FRICTION FORCE

Why is there friction force?

Friction force is always opposing the motion

Example 1

Friction: static and kinetic. A 10-kg box rests on a horizontal floor. The coefficient of static friction is 0.4 and the coefficient of kinetic friction is 0.3 Determine the force of friction acting on the box if a horizontal applied force is exerted on it of magnitude:

(a) 0, (b) 10 N, (c) 20 N, (d) 38 N, and (e) 40 N.

If coefficient of friction is 0.4 and m=2kg what minimum force F will keep the box from falling: (a) 100 N; (b) 80 N; (c) 50 N; (d) 20 N; (e) 8 N?

A 20-kg box is pulled along a horizontal surface by a force of 100 N applied at a 30° angle above horizontal. We assume a coefficient of kinetic friction of 0.3. Calculate the acceleration.

When do we have bigger friction force?

Two boxes are connected by a cord running over a pulley. The coefficient of kinetic friction between box A and the table is 0.2. We want to find the acceleration, a, of the system, which will have the same magnitude for both boxes.

Two crates, of mass 65 kg and 125 kg, are in contact and at rest on a horizontal surface. A 650-N force is exerted on the 65-kg crate. If the coefficient of kinetic friction is 0.18, calculate the acceleration of the system

In figure the coefficient of static friction between mass and the table is 0.40, whereas the coefficient of kinetic friction is 0.20.

(a) What minimum value of will keep the system from starting to move?

(b) What value(s) of will keep the system moving at constant speed?

Inclined plane with friction