Question

A 5 kg block rests on a flat surface with a coefficient of static friction of 0.5 and a coefficient of kinetic friction of 0.3.A force of 20 N pushes on the block does not move . what is the magnitude of the frictional force on the block

Answer 1

The block is in vertical equilibrium. i.e

So, the maximum value of the static frictional force is

The applied force is 20 N. So, the block does not move.

Now, here is a catch, the static frictional force is a self-adjusting force and it is only available to cancel other force.

Here,

Acceleration is zero. So,

So, the magnitude of the static frictional force is 20 N.