Continuing on John Denker's theme of energy that cannot do work when
is energy is the ability to do work:
This problem has bothered me for some time -
How much work is done during one repetition of a weightlifting exercise?
Keeping it simple, let's say a 1000 N barbell is bench pressed 0.5 m
up and then down at roughly constant speed. The bar was replaced on
the rack with a polite minimum of clanking.
The change in GPE for the bar is simple, it increases 500 J and then
decreases 500 J.
What energy expenditure does the person make? During the positive
part of the rep. upwards the person's chemical potential energy
decreased by 500J or they transfered 500 J to the bar (everyone happy
with the terminology of energy changes? no! - didn't think so).
On the negative stroke downward ( considered by many weightlifters to
be the more important in developing muscle mass and strength) the bar
is losing GPE, but the lifter is still pushing and decreasing their
supply of CPE to lower the bar in a controlled manner. This negative
controlled stroke is called the burn when the muscles are fatigued
and almost at the point of failure; it is believed by most to be the
process by which growth is stimulated. The decrease in GPE of barbell
is not available to help lower the weight as it might be it the bar
was raised and then lower by a counter weight system.
It would seem that the lifter must do work to gain energy.
How do I total out the work done by the lifter? If I am traditional
with my signs and I use W = Fd with d defined as displacement (as it
is in many but not all books), I get the lifter did no work - this
makes no sense to me and seems physically unreal.
As I see it conceptually, the decrease in CPE of the lifter due to
the exercise (barring other metabolic functions and efficiency
losses) was 1000 J from one meter of pushing 1000N. In addition, the
lifter increased in unusable thermal energy 500 J from the decrease
in GPE of the bar. It will now be necessary for the lifter to
transfer 1500J of thermal energy to the surroundings to maintain body
temp.
Has this ever been tested on a man sized calorimeter?
Comments?
Scott
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Scott Goelzer
Physics Teacher
Coe-Brown Northwood Academy
Northwood NH 03261
603-942-5531e218
sgoelzer@coebrownacademy.com
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