Date: Tue May 26 13:24:36 2009

Author: J. Terrence Klopcic

Subject: Re: Bad Physics ? in Text

Post:

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Right! Hooke's Law. (How else could an inanimate object like a floor
exert a force?)

But, wow, aren't we getting a little minute/obscure for the
algebra-based class for which this discussion was started?

The "correct" answer seems to depend upon what
assumptions/simplifications the prof wishes to make.

Terry

* J. Terrence Klopcic, PhD
Director of Laboratories
Departments of Physics and Mathematics
Kenyon College*



Richard Tarara wrote:
> But the floor deflects when you push down on it. The energy is transferred from your muscles to the floor and then back again. It is the trampoline effect.
>
> Rick
>
> ----- Robert DeSerio wrote:
>
>> I agree with your faculty. No work is done by the normal force
>> as it doesn't act through a distance. W (done by F) = int F dot dx.
>> To simplify without changing the physics too much,
>> just consider a model consisting of a compressed (massless) spring
>> resting on the floor with a point mass attached on top. When the spring is
>> released it exerts a force on the mass and does work on the mass
>> (just as your muscles do work on your mass when you jump) thereby increasing
>> the kinetic energy of the mass. In return, the spring loses internal or
>> potential
>> energy. Since the normal force is exerted by the floor, if the normal force
>> did work, wouldn't the floor have to have lost some amount of internal
>> energy?
>>
>> Regards,
>> Bob
>>
>> Robert DeSerio
>> Department of Physics
>> University of Florida
>> P.O. Box 118440
>> Gainesville, FL 32611-8440
>>
>> Phone: 352 392-1690 (office)
>> 352 392-0524 (fax)
>> Email: deserio@phys.ufl.edu
>> Web: http://www.phys.ufl.edu/~deserio
>>
>>
>>
>> William Maddox wrote:
>>
>>> From: WC Maddox
>>>
>>> A faculty member here who is reviewing the solutions manual for a new
>>> text at the algebra/trig level would like some additional opinions on
>>> this:
>>>
>>> According to the author of the solutions manual if someone jumps up
>>> there is work done by the normal force in the amount mgh as well as
>>> work done by gravity in the amount -mgh where h is how high your
>>> center of mass goes.
>>>
>>> Any comments ? The vote here is 4 to 0 that normal force part is wrong.
>>>
>>> End Message
>>>
>
>
>

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Right! Hooke's Law. (How else could an inanimate object like a floor
exert a force?)



But, wow, aren't we getting a little minute/obscure for the
algebra-based class for which this discussion was started?



The "correct" answer seems to depend upon what
assumptions/simplifications the prof wishes to make.



Terry



* J. Terrence Klopcic, PhD
Director of Laboratories
Departments of Physics and Mathematics
Kenyon College*






Richard Tarara wrote:
> But the floor deflects when you push down on it. The energy is transferred from your muscles to the floor and then back again. It is the trampoline effect.
>
> Rick
>
> ----- Robert DeSerio wrote:
>
>> I agree with your faculty. No work is done by the normal force
>> as it doesn't act through a distance. W (done by F) = int F dot dx.
>> To simplify without changing the physics too much,
>> just consider a model consisting of a compressed (massless) spring
>> resting on the floor with a point mass attached on top. When the spring is
>> released it exerts a force on the mass and does work on the mass
>> (just as your muscles do work on your mass when you jump) thereby increasing
>> the kinetic energy of the mass. In return, the spring loses internal or
>> potential
>> energy. Since the normal force is exerted by the floor, if the normal force
>> did work, wouldn't the floor have to have lost some amount of internal
>> energy?
>>
>> Regards,
>> Bob
>>
>> Robert DeSerio
>> Department of Physics
>> University of Florida
>> P.O. Box 118440
>> Gainesville, FL 32611-8440
>>
>> Phone: 352 392-1690 (office)
>> 352 392-0524 (fax)
>> Email: deserio@phys.ufl.edu
>> Web: http://www.phys.ufl.edu/~deserio
>>
>>
>>
>> William Maddox wrote:
>>
>>> From: WC Maddox
>>>
>>> A faculty member here who is reviewing the solutions manual for a new
>>> text at the algebra/trig level would like some additional opinions on
>>> this:
>>>
>>> According to the author of the solutions manual if someone jumps up
>>> there is work done by the normal force in the amount mgh as well as
>>> work done by gravity in the amount -mgh where h is how high your
>>> center of mass goes.
>>>
>>> Any comments ? The vote here is 4 to 0 that normal force part is wrong.
>>>
>>> End Message
>>>
>
>
>

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