Saturday, May 16, 2009

IIT JEE Level Revision Questions - 6. Friction


1. fill blanks

A uniform cube of side ‘a’ and mass ‘m’ rests on a rough horizontal table. A horizontal force F is applied normal to one of the faces at a point that is directly above the centre of the face, at a height of 3a/4 above the base. The minimum value of F for which the cube begins to tip about an edge is _______________ . (Assume that the cube does not slide)

2. A block of mass 1 kg lies on a horizontal surface in a truck. The coefficient of static friction between the block and the surface is 0.6. If the acceleration of the truck is 5 m/s², the frictional force acting on the block is _________ .
(JEE 1984)

Answer: 5 N

Reason: The horizontal force acting on the cube due to the acceleration of the truck is

F = ma = 1 kg * (5 m/s²) = 5 N

As the block is not moving, a frictional force of 5 N is acting on it.
We can verify that the static friction can go up to a maximum value of 1 kg *(10 m/s²)*0.6 = 6 N if we take g as approximately equal to 10 m/s².

Revision Question 1.

A 2 kg block is placed over a 4 kg block and both are placed on a smooth horizontal surface. The coefficient of friction between blocks is 0.20. Find the acceleration of the two blocks if a horizontal force of 12 N is applied to (a) the upper block, 9b) the lower block. Take g = 10 m/s².

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