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

["Harvey S. Leff" <hsleff@csupomona.edu>]

On 01/08/2012 06:42 PM, Paul Lulai wrote:

The net internal work is zero. The work done by hand on ball is the negative of work done by ball on hand. This assumes typical contact forces between hand and ball.

Any energy that is sound or heat is ephemeral, and ends up ultimately as internal energy of the total system. The relevant system equation is

U(initial) + K(initial) = U(final) K(final) = U(final)

where U(final) is "after the dust settles;" i.e., after all thermodynamic processes cease, and K(final) = 0. 

It is important to note that for deformable systems, which have variable internal energy, work is not needed to initiate motion of a ball or to stop it. All that is needed is a nonzero force on the ball. 

Consider for example pushing away from a rigid wall while on roller skates. Work on you is zero if the wall is rigid, and momentum change follows from Newton's second law, F = dp/dt, where p is your momentum, and F is the force of the wall on you.

Harvey Leff
Portland, OR
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On Jan 8, 2012, at 5:42 PM, Paul Lulai wrote:

> 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.