ball.

The golf ball is try.

A golfer might try and hit that golf ball straight towards the flag, but a force from the wind is gonna make that golf ball change direction.

So there is a zoomed in picture of the golf ball and here's the direction of the force from the wind on the golf ball.

So the golf ball's moving forwards but the force is pointing in the direction shown, which is not the direction of movement.

So forces show the direction of the change, not the direction of movement necessarily.

And here is an ice hockey puck sliding along ice, but the friction force of ice on the puck is gonna make that puck slow down.

So that would be the direction that friction acts on the puck.

The force arrow points in the direction of the change, which is to slow down the puck.

So the force points in the direction against the movement, which is the direction of the change.

Let's do a check of that.

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

The descriptions of the forces acting are all correct, but which force arrows have been drawn in the correct direction? So the first picture is supposed to show the gravitational force of earth on the child.

Is that direction correct? The second picture shows the drag force of the air on the child.

Has that direction been drawn correctly? The third picture shows.

Is supposed to be showing the friction forces of the floor on the child's socks.

Have they been drawn in the correct direction? And the fourth picture is supposed to be showing the normal contact forces of the floor on the child's feet.

Have they been drawn in the correct direction? Which of those have been drawn in the correct direction? I'll give you five seconds to make a decision for each picture.

Okay, make sure you've made a decision for each picture.

So I'll go through them all.

So the gravitational force on the child, that's the correct direction because gravitational forces act towards the centre of earth.

And the drag force of the air on the child will drag forces oppose motion.

So that's been drawn in the wrong direction.

It should be pointing the opposite direction to that.

The friction forces of the floor on the child's socks, again, friction forces point in the opposite direction to motion.

So that is wrong.

Those forces are in the wrong direction.

They should be drawn in the opposite direction.

Friction opposes the motion of the child and the normal contact forces of the floor on the child's feet while the child pushes down on the floor with their feet 'cause that's what objects do on surfaces.

If they're put on surface, they push down on that surface, but the surface pushes back and that's the normal contact force of the floor on the child's feet.

The floor pushes back on the child.

So that is correct.

So well done if you identified which of those force arrows were drawn in the right direction and which were drawn in the wrong direction.

Here's another important idea about where to draw force arrows.

They have to be drawn starting from the object the force acts on.

So let's say we want to draw the force of the racket on the ball.

Where do you draw it? Well, you have to draw it starting on the ball.

That's a good place to draw it, on the ball.

Because it's a force acting on the ball.

So you draw it starting on the ball.

So don't draw it off the ball.

Okay? Or off the ball or off the ball.

Sometimes students attempted to draw arrows like that last example because it looks like the force is pointing at the ball.

But we actually want to draw force arrows starting on the ball, not finishing on the ball, okay? Make sure you understand that correct placement.

The next slide gives one other important point as well.

Force arrows should also be drawn at a point on the object that shows where the force acts.

So the force of the racket on the ball is drawn starting from the point where the force acts, which is where the racket touches the ball.

So that's why that force arrow has been drawn starting in that position and not in that position or that position or that position or that position.

Because in those four non-examples, the force arrow doesn't start where the racket touches the ball, which is where the force acts.

You need to draw force arrows starting from a point on the object that shows where the force acts.

And in this case, if you look at the correct example, the force of the racket on the ball is gonna act on that kind of bottom surface of the ball, which is the bits that the racket is in contact with.

So that's where to draw force arrows.

So let's do a little check of that.

So Lucas lifts up his bag using the handle.

So which arrow is correctly placed to show the force of Lucas's hand on the bag? Which one letter correctly shows the force of Lucas's hand on the bag that shows where that force acts when he lifts the bag with his hand? So it should be fairly obvious.

Five seconds to choose the correct letter.

Let's see how you got on.

Well done if you chose arrow B.

Arrow B is correctly placed to show the force of Lucas's hand on the bag because that is the only arrow that shows where the force of his hand acts on the bag.

Let's do another check.

This person is exerting a pushing force on the box with their hands.

Which arrow is correctly placed to show this force? A force pushing on the box with the hands.

