Question

A system of two blocks as shown in the figure is initially at rest. Find the frictional force $f$ between the blocks as they are pulled by the forces as shown in the figure.

• A. f = 25 N
• B. f = 10 N
• C. f =12 N
• D. f = 15 N

Answer 1

The correct option is B f = 10 N


Let blocks are moving together, so the friction force between blocks would be static.
$\text{Upper block}20-f=5a\text{lower block}10+f=10a⟹30=15a$

$⟹a=2m/s^2$
$20-f=5\left(2\right)=10$ N
$f=10$ N, but $f_{lim}=\mu mg=25$ N

As, $f\le f_{lim}$,
$⟹$ the blocks are movig together.