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I read the answer to Why does holding something up cost energy while no work is being done?

and wanting to know more, I asked my teacher about it without telling him what I read here. Instead of referring to muscle cells and biophysics, he answered my question in terms of entropy. He told me that while my arm muscles are stretched when I hold the object, they are more ordered. When my arm is at rest and muscles are not contracted, the muscles are less ordered (more entropy). So his conclusion was that the energy is required to keep the system (my arm muscles) from going to a state of higher entropy.

However, the answer in terms of muscle cells doing work on each other (i.e the answer to the hyper-linked question) made more sense to me. Could someone please give me some intuitive sense to my teacher's answer or explain the link between the two answers if there are any...

Community
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Eliza
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2 Answers2

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Your teacher's explanation is incorrect. A simple counterexample can be constructed to illustrate this by considering what happens when the role of your arm is replaced by that of a rubber band.

When a weight is suspended from the ceiling by a rubber band, the band stretches and its polymer chains become more ordered, in exact analogy to your teachers claim for an arm holding a weight. However, the rubber band can suspend the weight indefinitely for as long as you leave it there, and it's obvious that no energy is expended during that time.

The correct answer, as you alluded to, is in biophysics, and the fact that keeping muscle cells contracted requires continual energy supplying; but this is a matter of biology, not physics.

DumpsterDoofus
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Put the object on a table. Nothing happens, table or object. Your meat is metabolizing about 2000 Calories/day basal metsbolism, 8.4 million joules/day. "There is only about 0.1 mole of ATP in the body, but daily energy needs require 100 to 150 moles, or about 50 to 75 kg. The average human adult will use their body weight in ATP each day – so ATP must be recycled as it is used."

http://gasexchange.com/notes/metabolism/

The physics is wholly independent of the biology.

Uncle Al
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