Friction

Friction originates from forces between atoms and molecules when surfaces are in contact. For example, friction occurs when a body moves on a rough surface or through a fluid medium (water, air, etc.). There are two types of friction:

1.

The Static force of friction ( f_s ) is the force of friction between two objects when there is no motion.

2.

The Kinetic force of friction ( f_k ) is the force of friction between two objects when there is motion.

Consider a block on a rough surface. Apply an external force F_ext to the block.

• if F_ext < f_s(max) the block won't move
• as F_ext increases, f_s will increase until it reaches its maximum value. When F_ext = f_s(max) the block will start to move (this is called the point of slipping).
• Once the block starts to move, the force of friction is given by f_k .