# The relation between mass, inertia and energy

I have trouble understanding the following concept:

I learnt that mass is a measure of inertia, and that seemed logical enough. Yet separately I learnt that mass is a form of condensed energy. If energy is the capacity to perform work, i.e. to change an object's coordinates (with the greater the amount energy, the greater the velocity), isn't there a contradiction here?

• There is no contradiction. Energy has inertia and energy can be used to gain speed. Both properties work together. While you are changing speed, you are working against the inertia. In fact, the meaning of inertia is that you must spend some energy to change the speed. So it all works out just fine. – safesphere Sep 27 '18 at 18:23
• @safesphere So when accelerating one is using energy in order to overcome the effects of (condensed) energy, i.e. a certain degree of inertia? – Pregunto Sep 27 '18 at 18:33
• Yes, this is correct. You can see this as a process of converting one type of energy to another while the total energy conserves. – safesphere Sep 27 '18 at 18:35
• For example, a compressed spring needs more energy to be accelerated to some speed than the same spring uncompressed, because also the energy that was added during the compression is heavy and needs some energy to be accelerated. – md2perpe Sep 27 '18 at 20:03
• @md2perpe A very good example, thank you! – Pregunto Sep 27 '18 at 20:26