I stand at the window of a railway carriage which is travelling uniformly, and drop a stone on the embankment, without throwing it. Then, disregarding the influence of the air resistance , I see the stone descend in a straight line. A pedestrian who observes the misdeed from the footpath notices that the stone falls to earth in a parabolic curve. In reality there is only one path that the stone follows. The path of the stone begins when Mr.
All movement is relative to non movement.
The movement of the stone takes place in two directions, vertical and horizontal, the path is a combination of the two directions. Only if the carriage is stationary or if the stone is dropped onto the floor of the moving carriage is it possible for the path of the stone to follow a straight line. In the last mentioned case the co-ordinate system will be the carriage which is then regarded as being stationary. So much for Mr. Einstein's 'serious reflection'.