Solving a Car Coasting Downhill Problem: Average Retarding Force

[TeeNaa]

A 1200 kg car start from rest and coast down a uniform grade, at the bottom of which ti has a speed of 20 m/s. If the car has traveled 800 m along the grade and has descended 35 m, what is the average retarding force(friction) encountered by the car? How would I solve this problem? I know Fx = ma = Ff - mgsintheta Fy = 0 = N - mgcostheta Ff = muN I don't have mu though. Thanks

 

[rude man]

I would consider energy balance.

What is the kinetic energy at the bottom?
What would be the kinetic energy in the absence of friction?
What happened to the difference?

 

[TeeNaa]

Is there any other way to solve this problem beside KE? I haven't learn that yet. Thanks

 

[rude man]

Is there any other way to solve this problem beside KE? I haven't learn that yet. Thanks

Sum of forces along the ramp = ma
One force is gravity.
Second force is friction.
(Third force is force pushing up against the car. Why is this irrelevant?)

Get a from ramp length s and final velocity v.
Hint: dv/ds = dv/dt dt/ds = a/v.