ABSTRACT
Returning to our ideal train moving to the right, now assume there is
gravity. A ball thrown straight up will, according to an observer on the
train, eventually turn around and fall back down, moving along a straight
line, as shown in Figure 13.1(a). Since the acceleration due to gravity at the
surface of the earth is taken to be constant, the exact motion is described
by a quadratic equation in t. From the ground, the same effect is seen, but
combined with a constant motion (linear in t) to the right. The motion
therefore takes place along a parabola; see Figure 13.2.
