Moving Force. 125 
In the first case, the sum of the products of 
the masses into the squares of their respective 
velocities, is the same before and after collision ; 
but in the second case, that sum is less after 
than before collision ; and it must, I presume, 
be admitted, that the rotatory force in that case 
is diminished by the collision. 
13. If an iron prism AB, (fig. 13.) move- 
able on a fixt centre at A, be let fall on a piece 
of soft clay C, the greatest impression might be 
expected to be made when the clay is placed 
under P, the centre of percussion of the prism. 
But if the experiment be made, the impression 
will be found to be the same, whether the clay 
be placed at C, D, or E, or at any other dis- 
tance from the centre of motion. 
14. Let two equal elastic balls A and B, 
(fig. 14.) be connected by an elastic rod, and 
be at rest in free space, and Jet G be their com- 
mon centre of gravity. If another elastic ball C, 
whose mass is equal to the joint masses of A, 
B, and the rod, moving with the velocity 
v in the direction C G at right angles to the 
rod, strike it at G; C will be brought to rest, 
and G will move off with the velocity v, in the 
direction CG. But if we repeat the experi- 
ment, applying the force of D instead of C, 
the mass of D being equal to that of A or Band 
half the rod ; and its velocity equal 2 v, striking 
