## 24 Adding Energy means Adding Momentum and Kinetic Energy

In STR, relativ velocity *v* is supposed to be constant. Therefore the Epstein angle *φ* with sin(*φ*) = *v* / *c* is constant, too. The energy-momentum diagram shows clearly that any increase Δ*E* of the rest energy *E* goes along with an increase Δ*p* of momentum:

If some energy is added to *E* then *E'* j= *E* / √ **and** *p* will increase ! In another paper we have worked out that in STR kinetic energy and momentum are proportional to each other if the relative velocity *v* remains constant:

*E'*jinto an increase of the

*heat content*jof the gas and an increase of the

*kinetic energy*jof the gas. Eddington

[4, p. 34] and others oversimplify the situation if they directly deduce Δ

*Q'*= Δ

*Q*/ √ from Δ

*E'*= Δ

*E*/ √ . It is wrong to interpret the total increase Δ

*E'*as an increase of thermal energy - there is always an increase of kinetic energy, too.