(a) Consider a person standing on a freely rotatable disk. He stretches his hands in horizontal direction and ask his friend to pull one of his hand in tangential direction (of the disk) setting the person on the disk in angular motion. Once in motion, if he moves his arms and align them in vertical direction above his head what will be the effect on his angular speed? (b) Will the speed remain constant or will it change. Note that we do not have any force acting on him that can change the torque.
Rigid Body
A rigid body is an object which does not change its shape or undergo any significant deformation due to an external force or movement. Mathematically speaking, the distance between any two points inside the body doesn't change in any situation.
Rigid Body Dynamics
Rigid bodies are defined as inelastic shapes with negligible deformation, giving them an unchanging center of mass. It is also generally assumed that the mass of a rigid body is uniformly distributed. This property of rigid bodies comes in handy when we deal with concepts like momentum, angular momentum, force and torque. The study of these properties – viz., force, torque, momentum, and angular momentum – of a rigid body, is collectively known as rigid body dynamics (RBD).
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