# Trying to find static friction

1. Oct 6, 2008

### crhscoog

1. The problem statement, all variables and given/known data

3. The attempt at a solution

i've tried

u(mg+fcos(theta)
u(mg+fsin(theta)
u(mg+fsin(theta)

they have all been wrong. before i guess using: u(mg-cos(theta) and risking missing more points, can you tell me if it's right and if not, what do i do to solve this problem.

Last edited: Oct 6, 2008
2. Oct 6, 2008

### Rake-MC

Forces in y direction = gravity, normal force and y component of applied force.
To calculate static friction you can ignore the x plane (in this example).

The magnitude of normal force is of course the y component of applied force + gravity.

y componenet of applied force = 107sin(theta).

Ffriction = u[mg + 107sin(theta)]

3. Oct 6, 2008

### crhscoog

yeah i forgot to put the f in the equations that i used. i used it and still got it wrong... ahh

4. Oct 6, 2008

### Rake-MC

Could you please show exactly what you did..?

0.67[(21.5)(9.81) + (107)(sin(35))] ?

5. Oct 6, 2008

### crhscoog

that is what i used but i found the answer. its 88.707 N

apparently u just do fcos(theta)

107cos34... i dont get it but im read over some notes on what exactly force of static friction is...

6. Oct 6, 2008

### Rake-MC

Interesting, if the question is posing that the gravitational force down is cancelling out its equal and opposite component in the positive y direction, that would mean that if the applied force was horizontal, then there would be no static friction.