Page 60 - Physics - XI

4 Laws of Motion
Laws of Motion

EXPERIMENT - 9

Aim

To study the relationship between force of limiting friction and normal reaction and to fi nd the co-effi cient
of friction between a block and a horizontal surface.

Apparatus and Materials Required
Horizontal surface (table) with a frictionless pulley attached at one of its edge, a wooden block with a hook,

a spring balance, a weight box, string, a pan, a spirit level, a strong thread, and a horizontal plane with a
glass top.

Terms and Definitions

Friction: The force which comes into play when a body actually moves (slides or rolls) or tends to move
over the surface of another body is called force of friction. It always acts in the direction opposite to the
direction of the relative motion.
Origin of Friction

According to old point of view, friction is due to interlocking of the surface irregularities between the two
surfaces. But according to modern point of view, the force of friction arises on account of strong atomic or
molecular forces of attraction between the two surfaces at the points of actual contact.
Static Friction

The opposing force which comes into play when one body tends to move over the surface of another body,
but the actual motion has yet not started is called static friction.
Limiting Friction

Limiting friction is the maximum opposing force which comes into play when one body is just at the verge
of moving over the surface of another body in contact with it.
Dynamic Friction
Dynamic friction or kinetic friction is the opposing force that comes into play when one body is actually
moving over the surface of another body. Dynamic friction is always less than static friction.

Rolling Friction and sliding friction
The dynamic friction may be of two types:

(i) Sliding friction: The opposing force that comes into play when one body slides over the surface of
another body is called sliding friction.
(ii) Rolling friction: The opposing force which comes into play when one body actually rolls over the
surface of another body is called rolling friction. The rolling friction is much less than sliding friction.

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