Question

Blocks $A$ and $B$ of masses $2kg$ and $3kg$ respectively are placed on a smooth ground. If an external force $P$ of magnitude $10N$ acts on $A$ as shown, find the force exerted by $B$ on $A$ is

• A. $6N$
• B. $3N$
• C. $9N$
• D. $1N$

Answer 1

The correct option is A $6N$

Block $A$ and $B$ as a system, FBD of block $A+B$ is

(Here normal force between $A$ and $B$ is internal so we cannot show it in FBD) there is no vertical motion so the net force on system in vertical direction is zero. In horizontal direction there is an external force $10N$ so the system will accelerate with acceleration $a$. $a=\frac{10}{5}=2\frac{m}{s^2}$
FBD of block A is,
(Here block A and block B are the separate systems so the normal force N is external, normal force N must be shown in each FBD)
$10-N=ma=2\times 2⟹N=6N$
FBD of block B is,

$N=ma=3\times 2=6N$