Question

Why is gravity considered in the torque?

Answer 1

Torque is a measure of the force that can cause an object to rotate about an axis. Just as force is what causes an object to accelerate in linear kinematics, torque is what causes an object to acquire angular acceleration.

    Now for a stretched arm if no force is applied on the end of the arm still there will be a force acting on that point due to the weight of the hand and as the other end is fixed with the body, the weight which is Mass (m) multiplied by gravitational acceleration (g) will create torque on the arm causing it to rotate about the joint. That's why gravity plays a role in torque.