Frames of Reference (1960) Educational Film

Frames of Reference is a 1960 educational film by Physical Sciences Study Committee.
The film was made to be shown in high school physics courses. In the film University of Toronto physics professors Patterson Hume and Donald Ivey explain the distinction between inertial and nonintertial frames of reference, while demonstrating these concepts through humorous camera tricks. For example, the film opens with Dr. Hume, who appears to be upside down, accusing Dr. Ivey of being upside down. Only when the pair flip a coin does it become obvious that Dr. Ivey — and the camera — are indeed inverted.

The film's humor serves both to hold students' interest and to demonstrate the concepts being discussed.
This PSSC film utilizes a fascinating set consisting of a rotating table and furniture occupying surprisingly unpredictable spots within the viewing area. The fine cinematography by Abraham Morochnik, and funny narration by University of Toronto professors Donald Ivey and Patterson Hume is a wonderful example of the fun a creative team of filmmakers can have with a subject that other, less imaginative types might find pedestrian.
Producer: Richard Leacock
Production Company: Educational Development Corp.
Sponsor: Eric Prestamon

We're used to seeing things from a particular point of view.

That is from a particular frame of reference. Things look different to us under different circumstances.

At the moment, things look...

Dr. Hume: You look peculiar.

You're upside down.

Dr. Ivey: No, you're the one that's upside down.

Dr. Hume: No, you're upside down.

Dr. Ivey: No, I'm not.

He's the one that's upside down, isn't he?

Dr. Hume: Well, let's toss for it.

Dr. Ivey: All right.

Okay.

Dr. Hume: You lose. He's the one that's really upside down.

You'd better come into my frame of reference now.

Dr. Ivey: (Laughs)

My frame of reference was inverted from what it usually is.

That view of things would be normal for me if i normally walked on my hands.

Dr. Hume: This represents a frame of reference.

Just 3 rods stuck together so that each is at right angles to the other 2.

Now, I'm going to move in this direction. You see the frame on the same spot on the screen.

But you know I'm moving this way, because you see the wall moving this way behind me.

But how do you know that I'm not standing the still, and the wall is moving?

It was the wall that was moving.

Now the wall has disappeared and you have no way of telling whether I'm moving or not.

But now, you know that I'm moving. The point of this is that all motion is relative.

In both cases I was moving, relative to the wall.

And the wall was moving relative to me.

Dr. Ivey: All motion is relative.

But we tend to think of one thing as being fixed and the other thing as being moving.

We usually think of the Earth as fixed, and walls are usually fixed to the earth.

So, perhaps you were surprised the first time when it was the wall that was moving, and not Dr. Hume.

A frame of reference fixed to the Earth is the most common frame of reference

in which to observe the motion of other things.

Dr. Hume: This is the frame of reference that you're used to.

The frame is fastened to the table. The table is bolted to the floor.

The floor is anchored in building and the building is firmly attached to the Earth.

Of course, the reason for having 3 rods is the position of any object, such as this ball

can be specified using these 3 reference lines. This reference line points in the direction which we call up.

Which is a different direction here than it is in the other side of the Earth.

And these 2 reference lines specify a plane, which we call horizontal, or level.

In this film we're going to look at the motion of objects in this Earth frame of reference, and in other

frames of reference, moving in different ways relative to the Earth frame.

Well, let's look at a motion. This steel ball can be held up by the electromagnet.

Now I'm going to open the switch and you watch the motion of the ball.

The ball is accelerated straight down by gravity, along the line parallel to this

vertical reference line.

Dr. Ivey: As you can see. The electromagnet is mounted on a cart that can move.

Now, I'm going to exactly the same experiment that Dr. Hume did.

But this time, while the cart is moving at a constant velocity.

The cart is pulled along by a string which is wound around this phonograph turntable.

And that pulls it with a constant velocity.

When the cart passes this line, the ball is released, as you can see.

I'm going to start the cart down at the end of the table, so that by the time it gets to this point

I can be sure it's moving with a constant velocity. Now, I want you to watch right here

so that you will see the ball falling.

I think you can see that the ball landed in exactly the same position that it did before

when Dr. Hume did the experiment with the cart fixed. But this time, the ball could not have fallen

straight down. Let me show you why.

The ball was released at that point.

If it had fallen straight down, because the cart moves on in the time it takes to fall

it would have landed back here somewhere, but it didn't.

Now, I'm going to do the experiment again. This time I'm going to let you watch the motion through a

slow motion camera, which is fixed here. As the cart moves by, the ball will fall

and you can watch in the slow motion camera.

I'll show you this again. This time there will be a line on the film so that you can see the path.

I think that you can see that the path of the ball is a parabola.

But, all of this has been in a frame of reference fixed to the Earth.

How would this motion look in a frame of reference that was moving along with the cart.

A frame of reference like that.

Well, so that you can see what it looks like I'm going to fix this slow motion camera

so that it moves with the cart.

Like this.

I'm going to do the experiment again.

Incidentally, I'll start it, and then I'm going to stand here so that when the ball

falls you will have something which is fixed as a reference point.

In the moving frame of reference, I think you could see that the path of the ball is a vertical, straight line.

It looks exactly the same as it did before, when Dr. Hume did the experiment with the cart fixed.

If we were moving along in this frame of reference and we couldn't see the surroundings

then we wouldn't be able to tell by this experiment that we were moving at a constant velocity.

As a matter of fact, we wouldn't be able to tell by any experiment that we were moving

at a constant velocity.

I'm going to do the experiment once more, and this time,

I'm not going to stand here behind the ball as it falls.

