Re: positive and negative work

["Waggoner, Bill" <BWaggoner@METROPO.MCCNEB.EDU>]

First,  I realize you have a right to an arrrrrrrrrrrghh... but is that
really called for?

Second, a person leaning against a wall doesn't have a cm that is
displaced.. so I am not sure where you are going with this.

I guess you are also going to say that the floor wouldn't crush an egg
that was dropped from a height, as it does no work?



-----Original Message-----
From: Joe Heafner [mailto:heafnerj@VNET.NET]
Sent: Friday, November 09, 2001 3:26 PM
To: PHYS-L@lists.nau.edu
Subject: Re: positive and negative work


> From: "Waggoner, Bill" <BWaggoner@METROPO.MCCNEB.EDU>
>
> From an energy point of view the energy of the "system" is decreasing.
> By system I mean the entire person.
Right.

> Assuming no change in KE of the system, I would tell a student the
> normal force of the floor did negative work on the system. (That is if
> we don't view the "system" as a spring that's being compressed.)
NO NO NO!!!!! ARRRRRRRRRRRRRRRRRGH! The force on the student due to the
floor does not act through a displacement, so that force transfers no
energy into or out of the system (student). If it DID, then we could
extract nourishment from just standing on the floor or leaning against a
wall.

Read Arons and/or Chabay and Sherwood.

> Does this necessarily mean the reaction forces of the feet pushing on
> the floor did postive or negative work on the floor. That depends on
how
> you model that surface, and the initial conditions. Is the floor a
> "spring" as well?
See above.


Cheers,
Joe

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