Is it arrow A, is it arrow B? Is it arrow C or is it arrow D? Which force correctly shows.

Is correctly placed to best show the force of the hands on the box.

Choose an answer now.

Okay, well done if you said answer B.

We're looking for a force on the box.

So the arrow has to start on the box close to where the force acts and the force comes from the hands so it should be drawn on the box close to the hands.

C is not quite right because the arrow doesn't start on the box, the arrow starts off the box.

So the arrow in C looks like it's acting on the person's wrists.

That's not the force we're drawing.

We're supposed to be drawing the force on the box.

So the arrow must start on the box.

So well done if you went for option B.

Final couple of points about placing force arrows.

We've mentioned this already, but just to make it really clear.

Gravitational forces are drawn starting from the centre of an object and they point down towards the centre of the earth.

So that is gravitational force of earth on the apple and this is gravitational force of earth on the parachutist.

Okay, gravitational force arrows are drawn starting from the centre of the object 'cause we think of gravity as acting on the object centre of mass.

So we try and show that by starting the arrow from the centre of the object for a gravitational force.

Here's an example of that.

We're gonna draw the gravitational force of earth on the model aeroplane.

We need to draw it starting from the middle of the aeroplane.

So it's a gravitational force, so it's gonna act from the object centre of mass.

So we draw the arrow starting from a point at the centre, you can see the red dot has just appeared.

The gravitational force is gonna ally downwards.

So we're gonna get out our ruler to draw the arrow the right length.

This looks like it's gonna be a 4.

5 newton force.

We're using one centimetre to represent one newton again.

So the arrow represents a 4.

5 newtons and that's it.

We've drawn the gravitational force in the right direction, acting from the right point on the object for gravitational force and we've drawn at the right length for that force.

So over to you, it's your turn, add one arrow to each diagram to represent the force described.

Use one centimetre for one newton for all of these.

And again this task is designed to be done on the accompanying worksheet to the lesson which has the diagrams already printed.

So number one, I'd like you to draw the tension force of the string on the pencil case.

So three newton force.

So think about where to start.

Where's the starting point for that force arrow for that force.

Force two, I'd like you to draw the gravitational force of earth on the moving ball.

So two newton force.

So you need to think about where is the right place to start that gravitational force arrow from on the ball.

And number three, I'd like you to draw the friction force of the ice on the sledge.

That's a 7.

5 newton force.

You'll need to think about the right place to draw that 7.

5 newton force starting from on that diagram.

So pause the video now, you need your ruler and your pencil and have a go at drawing force arrows for those three forces.

The right length, the right direction, and starting from the correct point on the diagram.

Off you go.

Right, well done for your efforts on that task.

So I'm gonna give feedback on each one.

So for the first one, the tension force of the string on the pencil case, it needs to be drawn starting from the point on the pencil case where the force acts from the string, which I think would be about that point there.

And a three newton force, we're gonna need to draw it three centimetres.

So you need to use your ruler, draw a three centimetre arrow and that's it.

Well, done if yours looks like that.

And it's three centimetres long drawn from about that point.

Number two, the gravitational force of earth on the moving ball, that needs to be drawn starting from the centre of the ball because gravitational forces, we think of them acting from the centre of mass of the objects.

So we draw gravitational forces acting from the centre and they always act virtually downwards.

And this is a two newton force so we need to draw it two centimetres long.

So it should be two centimetres long on your diagram.

So well done if yours looks like that.

And it's two centimetres.

And the third one was the friction force of the ice on the sledge.

That is gonna act on the lower surface of the sledge.

So choose a point on that bottom edge of the sledge but make sure it's on the sledge, not on the ice 'cause this is a force acting on the sledge at about that point on the bottom surface.

7.

5 newton.

So it needs to be drawn about 7.

5 centimetres long.

So it should look like that.

So take a moment to check all of your arrows and very well done for your effort on that task.

Okay, so we're now ready to put it all together in the last section of the lesson where we're gonna draw and label force arrows to represent forces acting.

So we need to say that different forces can act on different objects.

But to understand or predict an object's motion, you only need to look at the forces acting on that one object.

