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Re: Atwood's machine problem



On 1 Oct 2004, at 15:07, Herb Schulz wrote:

Howdy,

Consider the system to be the Monkey, Bananas, Rope (assumed massless) and
Pulley (assumed massless and frictionless and rigidly attached to a wall).
When the monky pulls down on the rope there ends up being anet externaL
force on the system through the pivot of the Pulley (i.e., P-2mg not = 0) so
the CM of the system is free to move. However, the net external torque on
the system measured about the pivot is zero so the Angular Momentum of the
System ABOUT THE PIVOT must be conserved and it was zero before. Therefore
the Bananas must have the same velocity (m and r are the same) at any given
instant as the monkey so the two Angular Momentum Vectors add to give zero.

Good Luck,

Herb Schulz
(herbs@wideopenwest.com)

No need to invoke conservation of angular momentum. In the
absence of external forces the CM must move with the constant
velocity it has acquired when the monkey initiated motion.

If dP/dt = 0, P is constant only and not necessarily zero.

Therefore both the monkey and the bananas must move upwards.

regards,
Sarma.

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