Why do we feel tired when holding something without any movement?

[Vicol]

Hello everyone :)

Could someone explain me how is it possible that we "lose energy" when we hold something in our hands though there is no shift/displacement? We feel it physically - tiredness.

I can generalize this question - to keep something at certain height we have to use force. But work is scalar product of force and shift, so in this case it seems to be zero. Do we need energy to "generate" force which keeps something in fixed possition?

 

[BvU]

Yes. Same as pushing against a sturdy wall. Maintaining muscle tension requires metabolism to use up calories but doesn't yield more potential energy so physically you don't do work.

 

[Doc Al]

But work is scalar product of force and shift, so in this case it seems to be zero.

The mechanical work done on the object is zero, as there is no displacement.

Do we need energy to "generate" force which keeps something in fixed possition?

Yes we do, as we are biological systems. It requires chemical energy ("food") for us to keep our muscles under tension.

Of course, you could just place the object on a shelf.

 

[Vicol]

Our muslces work like spring, don't they? I thought we lose energy only to squeeze spring, not to mantain it squeezed :)

 

[Doc Al]

Our muslces work like spring, don't they?

Not really. Your muscle fibers continually contract and relax to maintain tension. That takes chemical energy.