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 Jul 7th 2015, 03:39 AM #11 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 708 Since I am trying to help someone, rather than get involved in an argument I tried to use gentle language and certainly have not directly blamed anyone. I see no cause for blame here anyway. I do not know what answer is expected in Pre-University Physics and your post#8 analysis is correct as far as it goes. But I do not believe, even at high school level, that we teach or want to teach that energy comes from nowhere. Yes, in straight and level flight in uniform motion, the kinetic and potential energies are constant ie there is no change there. However for both birds and aircraft continual energy generation and expenditure is necessary in order to maintain that flight. A simple balance of forces show why this is so. Without going into the mechanics of flight (which is different for birds and aircraft) A 'lift force' is required to balance the gravitational force. This lift force is generated by interaction between the bird or aircraft and the air. Unlike gravity which does not require energy to generate it, the lift force is generated by expending biochemical energy in the case of the bird and plain chemical energy in the case of the aircraft. So all the while that the bird is flying forwards it has biochemical energy is being changed into muscular activity and thence into lift. This is no different from realising that a battery turns chemical energy into electrical energy all the while that a circuit powered by the battery is switched on. As soon as forward motion ceases and the bird stoops the need for biochemical energy stops and so is switched off, like the battery, and potential energy is converted to (vertical) kinetic energy. I have put the vertical in brackets because strictly energy is a scalar. This is my explanation at high school level.
Jul 9th 2015, 12:38 PM   #12
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 Originally Posted by studiot But I do not believe, even at high school level, that we teach or want to teach that energy comes from nowhere.
Energy is not a thing that it had to come from a particular place. Energy is merely a bookkeeping system and nothing more. So there is no physical meaning to say that energy came from somewhere. As Feynman said in his Lectures We do not have a picture that energy comes in little blobs of a definite amount. It is not that way.

The OP would be wise to learn what energy is by reading the webpage I wrote about it at:
http://home.comcast.net/~peter.m.bro..._is_energy.htm

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