Frames of Reference

822 words | 3 page(s)

Professor Patterson Hume and Professor Donald Ivey from the University of Toronto present this video about how things look different under different circumstances. They begin the video arguing about who is upside down, and because Dr. Hume is at the top of the screen (upside down), most people would assume that he is actually the one that is hanging from the ceiling. This was not the case; however, because the screen was reversed (so that the ceiling was at the bottom of the screen and the floor was on top of the screen), so it was actually Dr. Ivey who was hanging upside down by his legs from a pole. When he jumped down and the screen was reversed to where we would typically expect the floor and ceiling, it made sense that Dr. Ivey was the one upside down.

The video then progresses into another scenario illustrating how all motion is relative, using a moving wall. It gives the impression that the professor is moving, but then we see that the wall behind him was actually movie. This tricks the mind because according to Dr. Ivey, a frame of reference fixed to the earth is one that is most common. In other words, we tend to think that things that are fixed or attached (e.g., walls, tress, houses) are still and it is the other thing near them that is moving.

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Their next experiment has to do with the motion of a ball on a plane. There is a wooden cart that is on a track, fastened by strings to a turntable that is able to pull the car at a constant velocity. Attached to the cart is a vertical pole that holds the mall with an electromagnet. When the ball is released and the cart is still, the ball falls straight down due to gravity. Dr. Ivey then performs the same experiment, but this time the car was moving with a constant velocity. This time, the ball landed in the same spot (even though the cart was moving), but when seen through a slow-motion camera, it is clear that the balls path was that of a parabola and not a straight path as before. It was interesting; however, when he put the camera on track so that it was moving at the same velocity, the ball now seemed to fall in a straight line.

Also, when he was standing behind the cart (as a fixed frame of reference) it was clear that the camera was moving too, but when he left, it did not look to be moving (just the cart). He stated that this was because “all frames of reference moving at a constant velocity with respect to one another are equivalent.” Dr. Hume did add that when the motion is simpler, we do not use the Earth as a frame of reference; rather, we use the simpler object that is moving and put ourselves in that frame of reference instead. Also, when the frame of reference for the cart is accelerated, it does not represent an “inertial frame,” so this is why the ball does not move in a straight path down. He stated that we automatically make up a “fictitious force” that is acting on the ball.

The next experiment focused on how to compare the velocity of an object in one frame of reference to its velocity in another frame of reference. When they were on a moving cart and they pushed a puck across the table and the camera was fixed to the moving cart, then the puck seemed to be moving at the same velocity in both directions. On the other hand, when the camera was fixed to the Earth, then the puck seemed to be moving slower when it was passed in the direction opposite the moving cart (i.e., if the cart was moving to the right and the puck was moved across the table to the left) than when it was passed in the same direction. He explained this by saying that the velocity is the same for the puck object in both directions when the frame of reference is the moving cart; however, when the frame of reference is the Earth, then the velocity changes to V+U and V-U. They also show what happens when the table turns in a circular motion. They state that this creates an “unbalanced force,” which makes it seem like the ball is also moving in a circular pattern across the table, when in fact it is still moving straight. They point out that a rotating frame of reference is also not an inertial frame.

Lastly, Dr. Hume explained how the Earth is rotating on its axis, and because this is very sow, we do not observe these fictitious forces. He also demonstrated how we can tell that the Earth is actually moving and not the stars. He explained this using a pendulum.

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