# Doppler effect/ distance

1. May 25, 2010

### jwbehm

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

A stuntwoman is preparing to take a punch, crash through a "candy glass" window, and fall a long distance. The script calls for her to emit a piercing scream just before she hits the "ground." In reality, she will land on a waiting airbag. Lights! Camera! Action! The primary camera crew, filming from her starting height, hears her last-instant scream at a frequency 3.77 kHz. Her scream has a frequency of 4.05 kHz when she is at rest. How far did she fall? Report this as a positive distance.

2. Relevant equations

Fl=Fs(v-Vl/v+Vs)

3. The attempt at a solution

I have easily found the speed of the girl falling to be 25.5m/s but I dont even know where to start as far as finding her distance. I have considered finding the wavelength but I dont know where to go from there. Do I need to you an echo equation for this? I've been looking at it for a solid 2 hours convinced I could figure it out but I'm about to the point where I need a little guidance. I dont need an answer I just need an idea of where to go from here. This isnt due for another 3 weeks but its driving me insane.

THANKS!

2. May 26, 2010

### collinsmark

Good 'ol kinematics might help. You know g = 9.8 m/s2. Assuming her initial position and velocity were both approximately zero, how far would she have to fall to reach the target velocity, when accelerating at g?

[If you don't remember the single, direct equation for this, you can use a couple other kinematics equations. Use one equation to calculate the time it takes to reach the final velocity, then another equation to calculate the distance.]

3. May 26, 2010

### jwbehm

I was afraid it would be something like that. . . thanks. I should be able to get it from here but if not ill be back within the next 20 minutes haha

4. May 26, 2010

### jwbehm

Wow, that was ridiculously easy. 33.15m. Thanks for the help!