Homework Help: Static/Kinetic Friction Problem

1. Sep 18, 2008

Spartan Erik

1. The problem statement, all variables and given/known data
"A block that weighs 50N is initially at rest on a rough horizontal surface. A 20N horizontal force is then applied to the block. If the coefficients of friction (static = 0.6, kinetic = 0.3), what is the magnitude of the frictional force on the block?"

2. Relevant equations
Frictional force = coefficient of kinetic friction x normal force
Max static frictional force = coefficient of static friction x normal force

3. The attempt at a solution
Once again I'm worried this problem is misleadingly easy:

Max static frictional force = coefficient of static friction x normal force
Max static frictional force = 0.6 x (50N - 20N)
Max static frictional force = 0.6 x 30N
Max static frictional force = 18N

Is this method correct?

2. Sep 18, 2008

LowlyPion

No.

You need to draw a force diagram on the block.

What are the normal forces?

What is the horizontal forces.

If the block weighs 50N what is the maximum static frictional force it can resist?

3. Sep 18, 2008

Spartan Erik

I think I just realized my mistake..

The normal force of the block is the given 50N, and the horizontal force applied is 20N, so
Max static frictional force = 0.6 x 50N
Max static frictional force = 30N

The max static frictional force exceeds the force applied horizontally (30N > 20N)

4. Sep 18, 2008

LowlyPion

I think you are on to something.

Good Luck.

5. Sep 18, 2008

Spartan Erik

So if the max static frictional force is 30N, and you are only applying 20N of force to it, won't that make the magnitude of frictional force 20N (since the block won't move anyway)?

6. Sep 18, 2008

LowlyPion

Yes indeed.

7. Sep 18, 2008

Spartan Erik

Thanks I appreciate it!