Friction and force

1. Sep 25, 2007

johnkd6

I know I got at least two out of the three incorrect and cannot seem to understand why.
The question states:
The coefficients of friction between a block of mass 1 kg and a surface are μs=0.64 and μk=0.42. Assume the only other force acting on the block is that due to gravity, what is the magnitude of the frictional force on the block

A)if the block is at rest and the surface is horizontal?
B)if the block is at rest and the surface is inclined at 22o?
C)if the block is at moving downward and the surface is inclined at 49o?

For a) I said the magnitude is 0 because there is no force acting on it other than gravity and for b) I put .64-9.8sin(22) and c).42-9.8sin49. I got two negative numbers for b) and c) and probably think those are the ones I screwed up on but cannot understand how if the only other force acting on it is gravity and not a normal force. Any help would be greatly appreciated.

2. Sep 25, 2007

Staff: Mentor

The maximum friction force is the normal force times the friction coefficient. The teacher has already given you half of the answer for each problem. For the first, if the block is stationary, you don't even need to use the friction coefficient except maybe to check to make sure you have enough to keep it stationary. The force is g*sin(22). When inclined at 49 degrees, it is uk*g*cos(49).