Motion of the Center of Mass
The collision force acting between the target and the projectile is an internal force of the system
under consideration consists of these two objects. The motion of the center of mass of a number
of objects is solely determined by the external forces acting on the system (see Chapter 9).
This equation shows that if no external forces act on the system, the velocity of its center of mass
is constant.
Sample Problem 104
In a nuclear reactor, newlyproduced fast neutrons must be slowed down before they can
participate effectively in the chainreaction process. By what fraction is the kinetic energy of a
neutron (mass m
1
) reduced in a headon collision with a nucleus of mass m
2
(initially at rest) ?
Suppose v
1i
is the initial velocity of the neutron. Its final velocity, v
1f
, can be obtained using one
of the previously derived equations:
The initial kinetic energy of the neutron is given by
The final kinetic energy of the neutron is given by
The fraction of the kinetic energy of the neutron lost in the collision is given by
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 Fall '09
 Center Of Mass, Force, Mass, 100 %, 0 m/s, 28 %, 1.9 %

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