Homework Help: Simple force question that I cant get!

1. Jun 12, 2006

mikejones2000

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. Jun 12, 2006

Staff: Mentor

I think you took them too literally when they said "Assume the only other force acting on the block is that due to gravity". Of course there's a normal force, otherwise the frictional force will always be zero. They meant no other applied forces, like someone pushing or pulling the block.