Force --- Friction

Friction

What is Friction?
Is "friction" trouble?
Is "friction" useful?

What is Friction?
Friction is a force which resists motion when one surface slides over another.
It acts in a direction so as to oppose movement of surface.

Fricion exists because surfaces are never perfectly smooth. There are irregular undulations.
When one surface slides or tends to slides over another surface, the undulations on the two surfaces lock up. Friction arises and tends to oppose the possible motion of the surface.
Some of the energy used to overcome friction is converted to heat and sound.
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Is "friction" trouble?
Friction often causes troubles. It makes the moving parts of a machine stiff and wastes energy.
It may even cause wear in the moving parts. Friction may be reduced by separation the contact surfaces with lubricants.
Lubricating oil is used for this purpose in joints and hinges.
An ice block slides with little friction on the table because the block and the table are separrated by a layer of water

When a rocket is launched, it must overcome the friction that acts between the rocket and the air. It is alos called air resistance. The movement of the rocket through air produces a lot of heat.
Streamlining helps to reduce air friction and the amount of heat produced.
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Is "friction" useful?
Friction is often useful too.
It helps to stop a carwhen a force is applied to the braking systme.
When we walk we exert a force on the ground. Friction then occurs between the ground and our shoes. It is fricion that pushes us forward.
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Friction Force of Gravity Newton's Laws of Motion Suggested Experiments