6.1    Friction

Friction plays an important role in our daily life. Sometimes it is a nuisance if, for example, you want to pull or push a heavy box across the floor. However, think what will happen if there were no friction. You will not be able to use the pen to write notes. The brakes in your car wouldn't work, etc. We look at friction in a simple case, i.e. between two dry surfaces. A box (50 kg) rests on a horizontal floor (see the physlet below). The two vertical forces, the gravitational force and the normal force balance each other. The block is pulled to the right by a constant external force, , but it does not move! According to Newton's 2nd Law there must be an equal and opposite horizontal force present to keep the block from moving. This is the force of static friction, .

Script by Dr. Mohamed S. Kariapper using Physlets from

Move the slider to increase the applied force, you will observe that the staic friction force also increases by the same amount and the block remains at rest. When the applied force exceeds a certain magnitude (195 N in this example), the block starts to accelerate! The static friction force has reached its maximum possible value and couldn't match the applied force anymore. The block starts to move and the static friction force is replaced by a smaller friction force called the force of kinetic friction , which remains constant as the applied force is increased any further.