Friction

Friction

Friction can be defined as the force that opposes relative motion. It can also be defined as a force that acts on the surface of separation of two bodies in contact and tends to oppose the motion of one over the other. When surfaces come in contact with one another, it will be noticed that one of such surfaces is opposing the other from moving. This opposition to motion is known as friction.

For example, if a block of wood that rests on a table is pulled, the spring balance will record the force applied to it.

Types of Friction

Starting or Limiting Friction: This is the frictional force that is caused when one body is just about to move over another. Hence, it is the maximum force that opposes the motion of a body at rest, at the point when the motion is just about to begin. Static friction equals the force causing motion, and the body is then said to be in equilibrium.

Sliding or Kinetic Friction: This is the frictional force that is caused when a body is sliding over another body with uniform speed. Usually, dynamic friction is slightly less than static friction.

Characteristics of Friction

• Static friction occurs when the force causing the motion is just equal to the frictional force of the surface in contact.
• Dynamic frictional force is less than static frictional force.
• Frictional force opposes motion.
• Frictional force is independent of the area of surface in contact.
• Frictional force depends on the nature of the surface.
• Frictional force is independent of the relative velocity between the surfaces.
• Frictional force is proportional to the normal reaction (R) or the weight of the body.