# Time elapsed between seeing a crash and hearing the crash

1. Jan 24, 2007

### danshodan

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

A race car fan is sitting in the first row at the beginning of a 1.5 km front stretch of a racetrack. A race car driver coming around the final turn is approaching the fan’s position at 99.5 m/s. The driver sees a car wreck ahead and slams on the brakes causing the car to slow at 4.7 m/s2. Unfortunately, the driver crashes into the wreck at 27.3 m/s. If the speed of sound in the air is 343 m/s, how much time elapses from the moment the fan sees the crash and when the fan hears the crash?

2. Relevant equations
v=fλ
d=vt

3. The attempt at a solution
using the info:
v1= 99.5 m/sec v2=27.3 m/sec
i tried to use d=vt to find the t(ime) but i couldnt figure out d.
Eventually i got that t=1.53 by using v2=vi+at and d=vit+0.5at^2, but the correct answer should be 2.8s??? I dont know how to get there

2. Jan 24, 2007

### mukundpa

Find the distance at which the crash took place from the fan. Time taken by the sound to cover this distance will be the difference( neglecting the time taken by the light comparatively.