What is acceleration of gravity on board a train moving at speed c/2?
I don’t think other answerers are realizing that “c/2” means half the speed of light, and therefore you have to take relativity into account. The acceleration of gravity on board the train would be about 13.1 m/s². Here’s why: Suppose you were on board the train and dropped a ball from a height of 4.9 meters. (it’s a tall train). Let’s see what that would look like from your point of view and from the point of view of somebody standing outside the train. The outside observer would see the ball seem to follow a parabolic arc downward, and he would measure it as taking 1.0 second to hit the ground. This is consistent with his observation that the value of gravity is 9.8m/sec² But, due to the train’s great speed, the clocks of the people on the train are running slowly–only about 87% as fast as the outside observer’s clock (this is a prediction of the theory of relativity). So, when the person INSIDE the train measures the time it takes for the ball to fall, HIS stopwatch reaches only 0.
