We now proceed to tackle energy conservation for a weigth lifter.
Consider a work-out in which a
m= 50 kg mass is taken from its stand, lifted up and down
N=20 times
and then returned to its stand. No change in mechanical energy but body is only
% efficient and cannot absorb mechanical energy like a spring hence the internal
energy changes by amount
Putting in numbers we get
This energy ultimately comes from oxidation of food molecules. The glucose
sugar molecule for example is oxidized as follows:
The process yields approximately 3.81 Cal=16 kJ/g
for metabolic use. note that in the life-sciences
the unit 1 Cal=1000 cal= 4186 Joule is typically used. Thus a diet coke
harmlessly contains ``one calorie'' which in fact means 4186 Joule.
Other types of food yield
To replenish his supplies the person thus needs to eat
of glucose. All this energy is ultimately converted into heat since there was
no mechanical energy gain. If the work was done in 2 minutes then in order for the
body temperature to stay constant heat must be radiated at a rate
You may have a feel for 408 Watt from the light and heat generated by a 75
Watt bulb. It seems like a high power level but in fact a 70 kg man resting consumes energy at approximately 80 W
so during the workout the energy consumption associated with lifting
the weight exceeds the base metabolic rate by a factor 5.