Force necessary to create enough friction.

1. Jul 26, 2009

Elmnt

1. The problem statement, all variables and given/known data
The two blocks (m = 18 kg and M = 101 kg) in the figure below are not attached to each other. The coefficient of static friction between the blocks is µs = 0.59, but the surface beneath the larger block is frictionless. What is the minimum magnitude of the horizontal force required to keep the smaller block from slipping down the larger block?

http://www.webassign.net/hrw/6-36.gif

2. Relevant equations

3. The attempt at a solution

My thought process involved me finding the force of friction that would be large enough to cancel out the the force due to gravity and hold the block in place. The friction force would have to have a magnitude of 176.4N (18kg*9.8m/s$$^{2}$$) since the static friction coefficient is .59 I just solved: friction force = static coefficient * Normal force , for the normal force, which = 298.98 N. Now that I know what the normal force needs to be, I am confused on how to solve for the force necessary to produce that normal force.

2. Jul 26, 2009

rock.freak667

I think the minimum force would be the normal force.

3. Jul 27, 2009

Elmnt

I was thinking along those lines, but it is wrong. I think it has something to do with the frictionless plane.

4. Jul 27, 2009

Staff: Mentor

So far, so good. Hint: What's the acceleration of the blocks?