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19. ssm A lumberjack (mass = 98 kg) is standing at rest on one end of a floating
log (mass = 230 kg) that is also at rest. The lumberjack runs to
the other end of the log, attaining a velocity of +3.6 m/s relative to the shore, and then hops onto an
identical floating log that is initially at rest. Neglect any friction and
resistance between the logs and the water. (a) What is the velocity of the first log
just before the lumberjack jumps off? (b) Determine the velocity of the second log
if the lumberjack comes to rest on it. |
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If we
consider the first log and the lumberjack as our system, then the force
between the lumberjack and the log and the log and the lumberjack are
internal forces. So
there are no external forces, so momentum must be conserved. |
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Note this is in the opposite
direction of the lumberjack’s velocity |
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When the lumberjack jumps, we
can consider this a collision and so again momentum
is conserved. |
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