Question

Define a rigid body?

Answer 1

A rigid body is a collection of points in space. It is a three dimensional solid structure which has a position and undergoes motion in space. Practically, nobody can be perfectly rigid. A rigid body is said to be in equilibrium when net forces acting on its centre of mass is zero.

Complete answer:
A solid body which experiences zero or negligible deformations due to motion is called a rigid body. A rigid body is a collection of point masses and the distance between two fixed points always remains constant irrespective of the external forces applied on it.
It is assumed that the total mass of a rigid body is concentrated at the centre of mass to simplify the calculations. Net force acting on a rigid body may be thought of acting on the centre of mass.
The position of different points of a rigid body may be described by a vector with origin on the centre of mass. All points of a rigid body follow the same motion and move with the same linear or angular velocity or acceleration.
A rigid body undergoes motions like translational, rotational, vibrations etc. A rigid body is said to be in equilibrium if no net force or net torque is acting on the body.

Note:
Practically, no body can be completely rigid, Every body considered to be rigid undergoes some deformations, but the bodies in which the changes are negligible are called rigid. When a body experiences a deforming force, by Newton’s third law, a restoring force is generated in it.