A 7.0-kg block is at rest on a horizontal floor. If you push horizontally on the 7.0-kg block with a force of 21.0 N, it just starts to move.
(a) What is the coefficient of static friction?
(b) A 12.0-kg block is stacked on top of the 7.0-kg block. What is the magnitude F of the force, acting horizontally on the 7.0-kg block as before, that is required to make the two blocks start to move?