# Doppler effect problem

1. Dec 5, 2013

### nickb145

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

A parachutist leaps from a hovering helicopter, after 4.0s of freefall shouts back at 425Hz. What frequeny is heard at the helicopter?

2. Relevant equations

f'=f/(1+Vs/V)

Vsource=?
f'=?
f=425Hz
V=343m/s
3. The attempt at a solution

I'm stuck on finding the speed for the jumper. I'm sure it's simple but i just can't think of it for somereason.

Last edited: Dec 6, 2013
2. Dec 5, 2013

### ehild

Read the problem carefully. It is free-fall, the parachute is not yet opened . What is the speed of a falling body after 4 s?

ehild

3. Dec 5, 2013

### nickb145

thinking

Last edited: Dec 5, 2013
4. Dec 5, 2013

### nickb145

vy=Vyi-gt right?

initial speed is 0 so its just -9.8*4=-39.2

5. Dec 6, 2013

### ehild

Yes, it is the velocity if you consider upward positive.
The parachuter shouts back. How is his sound heard on the helicopter? (Hm. The helicopter is very noisy --think it is a super helicopter making only low noise so the parachuter's sound can be heard )

ehild

6. Dec 6, 2013

### nickb145

But it is strange that i get a higher frequency of what it should be. The answer is 381.4 and i keep getting 479. I'm getting my signs wrong somewhere...

I used f'=f/(1+Vs/V) for a source moving away from reciever.

7. Dec 6, 2013

### ehild

You need to calculate with the speed of moving away. The speed is positive, magnitude of velocity. Substitute Vs=39.2 m/s.

ehild