A108 kg clock initially at rest on a horizontal floor requires a 639 n horizontal force to set it in motion. after the clock is in motion, a horizontal force of 521 n keeps it moving with a constant velocity.

Explanation:

It is given that,

Mass of the clock, m = 108 kg

Force acting on it when it is in motion,

After the clock is in motion, a horizontal force of 521 N keeps it moving with a constant velocity, F' = 521 N

It is assumed to find the coefficient of between the clock and the floor. The force of friction is given by :

So, the coefficient of static friction between the clock and the floor is 0.6. Hence, this is the required solution.

a and b sometimes have a positive value

i think the answer is a, hope this : )