Question

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

• A. $20N$
• B. $60N$
• C. $50N$
• D. $100N$

Answer 1

The correct option is A $20N$

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 as combined mass,

$a=\frac{netforce}{totalmass}a=\frac{120-50}{2+4+1}=\frac{70}{7}=10$

Apply force balance equation to all the masses individually

$formass1kg120-N_1=1X10120-N_1=10N_1=110Nformass4kg{N}_1-N_2=4X10N_1-N_2=40here{N}_1=110Nthen,N_2=110-40=70N$

for mass $2kg$ net force in the direction of $N_2$ then net force on mass $2kg$ is

${F=N}_2-50F=70-50=20N$