And that's a really important principle that we do throughout physics topics.

To understand how one object is moved, so how its motion changes, all we need to do is look at the forces acting on that object and then we can predict how that object's motion might change.

Or what will happen next.

So let's look at this tug of war.

There's two people pulling a rope.

The force of person A on the rope is six newtons.

So that force is gonna act on that point on the rope, that's where they're pulling the rope.

We need to get our ruler, draw a six newton force.

So six centimetres and that's that force.

What about the force of person B on the rope? 6.

5 newtons.

They're just pulling with a slightly bigger force.

That is where we're gonna start the force arrow from.

So where their hands are pulling on the rope, we can draw the force on the rope starting from that point.

Use a ruler, 6.

5 newtons.

So we're getting the scale right and that's the force of person B on the rope.

And look how the force arrows have been labelled.

The force of person A on the rope and the force of person B on a rope.

That's how you described forces.

You say what provides the force and what the force acts on.

And of course you should be able to see what the force acts on because that's where the force arrow starts.

Now if we were to remove everything from that picture, apart from the rope and the forces on the rope, it would look like that.

And that kind of picture can help us then predict how the rope is gonna move.

We can see the rope is probably gonna speed up to the right in this example, but not by very much because the force pulling to the right is only half a newton bigger than the force pulling to the left.

So that is something we do in physics quite a lot.

If we want to understand or predict an object's motion, we work out what are all the forces acting on that object and then we get rid of everything else and just focus on that one object and then we can see how that one object is gonna move or what happens next to that object.

Right, let's do a quick check of everything we've just said.

Which three of the below are correct? So three of them are right.

Which three are correct when drawing and labelling forces acting? A, use a ruler.

B, draw force arrows the correct length (to scale).

C, draw each arrow starting from a point that shows where the force acts.

D, the width of the arrow shows the size of the force.

Which three of those are correct? I'm gonna give you five seconds to choose now.

Pause the video if you need longer.

Make sure you've chosen three that you think are correct.

So A, user ruler, that's correct.

We need to do that.

We need to use a ruler to draw force arrows.

B, draw force arrows the correct length (to scale).

Yeah, we need to do that.

That's why we need the ruler.

And also the ruler helps make sure we draw them straight.

C, draw each arrow starting from a point.

Starting from a point that shows where the force acts.

Yes, that's right.

We do need to do that.

So D is the one that's not correct.

It's not the width of the arrow that shows the size of the force, it's the length of the arrow that shows the size of the force.

So hopefully that was fairly straightforward and you did identify.

It was A, B, and C, which are correct.

And D is incorrect.

Let's do another check.

So identify two mistakes in this example of how force arrows have been drawn and labelled.

So we've got the drag force of the air on the parachute toy, which is supposed to be three newtons and the gravitational force of earth on the parachute toy, which is supposed to be six newtons.

What are two mistakes that have been made in this example of drawing force arrows? Pause the video now.

Make sure you can identify two mistakes.

Let's see how you've got on.

So did you spot a first mistake, which is that the arrows have not been drawn to scale? One force is supposed to be three newtons, the other force is supposed to be six newtons.

Double the force should be double the length.

One should be three centimetres for a three newton force.

The gravitational force should be six centimetres for a six newton force.

And that's not been done.

So that's the first mistake.

What about the second mistake? It was about the gravitational force? The weight, the gravitational force, the weight force, the gravitational force should be drawn starting from the centre of the object.

Gravitational forces should be drawn starting from the centre of the object because they're non-contact forces.

They act on the whole object or we think of them as acting from the objects centre of mass.

That's why we draw them starting from the centre.

So well done if you spotted both of those two mistakes with how those force arrows have been drawn.

So now it's your turn.

This is the last task of the lesson.

We're gonna put everything together and I want you to draw arrows for the following forces to each diagram.

Use one centimetre for one newton like we've done every time, and label each force arrow fully as the force of A on the B.

So say the object that provides each force that you draw and the object that each force acts on.

So number one, I'd like you to draw two pulling forces on the cracker.

Four newton and seven newton.

One force for each person's pulling force, one's four newton, one's seven newton.

