23t 
MOVING FORCE. 
Cases of diffi- 
culty in the 
doctrines of 
moving foice. 
This compression of the spring is comprehended by Mr, 
Siueaton under the term change of figure 5 and he has shown, 
by some well-chosen experiments, that when a non-elastic 
yielding body, moving with a given velocity, strikes directly 
another equal body at rest, exactly half the force of the strik- 
ing body is expended in producing change of figure*. 
The facts exhibited in the 7th case are similar to those which 
Mr. Sraeaton has described as the results of his experiments. 
According to the theory, the whole force of A (fig. 7) before 
collision, is to be found in the motion o{ A and B after collision. 
But if that be admitted, we must suppose the spring to have 
been compressed without force : yet we have no more reason 
to .suppose that the spring can be compressed without force, 
than that a body can be put in motion w'ithout force; and the 
amount of the force which has been expended in compressing 
the spring, is ascertained by its effects in producing motion in 
C and D ; and although these balls move in opposite directions, 
it cannot be supposed that their motion can be produced with- 
out force. 
In this explanation, however, of the action of the spring 
on C and D, Mr. Maclaurin understood a material inconsistency 
to be involved, which he stated in a treatise that obtained the 
prize of the Royal Academy of Sciences at Paris, in 1724t- 
Mr. Maclaurin supposes two equal bodies like C and D, with 
the compressed spring between them, to be situated in a space 
E F G H, w’hich, together with the balls, “ moves uniformly 
in the direction CD with the velocity as 1 ; and that the 
spring impresses on the equal bodies, C and D, equal velocities 
in opposite directions, that are each as 1. Then the absolute 
velocity of D (which was as 1) will be now as 2 ; and accord- 
ing to the new doctrine, its force as 4 ; whereas the absolute 
velocity, and the force of C (which was as l) will be now de- 
• Experimpiits on Collision- Pliil. Trans. 1782. 
'f Tl;p “ IJisconrs sur le mouvcmpul” of Jolin Bernoulli was of- 
fered for the same prize, but was njtcled, the preference feinggivea 
to the treatises of Maclaurin and Maziere. 
stroyed f 
