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*From*: "Bob Sciamanda" <treborsci@verizon.net>*Date*: Mon, 29 Jun 2015 17:50:32 -0400

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.

Thanks for your help.

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?

Thanks for your help.

Paul.

_______________________________________________

Forum for Physics Educators

Phys-l@www.phys-l.org

http://www.phys-l.org/mailman/listinfo/phys-l

Bob Sciamanda

Physics, Edinboro Univ of PA (Em)

treborsci@verizon.net

www.sciamanda.com

**References**:**[Phys-L] Rotational kinetic energy and work***From:*"Lulai, Paul" <plulai@stanthony.k12.mn.us>

**Re: [Phys-L] Rotational kinetic energy and work***From:*Philip Keller <pkeller@holmdelschools.org>

**Re: [Phys-L] Rotational kinetic energy and work***From:*"Bob Sciamanda" <treborsci@verizon.net>

**Re: [Phys-L] Rotational kinetic energy and work***From:*"Lulai, Paul" <plulai@stanthony.k12.mn.us>

**Re: [Phys-L] Rotational kinetic energy and work***From:*"Bob Sciamanda" <treborsci@verizon.net>

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