Mass and rest mass

A Releasing Your Unlimited Creativity discussion topic

Copyright 2009 by K. Ferlic,   All Rights Reserved

Mass is defined in one of two ways. One way is that it is defined and amount of material an object possesses. The second definition is that is the object’s resistance to a change in motion. Here the amount of mass an object possess is defined as the applied force on the object divided by the acceleration it undergoes as a result of that force.

Classically, or the understanding of physics before the Theory of Relativity, was that the mass of an object was fixed. That is, the amount of material an object posses never changed such that is always offered the same resistance to an applied force. However, with the understanding provided by the Theory of Relativity it was found, and subsequently verified by experiment, as an object moves closer to the speed of light its resistance to an applied force increases or, alternatively said, it mass increases. On one hand this seems very strange. But with the understanding that mass can be converted to energy, it only makes sense that as an object moves faster and faster it possesses more energy and that increase in energy can be also be experienced as in increase in mass.

When it was discovered that the mass of an object does change, it was found necessary to create the concept of rest mass. The rest mass is simply the mass of an object at rest. The rest mass of an object and the mass of an object are essentially one and the same as long as the speed of the object is much less than the speed of light. When the speed of the object approaches becoming a fraction of the speed of light, then one will see a difference between the mass of an object and the rest mass.

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