Frames of Reference (1960) Educational Film
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0:37 - 0:40We're used to seeing things from a particular point of view.
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0:40 - 0:48That is from a particular frame of reference. Things look different to us under different circumstances.
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0:48 - 0:51At the moment, things look...
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0:59 - 1:01Dr. Hume: You look peculiar.
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1:01 - 1:03You're upside down.
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1:03 - 1:05Dr. Ivey: No, you're the one that's upside down.
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1:05 - 1:07Dr. Hume: No, you're upside down.
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1:07 - 1:09Dr. Ivey: No, I'm not.
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1:09 - 1:13He's the one that's upside down, isn't he?
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1:13 - 1:14Dr. Hume: Well, let's toss for it.
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1:14 - 1:17Dr. Ivey: All right.
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1:18 - 1:20Okay.
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1:24 - 1:27Dr. Hume: You lose. He's the one that's really upside down.
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1:27 - 1:29You'd better come into my frame of reference now.
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1:29 - 1:32Dr. Ivey: (Laughs)
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1:40 - 1:46My frame of reference was inverted from what it usually is.
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1:46 - 1:55That view of things would be normal for me if i normally walked on my hands.
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1:55 - 1:57Dr. Hume: This represents a frame of reference.
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1:57 - 2:03Just 3 rods stuck together so that each is at right angles to the other 2.
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2:03 - 2:10Now, I'm going to move in this direction. You see the frame on the same spot on the screen.
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2:10 - 2:14But you know I'm moving this way, because you see the wall moving this way behind me.
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2:14 - 2:20But how do you know that I'm not standing the still, and the wall is moving?
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2:22 - 2:28It was the wall that was moving.
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2:28 - 2:34Now the wall has disappeared and you have no way of telling whether I'm moving or not.
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2:34 - 2:39But now, you know that I'm moving. The point of this is that all motion is relative.
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2:39 - 2:43In both cases I was moving, relative to the wall.
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2:43 - 2:49And the wall was moving relative to me.
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2:51 - 2:53Dr. Ivey: All motion is relative.
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2:53 - 2:58But we tend to think of one thing as being fixed and the other thing as being moving.
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2:58 - 3:02We usually think of the Earth as fixed, and walls are usually fixed to the earth.
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3:02 - 3:09So, perhaps you were surprised the first time when it was the wall that was moving, and not Dr. Hume.
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3:09 - 3:12A frame of reference fixed to the Earth is the most common frame of reference
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3:12 - 3:17in which to observe the motion of other things.
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3:17 - 3:20Dr. Hume: This is the frame of reference that you're used to.
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3:20 - 3:24The frame is fastened to the table. The table is bolted to the floor.
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3:24 - 3:29The floor is anchored in building and the building is firmly attached to the Earth.
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3:29 - 3:36Of course, the reason for having 3 rods is the position of any object, such as this ball
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3:36 - 3:45can be specified using these 3 reference lines. This reference line points in the direction which we call up.
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3:45 - 3:49Which is a different direction here than it is in the other side of the Earth.
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3:49 - 3:56And these 2 reference lines specify a plane, which we call horizontal, or level.
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3:56 - 4:03In this film we're going to look at the motion of objects in this Earth frame of reference, and in other
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4:03 - 4:08frames of reference, moving in different ways relative to the Earth frame.
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4:08 - 4:16Well, let's look at a motion. This steel ball can be held up by the electromagnet.
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4:16 - 4:22Now I'm going to open the switch and you watch the motion of the ball.
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4:22 - 4:27The ball is accelerated straight down by gravity, along the line parallel to this
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4:27 - 4:32vertical reference line.
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4:34 - 4:38Dr. Ivey: As you can see. The electromagnet is mounted on a cart that can move.
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4:38 - 4:41Now, I'm going to exactly the same experiment that Dr. Hume did.
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4:41 - 4:46But this time, while the cart is moving at a constant velocity.
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4:46 - 4:50The cart is pulled along by a string which is wound around this phonograph turntable.
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4:50 - 4:55And that pulls it with a constant velocity.
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4:58 - 5:07When the cart passes this line, the ball is released, as you can see.
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5:07 - 5:12I'm going to start the cart down at the end of the table, so that by the time it gets to this point
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5:12 - 5:16I can be sure it's moving with a constant velocity. Now, I want you to watch right here
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5:16 - 5:22so that you will see the ball falling.
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5:32 - 5:36I think you can see that the ball landed in exactly the same position that it did before
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5:36 - 5:41when Dr. Hume did the experiment with the cart fixed. But this time, the ball could not have fallen
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5:41 - 5:45straight down. Let me show you why.
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5:47 - 5:50The ball was released at that point.
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5:50 - 5:55If it had fallen straight down, because the cart moves on in the time it takes to fall
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5:55 - 6:00it would have landed back here somewhere, but it didn't.
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6:00 - 6:06Now, I'm going to do the experiment again. This time I'm going to let you watch the motion through a
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6:06 - 6:15slow motion camera, which is fixed here. As the cart moves by, the ball will fall
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6:15 - 6:22and you can watch in the slow motion camera.
