Sorry if it has been asked before I am sure. Anyways, if I hold a ball in the air, I do no work but I do spend energy ("chemical", heat... whatever happens in my muscle) and get tired. Where does the energy or whatever to cause me to get tired come from? For example how is an electrical charge capable to pull forever and never get "tired"?


This article explains well the mechanism of the skeletal muscles. Even though you don't see macroscopic movement and you expect your power "output" to be zero, because $\boldsymbol{P = F\cdot V}$ There is actually movement in your muscles.

  • $\begingroup$ Thanks for the link! However I am not sure if it answers my questions. I realize that muscles generate a power output to hold a ball stationary and their capacity is limited. My question was how is that my muscles generate a power output but nothing else does because a mass or electrical charge are capable of pulling another stationary mass or charge forever. $\endgroup$ – paradox Nov 30 '17 at 23:32
  • $\begingroup$ As far as Newtonian mechanics are concerned Energy is preserved (Not Force). If a stationary ferromagnet is pulling a stationary piece of iron there is no total acceleration, so there is no power output, so no energy is expended, nothing gets "tiered". To address the "but nothing else does" imagine a car pressed head-on against a wall, if you step on the gas the car won't go anywhere, the tyres might spin in a burnout fashion and the car is obviously consuming energy, the car's components are accelerating/decelerating, but there is no overall forward-backward motion, just like your muscles. $\endgroup$ – K. Kirilov Dec 1 '17 at 0:22
  • $\begingroup$ I guess, the question is what happens in that wall. It comes down to the electrical forces between the wall particles. The car consuming energy but the electrical forces can keep it stationary forever. I find it a bit counterintuitive that there is no "counter-energy" spent. $\endgroup$ – paradox Dec 1 '17 at 1:06
  • $\begingroup$ I'm going to be a bit over simplistic and say if there is no velocity there is no energy transfer. As long as you are not moving the ball/wall none of your energy goes into it. Look at the units: Energy - Joule, Power - Watt which is Joule/second, so Joule = Watt*second, so it takes energy to keep up power (NOT to keep up force). Watt on the other hand is Neuton * meter/second. If you are moving an object at 0m/s, your power output towards that object is 0W, so you can apply any force for any duration, as long as you're not moving it no energy goes into it. $\endgroup$ – K. Kirilov Dec 1 '17 at 9:42
  • $\begingroup$ Viewed from the wall/ball's perspective - they are not moving you, so their power output towards you is 0W, so they don't need to expend energy, because there is no power to keep up. $\endgroup$ – K. Kirilov Dec 1 '17 at 10:09

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