Previous Year NEET Question- Laws Of Motion L-5

Question: A force acts for $20 \mathrm{~s}$ on a body of mass $20 \mathrm{~kg}$, starting from rest, after which the force ceases and then body describes $50 \mathrm{~m}$ in the next $10 \mathrm{~s}$. The value of force will be:#

A) $40 \mathrm{~N}$

B) $20 \mathrm{~N}$

C) $5 \mathrm{~N}$

D) $10 \mathrm{~N}$

Answer: $5 \mathrm{~N}$#

Solution:#

$$ \begin{aligned} & m=20 \mathrm{~kg} \ & t=20 \mathrm{sec} . \end{aligned} $$ $$ \begin{aligned} & \text { Acceleration }=\frac{F}{20} \mathrm{~m} / \mathrm{s}^2 \ & \therefore v=u+a t \ & v=0+\left(\frac{F}{20}\right)(20) \ & =F \mathrm{~ms}^{-1} \end{aligned} $$

Now for next $10 \mathrm{sec}$. $$ \begin{aligned} & S=u t \ & 50=F(10) \ & F=5 \end{aligned} $$