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# Re: [Phys-L] Rotational kinetic energy and work

Sorry! This was replied to the wrong thread!
I'm getting old! It was meant for Carl Mungan's spherical cap thread.

-----Original Message----- From: Bob Sciamanda
Sent: Monday, June 29, 2015 5:37 PM
To: Phys-L@Phys-L.org
Subject: Re: [Phys-L] Rotational kinetic energy and work

OK, I'll milk it even further ==>
Consider the standard 2 parallel plate capacitor ==> C= A/(ed). Insert an
infinite plane midway between the capacitor plates and parallel to them.
This changes nothing electrical, but we now have two capacitors in series
and each has a capacitance C1 = C2 = A/(eaD) , where D = d/2, so C1 = C2 =
2C. But C1 and C2 are in series, so the end to end series capacitance is
Cseries, where 1/Cseries = i/C1 + 1/C2 . ==> 1/Cseries = 1/(2C) + 1/(2C) .
==> Cseries = C. So whether the situation is two plates or two spheres, or
two whatever, you can always insert the midstream plate and get an
equivalent series model.

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
treborsci@verizon.net
www.sciamanda.com

-----Original Message----- From: Lulai, Paul
Sent: Monday, June 29, 2015 4:39 PM
To: Phys-L@phys-l.org
Subject: Re: [Phys-L] Rotational kinetic energy and work

Bob,
Thanks for continuing the conversation! I had not (obviously) put that
together in this instance.
Have a great day.
Paul
On Jun 29, 2015 2:02 PM, "Bob Sciamanda" <treborsci@verizon.net> wrote:

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
To: Phys-L@Phys-L.org
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
kinetic energy.

On 6/29/2015 1:37 PM, Lulai, Paul wrote:

Hello.
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
done
to the system (from outside the system) to change the energy of the
system.
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.
When
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?