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I hope this question won't be ill received. I have a rather simple question in physics. I never got a satisfying answer to the following question: If a man holds a massive body in the air such that it is at rest with respect to the earth, then according to Newtonian Physics no work is being done. However everyone knows that this is a process which consumes energy. I thought of resolving this in a few ways:

One way was to say that it has to do with the fact that biological systems need to spend energy in order to exert a force.

The other one is that it is actually related to the equivalence principle. I read in wikipedia (I hope this source is reliable)

According to the equivalence principle it (a body resting in a gravitational field) should be indistinguishable from a particle in flat space being accelerated by a force.

So can the expenditure of energy in the case of a man holding a mass be explained by invoking the equivalence principle and saying that the situation is equivalent to the man exerting a force on the body in flat space? If the second explanation would be correct, then that would mean that the friction (electrostatic forces) of table would have to continuously lose energy in order to prevent a massive body from falling to the ground. That would be absurd?

Qmechanic
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eeqesri
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1 Answers1

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Your first statement is the true one. In order to counteract the object's gravity, your muscles must be held in tension, which ultimately means that there are moving fibers and cellular structures that are consuming energy. Muscle engagement would be why you have to work to hold something out perpendicular to your body, while holding it aloft in the palm of your hand is not doing any work, because you are leveraging your skeleton to provide the force.

Zo the Relativist
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