The normal force N is always perpendicular to the surface. The normal force is not always ‘up’. Also, frictional force works opposite in direction to the motion of the block.

For the coefficient of static friction, we need the force needed to get the block moving. In this case, 80 N.

Ff = μsN

N is equal to the weight of the block, so N = 200 N. Substitute-

80 N = μs

·200 N

or

μs = 0.4

For the coefficient of kinetic friction, the force needed to maintain a constant velocity was 40 N. Use the formula:

Ff = μkN

40 N = μk

·200 N

μk = 0.2

The two coefficients of friction for this system are μs = 0.4 and μk = 0.2.e