122 On the Measure of 
der; for the length of it forced into the bail 
will be —3 EF, and the velocity of both balls 
after collision will be $v. It is not easy to un- 
derstand how these last effects can be produced. 
by a force no greater than the first. 
9. It is argued that the mass into the velo- 
city must, be the proper measure of the force of 
a body in motion, because the sum of the pro- 
ducts of the various masses of any system of 
bodies into their respective velocities, is always 
the same in the same direction, unless acted 
upon by some external force. In other words, 
because the motion of the centre of gravity of 
any system of bodies cannot be changed or 
disturbed by any action of those bodies upon 
each other. 
If two equal non-elastic balls A and B, 
whose common centre of gravity is G, (fig. 9.) 
move with the velocities and in the direc- 
tions AC and BC, oblique to each other, 
they will meet at C, and after collision they 
will move on together with the velocity and 
in the direction GC. If the product of the 
mass into the velocity in the same direction 
be taken as the measure of the moving force, 
we have in the motion of these bodies, equal 
effects of force before and after collision. 
But it is obvious, that to produce the separate 
motions of A and B before collision, much 
