Question

A stone fall from a railroad overpass which is 36m high into the path of a train which is

approaching the overpass with uniform speed. If the stone falls when the train is 50m away from

the overpass and the stone hits the ground just as the train arrives at that spot, how fast is the

train (in kilometer per hour) moving?

Answer 1

Time of falling:

$$t=\sqrt{2h/g}.$$

Speed of the train:

$$v=d/t=d/\sqrt{2h/g}=18.45\text{ m/s},$$

or 66.4 km/h.