Question

Why the inertia of a body increases when the speed of the body is near to the speed of light.

Answer 1

It is not necessary to approach the speed of light for the mass to increase. Mass of all the moving object will be slightly more than the rest mass. Body having greater energy will have greater mass. This increase in mass will consequently increase the inertia of the body.

Rest Mass/Rest Energy → It is the mass/energy of body when it is at rest. Rest mass and rest energy are equivalent and proportional to each other. It is inherent property of matter and is same almost everywhere.

Relativist Mass/Energy→ When a body is in motion, it total energy is greater than its rest energy, and hence the relativistic mass is greater than rest mass. This relativisric mass is equal to the inertial mass of a body. So when a body is in motion, its inertial or relativistic mass increases and in turn increases the inertia of the body.