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1) If I place an object on a table, it will sit there forever. There's no change in potential or kinetic energy.

If I hold the object at the end of my outstretched arm, eventually I can't do it anymore. But the object is still not changing kinetic/potential energy.

Where is the loss coming from?

2) Related question: An airplane is flying level in the air. How much power is needed to keep it level?

It seems clear that any explanation will revolve around inefficiency in the force-providing mechanism, something to do with how your muscles work for the first question and something to do with drag in the airplane example.

Qmechanic
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Carlos
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    I've posted a link to a duplicate for your first question. The second question needs to be asked separately as a different question. – John Rennie Jul 28 '14 at 11:31
  • Having a read through that answer, it appears the loss is always a relative movement issue? Strands of muscle in one case, aerodynamic drag in the other? – Carlos Jul 28 '14 at 11:39

1 Answers1

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Concerning the air plane:

to stay at its current altitude, the plane needs to produce enough lift to cancel gravitation. Lift is produced by moving its wings forward which then push down air. To hold its velocity, the plane must compensate for frictional forces on the plane/air.

So the energy required for holding altitude is also the energy required for holding velocity.

M.Herzkamp
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  • How do you get an estimate of how much energy is needed? – Carlos Jul 28 '14 at 11:49
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    That would highly depend on the aircraft in question. Maybe you could take the mileage, energy density of its fuel and its velocity to get an approximation of the current power required to hold altitude. – M.Herzkamp Jul 28 '14 at 11:53