Rigid body:

Rigid body is composed of particles, which are at fixed distance with respect to each other. In simple words, if a particle "A" is at a distance of 10 mm (say) from another particle "B" within a rigid body, and then they continue to remain 10 mm apart during motion. Simulating the motion of rigid body is based on the simulating motion of a particle .This requirement is essential in describing rotational motion of a rigid body. The distribution of mass about its axis affects the rotational inertia of body. As such, change in inter-particle distance shall amount to changing "rotational inertia" of the body. The Rigid body can receive forces and torque to make your objects move in a realistic way. Any Game Object must contain a rigid body to be influenced by gravity, act under added forces.

Rotation of a rigid body can be characterized by the same angular velocity and acceleration of the particles comprising it. The situation is identical to the case of translation in which the linear velocity and acceleration of all the particles comprising rigid body are same. The position of a rigid body is position of all particles of which it is composed. Rigid body moves, its position and orientation vary with time both. In the kinematics sense, these changes are referred to as translation and rotation, respectively. Indeed, the position of a rigid body can be viewed as a hypothetic translation and rotation of the body starting from a hypothetic reference position .To simplify the description of this position, we destroy the property that the body is rigid, namely that all its particles maintain the same distance relative to each other. If the body is rigid, it is enough to describe the position of at least three no collinear particles. This makes it possible to reconstruct position of all other particles, given that their time-invariant position relative to the three selected particles is known.