Question

A block of mass 4 kg is placed on a rough horizontal plane. A time dependent horizontal force $F=kt$ acts on the block, $k=2$ Ns${}^{-1}.$ The frictional force between the block and plane at time $t=2$ s is $(\mu =0.2)$

• A. 4 N
• B. 8 N
• C. 12 N
• D. 10 N

Answer 1

The correct option is A 4 N

Normal Force, N = mg = 40N
Maximum Frictional Force, $f_{max}=\mu N=0.2\times 40=8N$
As at time $t=2s$,
$F=kt=2\times 2=4N$, which is less than the maximum frictional force which can be applied. So the block wont move and the friction force will be equal to the force applied which is 4N. (i.e $f=4N$)