# Have a question!

Printable View

• Jul 2nd 2006, 08:34 AM
tacubo
Have a question!
Two men, joel and jerry, push against an immoveble wall. Jerry stops after 10 min, while Joel is able to push for 5.0 min longer. Compare the work they do.
• Jul 2nd 2006, 09:18 AM
CaptainBlack
Quote:

Originally Posted by tacubo
Two men, joel and jerry, push against an immoveble wall. Jerry stops after 10 min, while Joel is able to push for 5.0 min longer. Compare the work they do.

The each do no work. To do work the force would have to move through
a distance, but here the wall is imovable and so no work is done.

RonL
• Jul 3rd 2006, 04:51 AM
topsquark
Quote:

Originally Posted by CaptainBlack
The each do no work. To do work the force would have to move through
a distance, but here the wall is imovable and so no work is done.

RonL

tacubo, what CaptainBlack told you is the correct "book" answer. Don't think that I'm telling you otherwise. His is the answer your teacher wants to see!

However... :) They both do work. The question above is working ( :D ) with the definition $W = \vec F \cdot \vec s$. So they do no work pushing on the wall since there is no displacement of the wall. BUT the more sophisticated definition of work is "the change in one form of energy to another." Both men are pushing on the wall, which converts chemical potential energy (in their muscles) to mechanical energy (their muscles are contracting or stretching depending on which muscle group you are talking about), even though the wall itself doesn't move. So they actually ARE doing work. This also explains why the weightlifter feels like he's doing work in supporting a barbell at constant height, even though the work formula says he's doing no work.

Don'tcha just LOVE how Physics teaches you one thing one year, then changes the definition for the next school year? :)

-Dan
• Jul 3rd 2006, 05:36 AM
ThePerfectHacker
Quote:

Originally Posted by topsquark
Don'tcha just LOVE how Physics teaches you one thing one year, then changes the definition for the next school year? :)

That is exactly why I hate physics.