# Q & A: moving inside a plane

Q:
Hello I am a 14 year old girl learning about physics in science class. My teacher brought up a question that got me interested. he said that if two people who had equal speed and equal strength both stood in the middle of a plane? (with a bathroom in the front and back side) going forward who would get to the bathroom the fastest My teacher said that they would get there at the same time because they were going at the same speed and were racing against the floor of the plane and not against the sky. But That got me thinking if they were running even though it would be almost immeasurable there would still be A millisecond of time where the person running to the back of the plane would have both feet off the ground so in that time the plane moving at over 5000 miles per hour would move under them making them go faster than the person moving to the front of the plane causing them to go in reverse. So I was wondering who would be correct?
- Meg McLean (age 14)
Chicogo Illinois USA
A:

Nice question. It gets at a very important principle: relativity. There aren't any basic physical effects that make one state of motion "rest" and others "moving". The only thing that matters is relative motions. So the question becomes, what physical thing would affect your motion in the plane differently for forward and backward motion? Other than the plane itself, the other thing that might influence your motion is the air inside the plane. But unless a window is open (bad idea!) that air is moving along with the rest of the plane. So there's nothing to make moving forward or backward in the plane different.

Mike W.

(published on 03/04/2016)

## Follow-Up #1: another look at old relativity

Q:
You said "[while the plane is] going forward who would get to the bathroom the fastest?" How do you know the plane is "going"? Because you hear the engines? But you can hear the engines also when the plane is on tarmac standing still. Because the plane is in the air? A blimp is also in the air but standing still. Because the air is passing by? The air (wind) also passes by the plane while on the tarmac. Because you see the earth below is passing by? Then it is the earth that is moving. But because earth is so much larger earth seems stationary and the plane moving; if they were the same size, e.g., if instead of earth you saw another plane in the opposite direction, then you would think it is the other plane that is moving. In fact many a time at the train station you think that your train is moving, only to find out that the other train was moving. In other (Einstein's) words "there is no way to detect a uniform rectilinear motion." The only time you detect a motion is when you change your speed; but a constant speed (of plane) to the guy on earth is the same as standing still to you (in the plane); neither of you can tell the other one is moving. This is known as Galilean Relativity (see Wiki) stated by Galileo in 1632.
- Mehran (age 65)
Arlington Heights, IL
A:

Mehran- Thanks for this further explanation.

Mike W.

(published on 03/07/2016)