Please Help With Physics!?

Question

I would really appreciate any help with this problem that I am stuck on. Thanks :)

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 20 degrees?

c) if the block is at moving downward and the surface is inclined at 47 degrees?

Answer 1

μs means static frictional force, so it only applies when the block is not moving *and a force is being applied*. The frictional force F = μN where N is the normal force.

a.) The frictional force equals the force due to gravity = 1 kg * 9.8 m/s^2 = 9.8 N

b.) Gravity is still acting on the block - so the frictional force is still 9.8 N.

μk is used when the block is moving. Frictional force F = μN where N is the normal force acting on the block.

c.) The block is acted on by gravity.
N = 1 kg * 9.8 m/s^2 * sin 47 (since it's going down the ramp)
N = 7.167 N
F = μN = .42 * 7.167 = 3.01 N

Answer 2

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