# Net Force

1. Oct 21, 2013

### 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

2. Oct 21, 2013

### 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?

3. Oct 21, 2013

### TeeNaa

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

4. Oct 21, 2013

### rude man

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.

Last edited: Oct 22, 2013