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6:27 - 6:34I'll show you this again. This time there will be a line on the film so that you can see the path.
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6:34 - 6:40I think that you can see that the path of the ball is a parabola.
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6:40 - 6:44But, all of this has been in a frame of reference fixed to the Earth.
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6:44 - 6:52How would this motion look in a frame of reference that was moving along with the cart.
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6:52 - 6:54A frame of reference like that.
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6:54 - 6:59Well, so that you can see what it looks like I'm going to fix this slow motion camera
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6:59 - 7:05so that it moves with the cart.
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7:06 - 7:08Like this.
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7:08 - 7:10I'm going to do the experiment again.
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7:10 - 7:14Incidentally, I'll start it, and then I'm going to stand here so that when the ball
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7:14 - 7:21falls you will have something which is fixed as a reference point.
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7:39 - 7:44In the moving frame of reference, I think you could see that the path of the ball is a vertical, straight line.
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7:44 - 7:50It looks exactly the same as it did before, when Dr. Hume did the experiment with the cart fixed.
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7:50 - 7:55If we were moving along in this frame of reference and we couldn't see the surroundings
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7:55 - 8:00then we wouldn't be able to tell by this experiment that we were moving at a constant velocity.
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8:00 - 8:03As a matter of fact, we wouldn't be able to tell by any experiment that we were moving
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8:03 - 8:05at a constant velocity.
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8:05 - 8:08I'm going to do the experiment once more, and this time,
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8:08 - 8:11I'm not going to stand here behind the ball as it falls.
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8:11 - 8:17So that you won't have any fixed reference frame.
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8:27 - 8:30[ball thuds]
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8:31 - 8:36As far as you're concerned that time, the cart wasn't necessarily moving at all.
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8:36 - 8:38That time, when you couldn't see the background,
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8:38 - 8:44then I think it was perhaps harder for you to realize that you were in a moving frame of reference.
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8:44 - 8:48Important thing to realize here is that all frames of reference
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8:48 - 8:52moving at constant velocity with respect to one another
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8:52 - 8:54are equivalent.
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8:54 - 8:58Dr. Hume: Dr. Ivy showed you what the motion of the ball that was released from the moving cart
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8:58 - 9:03looked like in the Earth frame of reference, and in the cart frame.
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9:03 - 9:06The motion looked simpler from the cart.
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9:06 - 9:13Now I want you to watch the motion of this white spot.
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9:14 - 9:19You probably see the spot moving in a circle.
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9:25 - 9:30But this is what it's path is actually like in the Earth frame of reference.
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9:30 - 9:37This is you're normal frame of reference. You saw the spot moving in the circle because your
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9:37 - 9:44eye moved along with the cart. You put yourself in the frame of reference of the moving cart.
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9:44 - 9:50So, you see, it isn't always true that we view motion from the Earth frame of reference.
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9:50 - 9:55When the motion is simpler from the moving frame, you automatically put yourself in that
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9:55 - 9:59moving frame.
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10:05 - 10:09Dr. Ivey: Now we're going to do another experiment on relative motion to show how to compare the
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10:09 - 10:15velocity of an object in one frame of reference to its velocity in another frame of reference.
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10:15 - 10:21If I give this dry-ice puck a certain start, it moves it straight across the table with a speed
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10:21 - 10:25which is essentially constant because the forces of friction have been made very small.
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10:25 - 10:27This is just the law of inertia.
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10:27 - 10:32An object moves with a constant velocity unless an unbalanced force acts on it.
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10:32 - 10:34Now, will you give it the same start backwards?
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10:34 - 10:37Dr. Hume: I will try.
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10:38 - 10:43Dr. Ivey: If Dr. Hume gives it the same start, it moves back in this direction with the same velocity.
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10:43 - 10:48Now, we are on a car here. A car which can move, and which really is going to move in this direction.
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10:48 - 10:53And we are going to repeat the experiment. All right, let's go.
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11:02 - 11:08If we were making measurements here, then we would observe the same velocities, that is the
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11:08 - 11:13same experimental results that we did before, and so would you, because you are observing this
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11:13 - 11:17experiment through a camera which is fastened to this cart.
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11:17 - 11:21That is, you are in the moving frame of reference with us.
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11:21 - 11:26But now we're going to do the experiment again, and this time you watch through a camera which
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11:26 - 11:30is fixed in the Earth frame of reference.
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11:30 - 11:34Now, concentrate on watching the puck. Don't let your eye follow us.
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11:34 - 11:39I think you will see that it will move faster that way, and not so fast this way, relative to
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11:39 - 11:44you and relative to the wall behind.
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11:52 - 11:59Here's the cart, which was moving along in this direction with a velocity "u."
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11:59 - 12:11We were sitting on the cart, at a table. Here I am over on this side, and Dr. Hume was on this side.
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12:11 - 12:18And we were pushing this puck back and forth on the table. When I pushed it went in this direction with
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12:18 - 12:26a velocity "v," and when Dr. Hume pushed it, it went in this direction with the same velocity "v."
