Laws of solid friction:
Sol.: The friction which exists between two surfaces that are not lubricated is called as solid friction. The two Surfaces can be at rest or one of the surfaces is moving and other surface is at rest. The following are laws of solid friction.
1. The force of friction acts in opposite direction in which the surface is having tendency to move.
2. The force of friction is equal to force applied to surfaces, so long as surface is at rest.
3. When surface is on point of motion, force of friction is highest and this maximum frictional force is called as limiting friction force.
4. The limiting frictional force bears constant ratio to the normal reaction between the two surfaces.
5. The limiting frictional force does not depend upon shape and areas of surfaces in contact.
6. The ratio between the limiting friction and normal reaction is a bit less when the two surfaces are in motion.
7. The force of friction is independent of velocity of sliding.
The above laws of solid friction are called as laws of static and dynamic friction or law of friction.