How to Find the Force of Friction on an Inclined Plane?

[simmonj7]

Homework Statement

A 12-g coin slides upward on a surface that is inclined at an angle of 12° above the horizontal. The coefficient of kinetic friction between the coin and the surface is 0.23; the coefficient of static friction is 0.35. Find the magnitude and direction of the force of friction after it comes to rest.

The Attempt at a Solution

So I thought the answer would be mg cos $$\Theta$$ * the coefficient of static friction however that isn't getting me the correct answer.

Help please.

Thanks. :)

 

[thrill3rnit3]

Have you drawn your free body diagram?

 

[simmonj7]

Yes.

 

[llello]

Hint: If the coin is moving, should you be using the static coefficient or the kinetic coefficient.

 

[simmonj7]

The coin isn't moving in my problem. I am trying to find the magnitude and direction of the force of friction after it comes to rest. So I should be using the static coefficient like I did.

 

[llello]

Ah. The force of static friction varies. The coefficient of static friction is used when you want the MAXIMUM value of static friction, ie just before the object starts to move.

As an example: If a block on an incline had a normal force of say 10N and the coefficient of static friction was say 0.5, then max static friction force would be 5N UP the ramp. Now if the angle is small enough to where the force of gravity DOWN the ramp is only say 1N, then just blindly using the static friction would say that the block would be pushed UP the ramp by friction, wouldn't it?

 

[simmonj7]

But mg cos(theta) is wrong as well...
Assuming I understood what you said correctly.

 

[llello]

I'm assuming its not something silly, like using grams instead of kilograms or having your calculator in radian mode as opposed to degree mode, correct?