Question

Three blocks of mass $1kg,4kg$ and $2kg$ are placed on a smooth horizontal plane as shown in figure. Find the normal force between $1kg$ block and $4kg$ block.

• A. $110N$
• B. $70N$
• C. $50N$
• D. $30N$

Answer 1

The correct option is A $110N$

By applying force balance equation on all the three free masses

The acceleration of all the three masses = $a$

let all the three masses are a system of $7kg$ moving in one direction

So, the acceleration of all the masses is same is same as combined mass
$a=\frac{120-50}{2+4+1}=\frac{70}{7}=10$

Apply force balance equation to all the masses individually
for mass $1kg120-N_1=1\ast 10120-N_1=10N_1=110$
So the normal reaction force between 1kg and 4kg is =110N