Do We Live In The Matrix?, page-32

If a net force is applied to a object it will increase in velocity relative to an observer. The increase in velocity will be accompanied by an increase in mass in such a way that relative to the same observer the body cannot exceed the speed of light. The body effectively absorbs the energy being imparted to it by gaining mass - hence the equivalence of mass and energy.
This increase in mass is not easily detectable at speeds that we are familiar with.