Question

Which of the following conditions are necessary for a rigid body to be in mechanical equilibrium?
Condition 1: Net external force acting on body is zero.
Condition 2: Net external torque acting on body is zero.
Condition 3: Internal forces and torque due to internal forces must be absent.

• A. Both condition 1 and condition 2 are necessary.
• B. Both condition 2 and condition 3 are necessary.
• C. Both condition 1 and condition 3 are necessary.
• D. All conditions are necessary.

Answer 1

The correct option is A Both condition 1 and condition 2 are necessary.

For translational equilibrium, net external force must be zero.
$\therefore a_{COM}=0$

For rotational equilibrium, net torque due to external forces must be zero.

Presence of internal forces and torque due to internal forces will not affect the mechanical equilibrium.
As according to Newton's $3rd$ law, internal forces and torque will be in pair and they will get cancelled out.

$\therefore$ Both condition 1 and condition 2 are necessary.