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Question
Is there any device that has, for a moment or so, zero momentum and possess kinetic energy?
Asked by: Ali Azfar
Answer
Yes, actually a lot of them. The only difficult
case is one with a single particle with no
internal structure, then you can't have zero
momentum, and have kinetic energy. However, with
things that contain more than one particle, it
is simple to think of such a device.
Take, for example, two equal masses connected to
two ends of a spring, and vibrating simultaneously
in-and-out. At any time, since the momenta of the
two masses are opposite and equal in magnitude,
the total momentum of the 'device' is zero.
However, each possesses kinetic energy, so the
total kinetic energy of the system is non-zero.
Another example would be a box filled with gas
(air, for instance). The total momentum is zero,
since the box isn't going anywhere as a whole.
However, the total kinetic energy of the molecules
is not zero.
Such a situation is always the case, when the
vector sum of the momenta of the pieces
constituting the system is zero, but the
individual momenta are not. It is obviously the
case with any device that has parts in motion,
but the device isn't going anywhere as a whole.
In rotary devices, the device will have non-zero
_angular_ momentum, but that is not part of the
question, as in the first example, it is possible
to have systems with zero momentum, _and_ angular
momentum, which still posses kinetic energy. And
they are not limited to a moment, and can be in
that state indefinitely, given the proper
conditions.
Answered by: Yasar Safkan, Ph.D. M.I.T., Software Engineer, Istanbul, Turkey
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