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2014-09-17T14:47:26+05:30

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Friction is not like other forces. It is a contact force and acts only when two surfaces are in contact. The atoms and molecules of the top layers of the two surfaces in contact attract or repel each other. These inter atomic and inter molecular forces cause friction.

When the surfaces are stationary, the attractive forces are more. If the surfaces are pressed against each other, then forces of attraction are more. Also, the molecules of one surface go into the small gaps in the other surface and so the edges of walls of gaps will oppose movement. So friction is more.

When the surfaces are moving (sliding) the forces of attraction are less. This is because the molecules have kinetic energy to overcome the attraction forces. Also, the kinetic energy of molecules can move easily the molecules of one surface embedded in to gaps in the other surface.

So the kinetic (sliding) friction is more than static friction.

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