Question

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 = 25.0 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 = 18.0 m/s.
(a) With what speed did car A hit car B if the coefficient of kinetic friction was 0.60 (dry road surface)?
____ m/s

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

Answer 1

F=mgsinα –F(fr)=

= mgsinα - μmgcosα

a=F/m =g(sinα – μcosα)

=9.8(sin14-0.57•cos14)= -3.04 m/s²

v²=v₀²+2ad

v=sqrt{ v₀²+2ad} =

=sqrt{18²+2• (-3.04) •25}=13.1 m/