## Wednesday, April 07, 2010

### AIPMT 2010 Questions on Friction

The following multiple choice questions involving friction were included in the AIPMT 2010 question paper: (1) A block of mass m is in contact with the cart C as shown in the figure. The coefficient of static friction between the block and the cart is μ. The acceleration α of the cart that will prevent the block from falling statisfies

(1) α < g/μ

(2) α > mg/μ

(3) α > g/μm

(4) α g/μ

When the cart moves forwards with acceleration α, the block of mass m presses against the vertical surface of the cart with a horizontal force of magnitude mα. Therefore, the normal force acting on the block is mα and the maximum frictional force trying to prevent the block from falling is μmα. This must be greater than or equal to the weight mg of the block.

Therefore we have

μmα ≥ mg from which α g/μ

(2) A gramophone record is revolving with an angular velocity ω. A coin is placed at a distance r from the centre of the record. The static coefficient of friction is μ. The coin will revolve with the record if

(1) r μg/ω2

(2) r = μ2

(3) r < ω2/μg

(4) r μg/ω2

Let the mass of the coin be m. The coin will revolve with the record if the frictional force which supplies the centripetal force (required for the circular motion of the coin) is sufficient. Since the maximum value of the frictional force is μmg, we have

μmg mrω2

Therefore r μg/ω2.

You will find more questions (with solution) on friction on this site. One of the posts on friction is

Similar useful posts in this section can be seen