Why are energy and momentum conservation needed to explain what is observed?
Momentum is an important quantity because it is conserved. Yet
it was not conserved in the examples in Impulse and Linear Momentum
and Force, where large changes in momentum were produced by forces
acting on the system of interest. Under what circumstances is
momentum conserved?
The answer to this question entails considering a sufficiently
large system. It is always possible to find a larger system in
which total momentum is constant, even if momentum changes for
components of the system. If a football player runs into the
goalpost in the end zone, there will be a force on him that causes
him to bounce backward. However, the Earth also recoils
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