Possible Duplicate:
Why does holding something up cost energy while no work is being done?
We all know the following Newtonian formulas:
PotentialGravitationEnergy=mgh
Kinetic Energy = 0.5m * v^2
W = F * distance
Therefore, if I put an apple on a table, it will not gain nor lose [potential] energy - because F = 0, and distance = 0.
But when I'm not using a 'mechanic lock' like that - but using my body muscle [without locking the bone] to hold the apple in the air - I'm damn right applying energy - my muscle is getting warmer [because glucose burning is very inefficient] - and I feel it stretching.
I DO apply a force towards up - but it's not a 'free force' like the Normal force - but I give no energy to the apple.
Can anyone please explain to me in detail what is actually happening? Thank you very much!
