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Paul, I take particular note of your final puzzlement:
"All forces are internal. No external
work is done to change the kinetic energy of the system."
I think you need to recall that external "work" is required to change the
"Center of Mass KE" (.5MV^2, where M is the total system mass and V is the
CM speed) - and that "work" is the line integral of external forces over
the CM trajectory. It takes an external force to accelerate the CM.
Otherwise, "internal KE" can be changed by internal forces. I think this
is what your subconscious was fuzzing over.
-----Original Message----- From: Philip Keller
Sent: Monday, June 29, 2015 2:17 PM
Subject: Re: [Phys-L] Rotational kinetic energy and work
To pull the weights in, the student did work. To get the energy to do
that work, they ate their Wheaties. The Wheaties are also part of the
system. Some of the decrease in bond energy went into the increased
On 6/29/2015 1:37 PM, Lulai, Paul wrote:
Ive run into a problem with my understanding of systems and energy.
Ive thought the if a system is to change its energy, then work must be
to the system (from outside the system) to change the energy of the
I hit a ball. I do work ti the ball giving it energy.
I have an example i cannot resolve.
The system is a person holding weights on the standard spinning chair.
the person pulls the weights in, the moment of inertia decreases and the
rotational kinetic energy increases. All forces are internal. No external
work is done to change the kinetic energy of the system.
What am i missing?
Thanks for your help.