Question

2. A 3.0 kg block rests on a level surface. The coefficient of static friction is µs = 0.50, and the coefficient of kinetic friction is µk = 0.20. A horizontal force F is applied to the block. As F is increased, the block begins moving. Describe how the force of friction varies as F increases from the moment the block is at rest to when it begins moving. Indicate how you could determine the force of friction at each value of F—before the block starts moving, at the point it starts moving, and after it is moving. Show your work.

Answer 1

Friction force is equal to the applied force F until the block starts moving. Hence, it will increase until the maximum value of 3.0*10*0.50=150 N. 

While it's moving, the friction will be 3.0*10*0.2 = 60 N, constant. The rest of the applied force will accelerate the block.