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Re: positive and negative work



But the systems cm is being displaced, both vertically and horizontally.

-----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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