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Friction Numericals are very important. One should understand the Friction Concepts very thoroughly. There are two surfaces involved which have the frictional force between them. You need two surfaces to have friction. A single surface is not enough.
Following Forces are involved while Solving a Friction Numerical:-
Weight of the Object =mg   ( m is the mass of the Object)
Normal Reaction =N ( Perpendicular Force)
Coefficient of Friction. Mu ( Read about Static and Kinetic
Coefficient of Friction). It depends on the two surfaces involved.
The applied force which is trying to move the object ( Push or Pull) =F ( External Force). The Intention of Applied force is Motion.
 - Frictional Force= f  ( The force between the two surfaces in contact). The frictional force is always opposite to Applied external force.
 One should draw a Free Body Diagram and show all the above forces.
- Moving Object If the object is moving, one has to overcome the Kinetic Force of Friction.
- Resting Object If the object is at rest, one has to apply external force to make it move. This force is called the Static Force of friction.

Important Point: The Static Force of friction is always larger than the Kinetic Force of Friction. If you have a resting object, you would like to overcome the Static friction before you can make it move.
Coefficient of Kinetic Friction for Aluminium on Steel is .41
Coefficient of Static Friction for Aluminium on Steel is  .61

Two identical ball bearings in contact with each other frictionless table are hit head-on by another ball bearing of the same mass moving initially with a speed v as shown in figure.
If the collision is elastic, which of the following (figure) is a possible result after collision?

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