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The friction, that exists between two surfaces which are not lubricated, is known as
solid friction. The two surfaces may be at rest or one of the surface is moving and other surface is at rest. The following are the laws of solid friction:

1. The force of friction acts in the opposite direction in which surface is having tendency to move.

2. The force of friction is equal to the force applied to the surface, so long as the surface is at rest.

3. When the surface is on the point of motion, the force of friction is maximum and this maximum frictional force is called the limiting friction force.

4. The limiting frictional force bears a constant ratio to the normal reaction between two surfaces.

5. The limiting frictional force does not depend upon the shape and areas ofthe surfaces in contact.

6. The ratio between limiting friction and normal reaction is slightly less when the two surfaces are in motion.

7. The force of friction is independent of the velocity of sliding.

The above laws of solid friction are also called laws of static and dynamic friction.