Question

A 20 kg square plate of side 4 m has uniform mass density and is hinged vertically at the midpoint of its side A. The torque about A is . Take $g=10m/s^2$

• A. 200 N-m
• B. 100 N-m
• C. 400 N-m
• D. 300 N-m

Answer 1

The correct option is C 400 N-m

Given -
Mass of the plate $=20kg$
Side of the plate $=4m$
Acceleration due to gravity $g=10m/s^2$
The force of gravity can be assumed to act on its centre of mass, which in this case is geometric centre of the square.
${\left|\right(\overrightarrow{{\tau }_A}\left)\right|}_{clockwise}=\frac{MgL}{2}$

$=\frac{20\times 10\times 4}{2}Nm$
= $400Nm$