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Tony's problem



On Fri, 20 Mar 1998 10:25:20 Tony Wayne <wayne@pen.k12.va.us> wrote:

I got a problem out of an older physics text (one without a solutions
manual). My students and spent a while debating the method of attaining
the solution. Can someone explain how they would solve it -using energy.

PROBLEM
Masses of 3 kg and 2 kg are hanging at opposite ends of a cord that
passes over a frictionless pulley. The system is held stationary for a
while, then released. What will be the speed of the masses when the 3 kg
mass has descended 0.5 m below its starting point? (USE ENERGY
RELATIONSHIPS TO SOLVE.)

Hi Tony:

Since you are insisting on the ENERGY approach I assume you know how
to solve this problem otherwise (net mass to accelerate=5 kg and net
force=9.8 N). The same answer should be obtained by equating the lost
potential energy, (m1-m2)*g*h,to 0.5*m*v^2. The mass of the pulley is
not given. It means it is negligible.
Ludwik Kowalski