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can you give me introduction on friction?

 Friction

          Whenever a body moves or tends to move over the surface of another body, a force comes into play which acts parallel to the surface of contact and opposes the relative motion.  This opposing force is called friction.
Example.  Consider a wooden block placed on a horizontal surface.  Give it a gentle push.  The block slides through a small distance and then comes to rest.  According to Newton's second law, a retarding force must be acting on the block.  This opposing force is called friction.  As shown in the below figure, the force of friction always acts tangential to the surfaces in contact and in a direction opposite to the direction of (relative) motion of the body.

    Force of friction always opposes relative motion.

       When two bodies are placed in contact, attractive forces act between their particles at the surface of contact.  As a result, each body exerts a contact force on the other.  These mutual contact forces are equal and opposite, obeying Newton's third law.  The component of the contact force F normal to the contact surface is called normal force or normal reaction N.  The component parallel to the contact surface is called friction.