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Double Atwood



I wonder if someone might help with the following student comments about
this problem given in my AP class this week.

A pulley (A) is attached to the ceiling with a string passing over it.
On the right end of the string hangs a mass M and on the left side hangs
another pulley (B) with a string holding a mass of 2kg on one side and a
mass of 3kg on the other side. The question is to find the mass M in
order that the 2kg mass remains stationary when the system is released.

I solved this in a pretty conventional manner making the downward
acceleration of pulley B equal to the upward acceleration the 2kg mass.

The student asks two questions:

1. Are the calculated tensions in the string around pulley B really
correct since the system of pulley B and its masses is an accelerating
system? Does this affect the answer to the problem?

2. Isn't pulley B behaving as a simple machine of mechanical advantage 2
so that movements in the string around pulley A are not equal to the
movements of the string around pulley B? Does this affect the answer to
the problem?

Any clarifying thoughts would be appreciated.

Thanks,
--
David Abineri dabineri@choice.net