Page 61 - Physics - XI

Coeffi cient of Limiting Friction
The coeffi cient of limiting friction between any two surfaces in contact is defi ned as the ratio of the force

of limiting friction and the normal reaction between them.

F L
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R
where µ is the coeffi cient of limiting friction between two given surfaces, F is the force of limiting
L
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friction and R is the normal reaction between the two surfaces.
Principle/Theory
When a body slides over the surface of another body, an opposing force called the force of friction comes
into play. The force of limiting friction F is directly proportional to the normal reaction R between the two
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surfaces, i.e.
F L ∝ R
or F = µ R …(1)
L L

where μ is a constant called coeffi cient of limiting friction which depends upon the nature of surfaces in
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contact. From equation (1), it is clear that if we plot a graph between F and R, it should be a straight line.
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Procedure
1. Put the horizontal plane with a glass R = (W + P)
top suitably on the table. Make it Wooden block (W) Additional weights (P)

horizontal by using a spirit level. Clean F Pulley
the glass surface and oil the pulley. Glass top
2. Find the least count and zero correction W + P
of the spring balance. Now, weigh the
wooden block and the pan with the

help of a spring balance separately. Pan (w)
3. Connect a string to the hook of the Weights (X)
wooden block on one side and to the F = (w) + X
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pan on the other side. Pass the string
over the pulley as shown in Fig. 9.1 Fig. 9.1: Experimental Setup
and see that the pan hangs freely in
the air. Also, ensure that the portion of the string between the hook and pulley is horizontal.

4. Put some weights in the pan and tap the glass surface gently. If wooden block does not slide, go on
increasing the weights in the pan till the wooden block just begins to slide on gently tapping the plane.
In this situation, the force of limiting friction F will be equal to weight of the pan w + weights (X)
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placed on the pan. The normal reaction R is equal to the weight of the wooden block (W).
5. Now, put a known weight P on the wooden block so that normal reaction R becomes (W + P) and repeat
step 4.

6. Repeat step 4 again by putting more known weights on the wooden block to get six diff erent sets of
observations.
7. Plot a graph between F and R.
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