- #1

GregAshmore

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**Equivalence of inertial & gravitational mass--I need a sanity check.**

Einstein, in his 1916 book Relativity, illustrates the equivalence of gravitational and inertial mass using the example of a braking train.

The example begins with the train at rest (of course) and the scenery moving to the rear at a constant speed. The passenger feels no force.

As the brakes are applied, the passenger says, "I feel a force. I am at rest in a gravitational field. The velocity of my surroundings is reducing at a constant rate as a result of the application of that field."

Well and good.

Now consider the case of the derailed train suspended over the side of a bridge. The passenger feels a force; both he and the surroundings are at rest. The passenger says, "I and my surroundings are at rest in a gravitational field."

The train comes loose and falls. The passenger feels no force; the surroundings accelerate upward. What does the passenger say?

Seems he would have to say, "I and my surroundings are no longer in a gravitational field. I am at rest, with no applied force. My surroundings must be under a force equal to their weight, for they are accelerating at a constant rate."

Is there a better relativistic answer?