# Homework Help: Force of friction

1. Oct 15, 2004

### kimikims

Any help?

A(n) 990 kg car is parked on a 4 degrees incline.
The acceleration of gravity is 9.8 m/s^2.
Find the force of friction keeping the car
from sliding down the incline. Answer in
units of N.

2. Oct 15, 2004

### quasar987

Newton's second law of motion states that the force on an object is equal to its mass times its acceleration.

$$F=ma$$

This means that if there is no (net!) force on an object (F=0), it has no acceleration either (a=0). This means that it's velocity doesn't change. This means that if the object is moving at a certain speed, it keeps doing so. And if the object is standing still, it keeps standing still.

With this in mind, if there is a force on our car (namely the force of gravity) and we want our car to stay still on the incline, this means we want the total force on it in the direction parallel to the incline to be 0.

You go from there.

(Hint: chose your system of coordinate parallel to the incline.)

P.S. It's A car

Last edited: Oct 15, 2004