Question

A block of mass 0.1 kg is held against a wall by applying a horizontal force of 5N on the block. If the co-efficient of friction between the block and the wall is 0.5, the magnitude of frictional force acting on the block is

• A. 2.5 N
• B. 0.98 N
• C. 4.9 N
• D. 0.49 N

Answer 1

The correct option is B 0.98 N

Static friction force acts in response to a force trying to cause a stationary object to start moving. If there is no such force acting on an object, the static friction force is zero. If there is a force trying to cause motion, the static friction force will increase up to a maximum value before it is overcome and motion starts.

$f_{static}$ $\le$ ${\mu }_{static}$ $\times$ $F_{normal}$

value of static friction varies from zero to maximum values of force when it just starts moving.

$f_s\le \mu N$

here normal reaction force is 5N

then

$f_s\le 0.5\times 5f_s\le 2.5N$

but weight force is

$F=mg=0.1\times 9.8F=0.98N\left(downward\right)$

thus friction force is only 0.98 N

Option B is correct