Physics

Problem

A train is moving towards a high vertical cliff at

20\,\text{m s}^{-1}

. The driver sounds the whistle and hears its echo after

4.0\,\text{s}

. If the speed of sound in air is

340\,\text{m s}^{-1}

, what was the distance between the train and the cliff at the instant the whistle was blown? a)

680\,\text{m}

720\,\text{m}

640\,\text{m}

740\,\text{m}