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12:26 - 12:33But this is the velocity relative to the cart. What about velocity relative to an observer on the ground,
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12:33 - 12:35in the fixed frame?
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12:35 - 12:50Well, if it is pushed in this direction, its velocity is u plus v. If it's in this direction, it's u minus v.
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12:50 - 12:53This is all very reasonable, there's nothing very hard to understand here.
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12:53 - 12:59The surprising thing about this expression is that it is not accurate in all circumstances.
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12:59 - 13:05At very high speeds, and by high speeds I mean speeds close to the velocity of light, this
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13:05 - 13:08expression breaks down.
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13:09 - 13:16At these very high speeds we have to use the ideas about relative motion developed by Albert Einstein
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13:16 - 13:19and his special Theory of Relativity.
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13:19 - 13:25However for all the speeds that we are ever likely to run into, this expression, u plus or minus v, is
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13:25 - 13:27completely adequate.
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13:27 - 13:32So far we have been talking about frames of references moving at a constant velocity relative to another.
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13:32 - 13:36Now I am doing the expriment with the dropping ball again,
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13:36 - 13:42only this time the cart will be accelerated relative to the earth's frame.
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13:42 - 13:48These weights will fall and give the cart a constant acceleration.
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13:53 - 13:57I put the ball all up and I now will release it in motion very fast,
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13:57 - 14:03and I want you to watch at the point where the ball is relaesed from the fixed camera.
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14:05 - 14:05Ready?
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14:08 - 14:12I don't know whether you saw it or not but the path of the ball was the same as it was before.
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14:12 - 14:16Only this time it landed in a different spot.
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14:16 - 14:21This is because hte cart kept on accelerating in this direction as the ball was falling.
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14:22 - 14:27Now I am going to let you see it again with the slow motion camera fixed onto the cart.
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14:49 - 14:52This time you saw the ball off to one side.
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14:52 - 14:59And not following down the vertical reference line as it did in the constant velocity case.
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14:59 - 15:03I suppose you were in the that accelerated frame of reference.
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15:03 - 15:06How could you explain this motion?
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15:08 - 15:12Gravity is the only force acting on this ball.
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15:12 - 15:16So it should fall straight down.
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15:16 - 15:22But if the law of inertia (Trägheitsgesetz) is to hold, there must be a force pushig sideways on the ball in this direction.
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15:22 - 15:26Because its a deviate (Abweichung) from the vertical path.
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15:26 - 15:28But what kind of a force is it?
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15:28 - 15:36It isn't a gravitational, or an electric, or a nuclear force, in fact it isn't a force at all as we know one.
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15:36 - 15:43So we're left to conclude that there s no force that could be pushing in this direction on the ball ,
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15:43 - 15:47but the law of inertia just does not hold.
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15:47 - 15:50This is a strange frame of reference.
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15:50 - 15:56We call a frame of reference in which the law of inertia holds an inertial frame (Inertialsystem) .
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15:57 - 16:03The law of inertia holds in the earth frame of reference, so it is an inertial frame.
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16:03 - 16:10The cart moving a constant velocity relative to the earth is an inertial frame.
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16:10 - 16:14But the cart which is accelerated is not an inertial frame.
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16:15 - 16:21Because the frame of reference that we are used to living in is one in which the law of inertia holds,
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16:21 - 16:27when we go into an non-inertial frame like the frame of the accelerated cart,
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16:27 - 16:38our 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.
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16:38 - 16:41So we make up a fiction that there is a force.
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16:41 - 16:45And sometimes we call it a ficticious force (Scheinkraft).
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16:46 - 16:51Ficticious forces arise in accelerated frames of reference.
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16:51 - 17:00The frame is accelerated in this direction so you in the frame see an acceleration of the ball in this direction.
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17:00 - 17:03And you say that there is a force causing it.
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17:12 - 17:14What's happening this time?
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17:14 - 17:18Why doesn't the puck move straight across the table as it did before?
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17:26 - 17:30As you can see it doesn't?
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17:30 - 17:39Though, if we believe it the law of inertia, then we must believe there is an unbound force to change the velocity of the puck.
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17:39 - 17:44But the puck is nearly frictionless (reibungslos), so what can be exerting this unbound force on it?
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17:45 - 17:52So how is it if you watch the motion this time through a camera that is fixed in the earth's frame of reference?
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18:19 - 18:24I think you can concentrate on watching just the puck, you can see that it is moving in a straight line.
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18:24 - 18:28And that therefore there is no unbound force acting on it.
- Title:
- Frames of Reference (1960) Educational Film
- Description:
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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 - Video Language:
- English
- Duration:
- 27:26
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Steven1L edited English subtitles for Frames of Reference (1960) Educational Film | |
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michela.lenzi edited English subtitles for Frames of Reference (1960) Educational Film | |
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michela.lenzi edited English subtitles for Frames of Reference (1960) Educational Film | |
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Bene Müller edited English subtitles for Frames of Reference (1960) Educational Film | |
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Bene Müller edited English subtitles for Frames of Reference (1960) Educational Film |