# Homework Help: Frictional force

1. Sep 25, 2011

### lalahelp

1. The problem statement, all variables and given/known data
A crate of potatoes of mass 13.0 kg is on a ramp with angle of incline 40° to the horizontal. The coefficients of friction are μs = 0.78 and μk = 0.43. Find the frictional force (magnitude) on the crate if the crate is at rest.

2. Relevant equations
F=ma
Ff=μmgcosα

3. The attempt at a solution

Ff=.78*13*9.81*cos40
=76.2 N

Why is my answer wrong?

2. Sep 25, 2011

### Staff: Mentor

You assumed that the static friction would be equal to its maximum possible value (μN). Instead, solve for the value of static friction considering that the crate is in equilibrium.

3. Sep 25, 2011

### lalahelp

What equation would I use to do that?

4. Sep 25, 2011

### Staff: Mentor

The net force is zero.

5. Sep 25, 2011

### lalahelp

I dont understand it still

6. Sep 25, 2011

### Staff: Mentor

What forces act on the crate?

7. Sep 25, 2011

### lalahelp

the mass and friction

8. Sep 25, 2011

### Staff: Mentor

The weight, the friction, and the normal force. What direction does each act?

9. Sep 25, 2011

### lalahelp

the weights acts straight down, friction to the right, and normal force up

10. Sep 25, 2011

### Staff: Mentor

Consider force components parallel to the incline and apply ΣF = 0.

11. Sep 25, 2011

### lalahelp

Ok thank you I got it !!