# Homework Help: Speed of a helicopter determined by sonar

1. Sep 20, 2007

### raman911

1. The problem statement, all variables and given/known data

A helicopter flies directly toward a vertical cliff. when the helicopter is 700 m from the cliff face it transmits a sonar signal. it recives the reflected signal 3.4 s later. if the signal travels at a wind speed 350m/s, what is the speed of the helicopter.

3. The attempt at a solution

250*3.4= 1190 m
plz help me what i need to do next? i spend 1 hour on it i did n't get answer.

2. Sep 20, 2007

### mgb_phys

How far did the signal travel in 3.4s?
If the helicopter was stationary it would have to have travelled 2*700=1400m
How much less did it travel?
The helicopter must have travelled that far in 3.4s

3. Sep 20, 2007

### raman911

what u mean by 2*700?

4. Sep 20, 2007

### mgb_phys

The distance the sound would have to travel 700m to the cliff and 700m back to the helicopter = 1400m if the helicopter was stationary.
In fact it only goes 3.4*350 = 1190m so the helicopter must have moved how much closer?