( 24.S ) 



tlic Ixxly, will |»ro|»jii2;alc with oilier vclocilio when cvaiiuiled fVom 

 ji system relative lu wliicli tlie ImkIv inuves with a xelocitv i\ Evaluated 

 fVoiii sncli a system the velocity of propagation in the direction of 



C' <•" 

 the motion is -, in ihe opposite direction il is . 



Lei ii> jiow lake a r(»d whose ends will he called ^1 and Jj. In 

 .1 and 1} two (Mjiial ajid oi)posite forces are ap|»lied. These forces 

 ai-e a{)[)lied at the same lime when evalnaled from a system relative 

 to which tiie rod is in resl. An observer relative to whom the rod 

 moves in the direction from .1 lowards B will find that the force 

 in A is applied earlier lliaii that in />. Call t' the moment in which 

 the force in A is applied, then he will find thai llie force in li is 



applied at the momenl f' -\- .,.''. The energy and the momentum 



cah'ulated hy Kinstkin and In Lokkntz are those (pianlities imparted 



to tiie body by the force xi during the interval ./', during which 



the force in B was not yet apjilied and could not cancel it. We 



have here however not yet taken into account that the effect of the 



force in B propagates in the rod with a negative velocity and that 



I» 

 it is fell in A before it is applied in ]l. During ihe interval ;^ .r' , 



during which il is not yet aj)plied, the force in B im])arts notwith- 

 standing energy and momentum to the body which exactly cancel 

 those amonnts which are imparled by the force in A. 



We see here again that the assumption of the existence of rigid 

 bodies leads in the theory of relativity to unacceptable conceptions. 

 We are therefore induced to assume that every bodj' is elastically 

 compressible and thai in such a way that the same law which holds 

 for the propagation of light in moving media also ajiplies to the 

 propagation of elastic disturbances. 



Let us apply to a body a system of equal and ojjposite forces, by 

 which it is compressed, and if we then set il in motion, in consequence 

 of which it contracts farther, then the forces will again do a certain 

 amount of work when this contraction takes place. This is perfectly 

 analogous to the case that we apply first a set of forces A, which 

 compress a body, and afterwards another set B, which compress it 

 still further. At this second compression the set ^4 will again do 

 some \York. So it pro\'es to be true that a set of equal and opposite 

 forces changes the energy of a moving (and also of a stationary) 

 body, but this energy is exclusively the consequence of the contraction 

 and change of form of the body. 



