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Re: momentum before force



Jack
You wrote:
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I wanted to sharpen the point. Perhaps a better way
to have done it would have been to ask for, or do, a derivation
of the rocket equation. Surely it involves d/dt(mv). No?
The sand leaking out of the wagon doesn't "really"
involve a change of mass. What is happening is that the mass is
just occupying a greater volume as it spreads itself out (until
it gets to the sidewalk -when there is no interaction with the
wagon). The mass "change" for the rocket is that a part of the
rocket's mass changes its velocity relative to the remainder of the
mass.
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It is my understanding that the rocket equation can not be
derived via d/dt(mv), where m and v are the mass and velocity
of the rocket + unspent fuel, respectively The rocket
equation can be derived by equating the change of momentum of the
system (rocket + unspent fuel + spent fuel) with the impulse acting
on the system by the net external force.

If the sand is replaced by water and if the hole is drilled
horizontally through the lower part of the back of the wagon then
the wagon is a rocket. However, all the arguments that I
previously made still remain valid.

Gene