Reference Frames

Reference Frame 1

Reference Frame 2


The above simulations present a classic physics thought experiment, sometimes called a Gedanken experiment (from the German).  An object is moving to the right in a straight line with constant speed.  The spaceship is accelerating upward. The animation on the left shows what a stationary observer would see as the rocket accelerates upward. The animation on the right shows what an observer in the spaceship would see.


How would someone in the spaceship describe the motion of the ball? 

How do you write the laws of the physics so that both frames are correct?   


Instructor Resources

Answer a: The motion of objects in an accelerating spaceship appears to be due to a force. Therefore the ball appears to be accelerating downward with a constant acceleration.
Answer b: In order to get observers in the two reference frames to agree, we must invent a force that acts on objects in the accelerating frame. Depending on the acceleration of the spaceship, we may call this fictitious force gravity. In fact, one cannot tell whether one is in a spaceship accelerating at 9.8 m/s2 far from the earth, or in a non-accelerating spaceship on the surface of the Earth (ignoring rotation of the Earth and other small effects). In fact, this equivalence between the two frames was Einsteinís starting point for general relativity.
Script Author: Mario Belloni