Friction and Its types. Static vs. Dynamic friction. Sliding vs. Rolling friction. Limiting friction. Coefficient of Friction.
A force acting in the opposite direction to the motion of the body is called force of friction or simply friction. It is of the following two types
1. Static friction ; and
2. Dynamic friction.
The friction, experienced by a body, when at rest, is known as static friction.
The friction experienced by a body, when in motion, is called dynamic friction. It is also called kinetic friction. It is of the following two types:
(a) Sliding friction ; and
(b) Rolling friction.
The friction, experienced by a body, when it slides over another body, is known as sliding friction.
The friction experienced by a body, when balls or rollers are interposed between the two surfaces, is known as rolling friction.
Limiting Friction:
The maximum value of frictional force, which comes into play, when a body just begins to slide over the surface of the other body, is known as limiting friction.
Laws of Static Friction:
Following are the laws of static friction:
- The force of friction always acts in a direction, opposite to that in which the body tends to move.
- The magnitude of force of friction is exactly equal to the force, which tends the body to move.
- The magnitude of the limiting friction bears a constant ratio to the normal reaction between the two surfaces.
- The force of friction is independent of the area of contact between the two surfaces.
- The force of friction depends upon the roughness of the surfaces.
Laws of Dynamic or Kinetic Friction:
Following are the laws of dynamic or kinetic friction :
- 1. The force of friction always acts in a direction, opposite to that in which the body tends to move.
- The magnitude of the kinetic friction bears a constant ratio to the normal reaction between the two surfaces.
- For moderate speeds, the force of friction remains constant. But it decreases slightly with the increase of speed.
Coefficient of Friction:
It is defined as the ratio of limiting friction (F) to the normal reaction (RN) between the two bodies. It is generally denoted by μ. Mathematically,
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