Where Energy is Lost?

einstor

I am still confused about this.
say, we are in a constant moving truck, we pushed the back of the truck, so we provide the force F on the walls of the truck, towards the rear. but the truck moving with constant velocity and has traveled a distance of say x. we do the work according to an inertial frame (the earth) is =-Fx, but what if we use our truck frame, according to the truck, we do not do work, but why do we become tired if you continue doing that work, where energy is lost?

vlado_skopsko

The energy depends on your reference frame. So from the truck point of view you dont do any work unless you open the door or deform the metal ( W=F*x - no x no work done ). And you feel tired because you produce heat energy by the way the muscles work (they are contracted by electric pulses, so they contract and relax many times in a second).
If you hold a a book on some constant height you will get tired without doing any work, and a table can do it for a long time.

russ_watters

That internal force has no effect on the motion of the truck. What's relevant isn't the truck's motion with respect to the ground, but your motion with respect to the truck (none). Your body is inefficient. It requires and energy input just to generate a static force.