1413 
supported the idea of absolute motion, later criticism led to the 
general conviction that motion is relative. The latter being assumed, 
we have the following two problems: 
1. How can inertial systems be fixed without the aid of an 
“absolute space”. 
2. Which are, according to this, the equations of motion of 
mechanics, if we require that absolute co-ordinates do no more 
occur in them, but exclusively relative co-ordinates (that fix the 
place of the material points with respect to each other) and their 
derivatives. 
FörpL gave a solution of the first problem, which shall be treated 
in the next $ *). In connection with this we shall give in § 4 a 
solution of the second problem. Here already can be remarked that 
this solution is quite different from the one given by the theory of: 
EINSTEIN, in § 5 we return to this. 
2. The hypothesis of A. Forr.*), by which special inertial 
systems are fined. 
From here we will deal with motion in plane instead of space, 
for the sake of simplicity ®). 
With FörpL we assume that the total matter in space consists of 
a finite number n of material points. For the co-ordinates 2’, y’ in 
an inertial system we then have the equations: 
m, w'y = X,. my,=Y EEN IE €) 
wle) 
We suppose further that for the quantities on the right side 
(components of force) the law of reciprocal action holds: 
Be Ol es a. as ne). | (Ba) 
SED ES et 2) 
The sign = includes all x points. This is the case e.g. if the points 
apply forces upon each other in the direction of the lines that 
join them. 
From (2), (8a) and (36) follows: 
1) Some short critical remarks on this and some other solutions shall be 
given in § 5. 
*) A. Förrr. Vorlesungen über technische Mechanik. VI. Erster Abschnitt. 
Die relative Bewegung. 
3) For space we have a quite analogic reasoning. Then the use of vectors 
can be recommended, : 
