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directed vertically downward. Thus, the vertical component of the linear momentum of the grapefruit 3 changes, but since no horizontal force acts on the grapefruit, the horizontal component of the linear
momentum cannot change.
Note that we focus on external forces acting on a closed system. Although internal forces can
change, the linear momentum of portions of the system, they cannot change the total linear momentum of
the entire system.
In some ways, the principle of conservation of momentum is more general than the principle of
conservation of mechanical energy. For example, it is valid, even when the internal forces are not
conservative, but mechanical energy is conserved only then the internal forces are conservative.
We have seen that for any type of collision, the total momentum of the system just before the
collision equals the total momentum just after the collision as long as the system may be considered
isolated. The total kinetic energy, on the other hand, is generally not conserved in a collision because
some of the kinetic energy is converted to internal energy, sound energy, and the work needed to
permanently deform the objects...
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