# Homework Help: Projectile motion

1. Jun 10, 2010

### mandy9008

1. The problem statement, all variables and given/known data
In Bosnia, the ultimate test of a young man's courage used to be to jump off a 400 year old bridge (now destroyed) into the River Neretva, 23 m below the bridge. (a) How long did the jump last? (b) How fast was the jumper traveling upon impact with the river?
(c) If the speed of sound in air is 341 m/s, how long after the jumper took off did a spectator on the bridge hear the splash?

2. Relevant equations
a. and b. change in Y=Voy t - .5 gt2

3. The attempt at a solution
a. v=21.23 m/s
b. t=2.167s
c. Vy=Voy - gt
341 m/s =21.23 m/s - 9.8 m/s2 t
t=-32.63s
I know that time cannot be negative, but I have tried the inverse of this number also, and I am still getting it wrong. Am I even using the correct equation?

2. Jun 10, 2010

### Staff: Mentor

OK.
No, you are not. The sound travels through the air at a constant speed. (The sound is not a projectile.)

3. Jun 10, 2010

### mandy9008

Could you point me in the right direction as to the equation I am supposed to use?

4. Jun 10, 2010

### Staff: Mentor

What equation relates speed, distance, and time for something moving at constant speed? That's what you'll need to find the time it takes for the sound of the splash to travel up to the bridge.

(To find the total time, don't forget to add in the time it takes for the man to fall, which you've already calculated.)