Question

A circus acrobat while performing spins brings his limbs closer to the body. Due to this ?

Answer 1

Hint: Students should know the concept of conservation of angular momentum. We know that the rate of change of angular momentum equals the torque. Here the external torque is zero. So, we know that the angular momentum remains conserved. So, we know that when an acrobat curls his body and brings his hands and legs as close to his body then his moment of inertia decreases and consequently his angular velocity increases.

Complete answer:
If a particle is moving along a circular path about a point at the centre, then the motion of that particle is called circular or rotational motion. The moment of linear momentum of a particle or a rigid body about an axis is known as angular momentum about that axis.If the torque acting on a body becomes $0$ that is torque becomes zero.

Then the rate of change of angular momentum also becomes zero.If no external torque acts on a system of particles, then, the total angular momentum of the system remains conserved. Therefore, a circus acrobat while performing spins brings his limbs closer to the body. Due to this angular momentum is conserved but K.E. is not conserved. Because when the moment of inertia decreases, rotational kinetic energy increases as work is done in decreasing the moment of inertia of the body.

Note: Angular momentum of a system is conserved as long as there is no net external torque acting on the system, the earth has been rotating on its axis from the time the solar system was formed due to the law of conservation of angular momentum.