# Homework Help: Forces problem

1. Feb 1, 2012

### InsTanCeZ

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

You testify as an "expert witness" in a case involving an accident in which car A slid into the rear of car B, which was stopped at a red light along a road headed down a hill. You find that the slope of the hill is θ = 12.0°, that the cars were separated by distance d = 23.5 m when the driver of car A put the car into a slide (it lacked any automatic anti-brake-lock system), and that the speed of car A at the onset of braking was v0 = 17.5 m/s.

(a) With what speed did car A hit car B if the coefficient of kinetic friction was 0.60 (dry road surface)?

(b) What was the speed if the coefficient of kinetic friction was 0.10 (road surface covered with wet leaves)?

2. Relevant equations

∑F = ma = μmg

v² = v₀² + 2aΔx

3. The attempt at a solution

∑F = ma = μmg
a = μg
= 0.600(-9.80m/s²)
= -5.88m/s²

v² = v₀² + 2aΔx
= (17.5m/s)² + 2(-5.88m/s²)(23.5m)
= 5.476m/s (part a)

haven't tried part b

How is this wrong? My online homework said 5.467 wasn't the answer for part a.

2. Feb 1, 2012

### SammyS

Staff Emeritus
It has to to do with you neglecting the slope of the road.

3. Feb 1, 2012

### InsTanCeZ

Thanks, I figured it out. :)