Question

A car travelling normally towards a cliff at a speed

of 50 m s–1, sounds its horn when it is 0.7 km

away. Taking the speed of sound to be 300 m s–1 in

air, find the time taken for the driver to hear its echo.

A. 0.04 s B. 4.0 s C. 4.7 s D. 28.0 s

Answer 1

Let x be the distance the car travels between blowing its horn and hearing the echo. So the sound travels 1000 meters from the car to the hill, and (1000-x) meters on the return trip. Thus the sound travels 2000+x meters in five seconds.

Now you can solve for x:

(2000-x)/5 = 340 → x = 300

So the car covers 300 meters in five seconds - it’s speed is thus 60 meters per second. That’s *fast*, by the way. :-)