Determining the Frequency Heard by a Helicopter from a Parachutist's Shout

[nickb145]

Homework Statement

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

Homework Equations

f'=f/(1+Vs/V)Vsource=?
f'=?
f=425Hz
V=343m/s

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.

 

[ehild]

Homework Statement

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

Homework Equations

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


Vsource=?
f'=?
f=425Hz
V=343m/s

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.

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

 

[nickb145]

thinking

 

[nickb145]

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

vy=Vyi-gt right?

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

 

[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

 

[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.

 

[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