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### FRICTION - Important Points to be remembered

1.  The force which opposes the relative motion of two surfaces of bodies in contact, is called as "frictional force".

2. Friction is tangential force between the contact surfaces of two bodies.

3. Friction is due to Electromagnetic Forces between the surfaces in contact.

4. Friction is due to molecular interaction at the surfaces in contact.Friction is due to adhesive forces between molecules of two surfaces in contact.

5. Friction depends on nature of surfaces in contact and on the impurities present on these surfaces.

6. Normal Force: When two bodies are in contact or when one body is placed over another body, the contact force which either body exerts on other normal to contact surface is called Normal Force or Normal Reaction.

7. Friction is proportional to Normal Force.

8. Limiting Friction is least force necessary to set a body into motion.

9. Sliding Friction is the friction which comes into play when the surface of an object moves relative to the surface of another object.

10. Static friction is the friction which comes into play when surfaces of the objects are at rest relative to each other even there is an external force acted upon.

11. Static friction is a self adjusting force.

12. Kinetic Friction is not a self adjusting friction.

13. The substances which reduce friction are called as lubricants.

14. Generally coefficient of static friction is less than 1 but in some cases it may exceed 1.

15. Frictional force is a "Non-Conservative" force.

16. If a body of mass 'm' is on the floor of a lift which is moving with uniform acceleration 'a', Normal force on body or its apparent weight is

N = mg ±ma = m(g±a)

a) If the lift moves up, then N = m(g+a)
b) If the lift moves down, then N = m(g-a)
c) If the lift falls freely, then N=0
d) If the lift moves with uniform velocity, then  a =0, and N=mg

17. When a person falls on a rough road, the frictional force exerted by road on him is along his direction of motion.

18. The angle made by resultant of Normal force and Limiting friction with Normal force is called angle of friction. The tangent of this angle gives coefficient of static friction.

19. The substances which reduce friction are called Lubricants.

20. A good lubricant must be highly viscous and low volatile in nature.

21. The frictional force exerted by fluids is also called as "drag".

22. Frictional force on an object in a fluid depend on its speed with respect to fluid, on the shape of the object and on the nature of fluid.

23. Friction can produce heat.