Friday, February 8, 2008

IIT JEE Physics Formula Revision 6. Friction

1. Normal force = Nf = Mg
M = mass of the object
g = acceleration due to gravity

2. fk = µk Nf

fk = magnitude of kinetic friction
µk = coefficient of kinetic friction

3. fmax = µs Nf


fmax = maximum static friction
µs = coefficient of static friction

The actual static friction can be less than maximum static friction if the force applied is less than fmax .

4. On an adjustable inclined plane, a block is kept and the angle is gradually increased so that the block begins to move.

Then fmax = mg sin θ
Nf = mg cos θ

There coefficient of static friction µs

µs = fmax / Nf
= tan θ = h/d

where θ = angle of incline when the block starts moving
m = mass of the block placed on incline
h = height of the incline
d= length of the incline

5. T find kinetic friction the angle of the incline is slightly reduced and the block is made to move with uniform velocity

In this case
µk = coefficient of kinetic friction = tan θ’ = h’/d’
(h is going to decrease and d is going to increase)

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