Number two, I'd like you to draw the gravitational force and the upper thrusts force on the toy boat.

Both of those forces are five newtons.

And number three, I'd like you to draw a driving force that's gonna push the plane forwards.

That's four newtons.

And an air resistance force, 6.

5 newtons acting on the model aeroplane.

So for each force you need to think about where should you draw each force starting from? What direction should it.

What direction does it act and how long should you draw it? Okay, you should pause the video now.

I should also say this task is also designed to be done on the accompanying worksheet to this lesson, which has the diagrams already drawn on.

So pause the video now, get your ruler, get your pencil, and have a go at drawing those force arrows.

Pause the video now.

Off you go, Right, I'm gonna give some feedback on how you've done that or how that should have been done.

So for number one, the two pulling forces on the cracker.

So I've done a four newton force to the left.

It didn't actually say which force goes to the left and which goes to the right.

So you might've done these the other way around.

I've done the four newton force to the left from this person.

As long as there's one force that's four centimetres and one force that's seven centimetres, then you followed the instructions correctly.

So just double check the lengths of those and that they're going in the right direction for each person's pull force on the cracker.

They should be labelled.

That's the force of person A on the cracker.

We could have just had the force of this person on the cracker and this is the force of person B.

The force of that person on the cracker.

So well done if you've got that and you weren't able to do that.

But I've just removed the people so we can just see the forces acting on the cracker 'cause this is what physicists kind of do when they're analysing forces.

Okay, number two.

The gravitational force and the upthrust force on the toy boat and they're both five newtons.

So the gravitational force acts from the centre of the boat 'cause that's where the force acts.

We think of it as acting from the centre of mass.

Five newtons five centimetres.

Lovely.

What about the upthrust force? Well, the upthrust force is the force of the water on the toy boat and the water pushes the boat on the bottom surface.

The upthrust force should be drawn starting from the bottom surface of the boat where the water pushes up on the boat.

That's what upthrust force is and does.

So that's a good starting point, starting on the bottom surface of the boat and pointing upwards five centimetres for a five newton force.

There we go, lovely.

And how should we label those? We should have the upthrust force of the water on the boat and the gravitational force of earth on the boat.

Okay, so number three.

That was a driving force of four newtons and an air resistance force of 6.

5 newtons.

So the driving force, where does that force act? Well, on this model aeroplane, I would assume that given that it's got a kind of propeller at the front that's driving it forward.

The propeller's gonna be what's driving this plane forwards.

'Cause the plane doesn't have any motors or engines under the wings, so the driving force must be provided by that propeller on this model aeroplane, must be spinning.

Maybe it's a remote controlled plane.

So that's where I've drawn the driving force acting from.

And it's four newton, so four centimetres.

Should look like that.

And the air resistance force.

Now remember that air resistance acts at every point on the object surface, but we only want to draw one arrow for the air resistance force.

So as long as you've drawn it somewhere acting from the surface of the plane, perhaps towards the front of the plane where the air resistance force acts, something like that.

But it doesn't have to be too precise.

As long as it's perhaps somewhere central on the plain surface, that would do.

So 6.

5 newtons needs to be 6.

5 centimetres.

So double check the length that you've drawn it and then how should this forces be labelled.

We've got the driving force on the aeroplane from the motor or from the propeller and the force of the air on the plane, which is air resistance or drag.

So as long as the forces are labelled something like those examples, that would be fantastic.

I'll just add, this wasn't part of the question, but I'll just add that there is a bigger backwards force on this plane than a forwards force.

Therefore this plane must be slowing down.

So well done.

That completes this lesson on how to use force arrows.

Here's a quick summary.

Force arrows are arrows drawn to represent the forces acting so of course you can't see the forces.

We have to draw arrows to represent the pushes and the pulls in any situation.

A force arrow is drawn starting from a point on the object that shows where the force acts.

Force arrow points in the direction the force acts.

And the size of a force is represented by the length of the arrow.

And you can see that in the diagram.

And finally, force arrows, they're are best labelled as the force of object A on object B, such as the friction force of the ice on the sledge.

Well done for completing this lesson on drawing force arrows.