# Homework Help: Static friction problem

1. Sep 23, 2011

A cart (with wheels) sits on a horizontal table. The front of the cart is a vertical wall, with mass M = 0.313 kg held against the wall. You want to push the cart forward with an acceleration of magnitude a so the mass M can be released and it will not slide down the wall. Assume the coefficient of static friction between the block and the wall of the cart is μs = 0.407. Find the minimum value of a for which the mass does not slide.

Forget equations for the moment. I just am having trouble reading this problem in the first place. I can't make sense of the situation physically - been trying for a while now. Anyone know of a picture of what's going on here? the wording seems a bit ambiguous. Does 'held against the wall' mean someone is pressing it into the wall of the cart? Or is it just sticking on the wall? My free body diagram just has gravity on the mass pulling down balanced by static friction between the mass and the cart's wall pointing up (before the cart is pushed). I am tempted to say there is a normal force but how can there be if nothing is pressing the mass into the cart? then again if nothing is pressing the mass into the cart how is it sticking to the wall? Static friction alone pointing up makes it stick to the wall such that there is equilibrium on the horizontal axis? Do I need a free body diagram for the cart, for which data is sparse? Or just for the mass? Thanks

2. Sep 23, 2011

### omoplata

I guess they expect you to only consider the case when the cart is moving forward with a constant acceleration.

A free body diagram for the mass would be enough in that case.

3. Sep 23, 2011