F-1 A box weighing 20 N rests on a horizontal surface. The coefficient of static friction is .4; the coefficient of sliding friction is .2.
a) How large is the friction force exerted on the box as it rests on the table?
b) How great will the friction force be a horizontal force of 5 N is exerted on the box?
c) What is the minimum force that will start the box in motion?
d) What is the minimum force necessary to keep the box in motion once it has started?
e) If the applied force is 10 N, how great is the friction force?

a)Analysis of forces in the y-direction shows that the forces acting on the object are w = mg acting downward and the normal force,, acting upward. Because t he object is resting on the table, we can conclude that= 20 N. There are no forces applied in the x – direction, so that there is no frictional force to oppose any motion caused by an applied force in this question.

b) Analysis of forces in the y-direction shows that the forces acting on the object are w = mg acting downward and the normal force, FN, acting upward. This has not changed from part a) above. Because the frictional force available is and is greater than the 5 N applied, only 5N of the 8 N frictional force comes into play here.  (If this were not the case, a 5N force applied to the right would be opposed by an 8 N force pointing left.  A net force of 3 N left would cause the object to accelerate left when we tugged right.
c) Clearly, some force > 8N will set up a situation where > 0 and the block will move.

d) Once the block has started to move, the co-efficient of friction drops from .4 to .2. Thus, the force necessary to start the block moving (8N) is reduced to 4N.

e) Once the maximum frictional force is reached, it remains the same regardless of how great the applied force might be.

This page edited 06/23/07