According to Newton's second law of motion,
F = ma
If a constant force 'F' is applied to a body for a very long interval of time, the velocity of the body will go on increasing due to the acceleration produced in the body. This velocity may become equal to, or even exceed the velocity of light, assuming that mass remains constant. But this is forbidden by the theory of relativity.
So, a question arises. What prevents a body from attaining a velocity equal to or exceeding the velocity of light? Einstein answered this question by stating that mass of a body is not a constant quantity. But, as the velocity increases, the mass of a body also increases.
So, as the velocity increases, force will produce less and less acceleration. When the velocity of a body becomes equal to the velocity of light, the mass of the body becomes infinite. Consequently, the force will be unable to produce any further acceleration in the body.
The mass of a body moving with a velocity 'v', relative to the observer, is given by
Where m0 is the rest mass of the body i.e., the mass of the body when it is at rest with respect to the observer.
It is clear from the above relation that the mass of a body in a moving frame, as measured by an observer in a stationary frame, is always greater than the rest mass m0 of the body.
