A body at rest does not strike another adjacent stable body.

This is obvious. Since the percussive action is compressive, it cannot be conceived without the effort and impetus of a displacement.

If two bodies moved at an equal velocity collide at right angles with the surface of a third body absolutely immobile, hard and rigid, the ratio of their percussions will be equal to the ratio of the masses of the colliding bodies.

Let two bodies A and B at equal velocities V collide at right angles with the surface C of a third body, absolutely fixed and stable (Table 3.​1, Fig. 22). All the bodies are absolutely hard and rigid. I claim that the ratio of the force and energy with which the body A strikes C to the percussion carried out by the body B is equal to the ratio of the mass of the body A to the mass of the body B. Since the velocities of the bodies A and B are equal, the ratio of the mass of the body A to the mass of the body B is equal to the ratio of the motive force present in all the mass of the body A to the motive force present in the body B. But the absolutely firm body C is impelled and compressed at right angles by all the particles of the bodies A and B impelled by as many small portions of impulsive virtue, and completely resists and refrains these impulses by its obstinate rest. Thus the ratio of the compression exerted by the body A to the impulse exerted by the body B is equal to the ratio of all the motive force expanded in the body A to all the motive force of B or to the ratio of the mass of the body A to the mass of the body B. Q.E.D.

If two equal bodies collide at right angles with the surface of another absolutely stable body and if all the bodies are hard and rigid, the forces of percussion are proportional to the velocities of the striking bodies.