Problem 6

A 8 kg block is at rest on a horizontal floor. If you push horizontally on the 8 kg block with a force of 20 N, it just starts to move.

(a) What is the coefficient of static friction?

(b) A 10.0 kg block is stacked on top of the 8 kg block. What is the magnitude F of the force, acting horizontally on the 8 kg block as before, that is required to make the two blocks start to move?

Solution:

The magnitude of horizontal force should be equal to the magnitude of the maximal static friction force, which is equal to the product of the coefficient of static friction and the normal force (gravitation force in the present problem).

(a)  The gravitation force is mg=8*9.8 = 78.4 N. Then the coefficient of static friction is

(b) Now we know the coefficient of static friction and we know the normal force: 18*9.8 = 176.4 N. Then we can find the magnitude of force F: