Question

When you push a piece of chalk across a chalkboard, it sometimes screeches because it rapidly alternates between slipping and sticking to the board. Describe this process in more detail, in particular explaining how it is related to the fact that kinetic friction is less than static friction. (The same slip-grab process occurs when tires screech on pavement.)

Short answer

Kinetic friction acts when there is a relative motion between the surfaces. Static friction works if there is tendency of motion but no relative motion between the surfaces.

Kinetic friction acts when we are writing on the board while static friction works if we stop writing.

Step-by-step solution

Definition of Kinetic friction

Kinetic friction is the friction that occurs when two or more things are moving in opposite directions.

Determining kinetic friction is less than static friction

Chemical bonding occurs between both the surface and the chalk when you push a piece of chalk throughout a chalkboard.

This bonding is responsible for static friction as well as kinetic friction. The friction force depends upon the nature of surface. Normally the coefficient of kinetic friction is less than static friction. The friction force is determined by the type of surface. Kinetic friction normally has a lower coefficient than static friction.

Therefore, value of kinetic friction is less the value of static friction.

While writing on the chalkboard there is a relative motion between the surfaces so kinetic friction works and lesser force is required to move the chalk.

But when the chalk sticking on the board static friction acts between the surfaces and it oppose relative motion so you require slightly greater force to write on the board.