Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-l] check your work (and kinetic energy)



Regarding work, energy and systems...

A person catches a ball. I include all objects within my system. I consider the initial case to be the ball in motion and the final case to be the stopped ball.
Initially, all energy is kinetic.
In the end case; some energy has left as heat, some energy did work to the person's hand catching the ball, maybe some sound etc...

If the person catching the ball is in the system, is it fair to say their was work done on something within the system? How do I account for that in my energy balance sheet?

Initial: 100 kinetic
Final: 20 heat; 5 sound; 75 work.
But the 75 units of work done to the hand... If the hand is in the system, the work is done to something in the system, so shouldn't it be energy of some sort then?
I have convinced myself that it doesn't have to become energy within the system by wondering the extreme case. If I catch (without a glove) a fastball thrown by CC Sabathia, I'd likely break something. So I suppose that in that case, the work the ball does to my (now broken) finger is likely energy that can't be considered heat.

The level at which I teach, most work done is typically done to something outside of a system. If not, the collisions are typically ideal (like ideal gases in thermodynamics) and all of the work that one particle would do to another typically becomes kinetic energy. Most of the time, if energy is lost in a collision like this, we typically read, state, or assume that the lost (un-useable) energy is thermal.

Thanks for your insight.

________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] on behalf of brian whatcott [betwys1@sbcglobal.net]
Sent: Saturday, January 07, 2012 11:14 PM
To: phys-l@carnot.physics.buffalo.edu
Subject: Re: [Phys-l] check your work (and kinetic energy)

On 1/7/2012 9:08 PM, curtis osterhoudt wrote:

Presumably, the same "extra-system object" that did work on the car to accelerate it in the first place (i.e. the ground) is doing the work on the object to slow it down. I must admit I thought about it for more than a femtosecond, but how is the ground NOT doing work on the car to slow it down if the brakes are applied?


I'm probably missing something when I say: when the car moves right,
the Earth necessarily moves left. Later, with the brakes applied,
the car slows to a halt, the Earth moves to the right, and warm air from
the disks wafts into the breeze.

Brian W
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l