IIT JEE , AIEEE Forum - Physics , Mechanics

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Frictional resistance to the relative motion of two solid objects is usually proportional to the force which presses the surfaces together as well as the roughness of the surfaces. Since it is the force perpendicular or normal to the surfaces which affects the frictional resistance, this force is typically called the normal force.

Remember these points :

1. Friction always opposes the motion or attempted motion of one surface across another surface.

2. Friction is dependent on the texture of both surfaces.

3. Friction is also dependent on the amount of contact force pushing the two surfaces together (normal force).

4. On a level surface, the normal force N is always equal and opposite to the weight.

5. The force of static friction (f_s) cancels out the applied force right up to and including when static friction reaches its maximum (f_{s max})

6. For applied forces greater than the maximum force of static friction the block starts to slip and then the value for friction becomes kinetic friction (f_k) and the box is then under a net force so it accelerates to the right.

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