Question

A block of mass $0.5kg$ is kept in an elevator moving down with acceleration $2m/s^2$. Find the magnitude work done (in Joule) by the normal contact force on the block in first second. Initially system is at rest $(g=140m/s^2)$:-

• A. -2 J
• B. -4 J
• C. -8 J
• D. 0 J

Answer 1

Answer: (C)

-8 J

Correct option is ( C )

By applying pseudo force $=$ ma so, for block ;

$\begin{array}{c}N+ma=mg\\ N=m(a+g)\\ N=\frac{1}{2}\times (2+14)\\ N=8N\end{array}$

Distance travelled in $1s=\frac{1}{2}$ at ${}^2$

$D=1m$

Work by Normal = -ND

$\begin{array}{c}=-8\times 1\\ =-8J\end{array}$