Conservation of mass is an intuitive concept but false. We already know that mass can be changed into energy and vice versa via the relationshipWe already know that nothing – strictly, no matter – can travel faster than the speed of light in a vacuum. What happens then as a body accelerates? Surely if we apply a force to an object it will continue to accelerate? If the speed of light is a fundamental limit, what happens as the limit is approached?

The mass of a body is not constant. What we think of as the mass of a body can probably most closely be described by the term 'rest mass' , the mass of a body when it is not moving. As the speed of the body increases, so does it's effective mass as shown in the diagram below,

and a greater force must be applied to keep it accelerating at the same rate. Newton's second law of motion is still relevant, in a slightly adapted form,whereis now the relativistic mass,As the speed increases the relativistic mass increases without limit, becoming infinite atimplying also that the applied force must be infinite to keep the body accelerating.