So that you won't have any fixed reference frame.

[ball thuds]

As far as you're concerned that time, the cart wasn't necessarily moving at all.

That time, when you couldn't see the background,

then I think it was perhaps harder for you to realize that you were in a moving frame of reference.

Important thing to realize here is that all frames of reference

moving at constant velocity with respect to one another

are equivalent.

Dr. Hume: Dr. Ivy showed you what the motion of the ball that was released from the moving cart

looked like in the Earth frame of reference, and in the cart frame.

The motion looked simpler from the cart.

Now I want you to watch the motion of this white spot.

You probably see the spot moving in a circle.

But this is what it's path is actually like in the Earth frame of reference.

This is you're normal frame of reference. You saw the spot moving in the circle because your

eye moved along with the cart. You put yourself in the frame of reference of the moving cart.

So, you see, it isn't always true that we view motion from the Earth frame of reference.

When the motion is simpler from the moving frame, you automatically put yourself in that

moving frame.

Dr. Ivey: Now we're going to do another experiment on relative motion to show how to compare the

velocity of an object in one frame of reference to its velocity in another frame of reference.

If I give this dry-ice puck a certain start, it moves it straight across the table with a speed

which is essentially constant because the forces of friction have been made very small.

This is just the law of inertia.

An object moves with a constant velocity unless an unbalanced force acts on it.

Now, will you give it the same start backwards?

Dr. Hume: I will try.

Dr. Ivey: If Dr. Hume gives it the same start, it moves back in this direction with the same velocity.

Now, we are on a car here. A car which can move, and which really is going to move in this direction.

And we are going to repeat the experiment. All right, let's go.

If we were making measurements here, then we would observe the same velocities, that is the

same experimental results that we did before, and so would you, because you are observing this

experiment through a camera which is fastened to this cart.

That is, you are in the moving frame of reference with us.

But now we're going to do the experiment again, and this time you watch through a camera which

is fixed in the Earth frame of reference.

Now, concentrate on watching the puck. Don't let your eye follow us.

I think you will see that it will move faster that way, and not so fast this way, relative to

you and relative to the wall behind.

Here's the cart, which was moving along in this direction with a velocity "u."

We were sitting on the cart, at a table. Here I am over on this side, and Dr. Hume was on this side.

And we were pushing this puck back and forth on the table. When I pushed it went in this direction with

a velocity "v," and when Dr. Hume pushed it, it went in this direction with the same velocity "v."

But this is the velocity relative to the cart. What about velocity relative to an observer on the ground,

in the fixed frame?

Well, if it is pushed in this direction, its velocity is u plus v. If it's in this direction, it's u minus v.

This is all very reasonable, there's nothing very hard to understand here.

The surprising thing about this expression is that it is not accurate in all circumstances.

At very high speeds, and by high speeds I mean speeds close to the velocity of light, this

expression breaks down.

At these very high speeds we have to use the ideas about relative motion developed by Albert Einstein

and his special Theory of Relativity.

However for all the speeds that we are ever likely to run into, this expression, u plus or minus v, is

completely adequate.

So far we have been talking about frames of references moving at a constant velocity relative to another.

Now I am doing the expriment with the dropping ball again,

only this time the cart will be accelerated relative to the earth's frame.

These weights will fall and give the cart a constant acceleration.

I put the ball all up and I now will release it in motion very fast,

and I want you to watch at the point where the ball is relaesed from the fixed camera.

Ready?

I don't know whether you saw it or not but the path of the ball was the same as it was before.

Only this time it landed in a different spot.

This is because hte cart kept on accelerating in this direction as the ball was falling.

Now I am going to let you see it again with the slow motion camera fixed onto the cart.

This time you saw the ball off to one side.

And not following down the vertical reference line as it did in the constant velocity case.

I suppose you were in the that accelerated frame of reference.

How could you explain this motion?

Gravity is the only force acting on this ball.

So it should fall straight down.

But if the law of inertia (Trägheitsgesetz) is to hold, there must be a force pushig sideways on the ball in this direction.

Because its a deviate (Abweichung) from the vertical path.

But what kind of a force is it?

It isn't a gravitational, or an electric, or a nuclear force, in fact it isn't a force at all as we know one.

So we're left to conclude that there s no force that could be pushing in this direction on the ball ,

but the law of inertia just does not hold.

This is a strange frame of reference.

We call a frame of reference in which the law of inertia holds an inertial frame (Inertialsystem) .

The law of inertia holds in the earth frame of reference, so it is an inertial frame.

The cart moving a constant velocity relative to the earth is an inertial frame.

But the cart which is accelerated is not an inertial frame.

Because the frame of reference that we are used to living in is one in which the law of inertia holds,

when we go into an non-inertial frame like the frame of the accelerated cart,

our believe in the law of inertia is so strong that when we see an acceleration of the ball sideways we think there is a force causing it.

So we make up a fiction that there is a force.

And sometimes we call it a ficticious force (Scheinkraft).

Ficticious forces arise in accelerated frames of reference.

The frame is accelerated in this direction so you in the frame see an acceleration of the ball in this direction.

And you say that there is a force causing it.

What's happening this time?

Why doesn't the puck move straight across the table as it did before?

As you can see it doesn't?

Though, if we believe it the law of inertia, then we must believe there is an unbound force to change the velocity of the puck.

But the puck is nearly frictionless (reibungslos), so what can be exerting this unbound force on it?

So how is it if you watch the motion this time through a camera that is fixed in the earth's frame of reference?

I think you can concentrate on watching just the puck, you can see that it is moving in a straight line.

And that therefore there is no unbound force acting on it.