**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!Â Hooke's Law.Â (How else could an inanimate object like a floor

exert a force?)Â

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.

Â Â Â Â Â Â Â Â Â Â Â Â Â Terry

*Â Â Â Â Â Â Â Â Â Â Â Â Â J. Terrence Klopcic, PhD

Â Â Â Â Â Â Â Â Â Â Â Â Â Director of Laboratories

Â Â Â Â Â Â Â Â Â Â Â Â Â Departments of Physics and Mathematics

Â Â Â Â Â Â Â Â Â Â Â Â Â 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

>

>> 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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