This chapter discusses various phenomena such as the centre of mass, linear momentum, collision, impulse and impulsive force. In the section "Centre of mass", a simple explanation of the centre of mass with a diagram is provided. In a two-particle system, the centre of mass divides internally the line joining the two particles in the inverse ratio of their masses. In a system having a greater number of particles, we can find the combined centre of mass if we know the centres of mass of parts of the system and their masses.
In the section "Centre of mass of continuous bodies", the location of Centre of Mass of a Uniform Straight Rod, Centre of Mass of a Uniform Semicircular Wire and Centre of Mass of a Uniform Semicircular Plate have been found out with the aid of formulae and diagrams.
The motion of the centre of mass has been discussed in the next section. If no external force acts on a two-particle system and its centre of mass is at rest initially, it remains fixed even when the particles individually move and accelerate. The motion observed at the centre of mass of the system is not affected by the internal forces. If the external forces add up to zero, the centre of mass has no acceleration.
Linear momentum and its conservation principle and rocket propulsion have been discussed in the following section. In the next section, the concept of collision is clearly explained with the help of two masses and spring, followed by the explanation with two balls.
The energy and momentum of particles during collision and types of collision has been explained. Elastic collision in one dimension with special cases and elastic collision in two dimensions have been discussed. Perfectly inelastic collision in one-dimension, final velocity and loss in kinetic energy during that type of collision and coefficient of restitution has been discussed. Finally, a simple and brief explanation about impulse and impulsive force is provided.