Physics Help Forum source of energy

 Energy and Work Energy and Work Physics Help Forum

 Mar 26th 2009, 02:58 AM #1 Senior Member   Join Date: Mar 2009 Location: Yadupatti Bazar,Sitamarhi Posts: 137 source of energy Let us consider a body is at a height of 2m from the earth.According to definition it has potential energy of magnitude mgh and kinetic energy is zero.Now what is the source of its energy ? is it due to gravitational force of eath or is any reason. What will be its potential energy if i bring it in universe? Now if body starts falling towards the earth,then P.E. will convert into K.E. On the earth velocity is zero.Hence P.E. and K.E. both will be zero.Is body now energyless.If it is mass, then it was at the all time. Further, on the earth ,body is at rest, but earth is moving?what about this?
 Mar 26th 2009, 04:12 AM #2 Senior Member   Join Date: Mar 2009 Location: Yadupatti Bazar,Sitamarhi Posts: 137 I didn't get any help till now? Please help anyone. Thanks for that
 Mar 26th 2009, 06:08 AM #3 Physics Team   Join Date: Feb 2009 Posts: 1,425 Yes the source of energy is due to the grav attraction of the earth. Work has to be done against this to raise it to the height h. It already is in the universe! All things known to us are already in the universe. If you mean Outer space even then the major contribution will be from the earths field. else you will have to calculate its P.E. with respect to all the other objects inthe universe and then total it. When it hits the earth the K.E. gets converted into sound energy, heat energy etc. so the total energy remains constant. So the energy of the body now wil be equal to its rest mass only. The earth always was moving .If you wish to consider its energy then include it in the problem right at the beginning and the total energy will be found constant. Whwnever you consider any problem fix a frame of reference first.
 Mar 26th 2009, 11:45 PM #4 Senior Member   Join Date: Mar 2009 Location: Yadupatti Bazar,Sitamarhi Posts: 137 thanks for reply. But in outer space if I want to calculate P.E. (say w.r.t MOON),then which formula we should use?
 Mar 27th 2009, 03:29 AM #5 Physics Team   Join Date: Feb 2009 Posts: 1,425 Well if there is a mass m very close to the moon, ( at a small height compared to its radius), then you can use P.E = mgh where g here is the acceleration due to gravity on the surface of the moon (which is roughly 1/5th that on earth i think) A more general formula is Grav P.E. = -G M m / R where M = mass of the moon, G = universal gravitation constant and R is the distance from the centre of the moon.
 Mar 28th 2009, 12:05 AM #6 Senior Member   Join Date: Mar 2009 Location: Yadupatti Bazar,Sitamarhi Posts: 137 thaks! You have given a -ve sign in ur formula. what indicates it? consider that there are only two heavenly bodies EARTH and MOON. can we say that the total P.E. in the body=P.E. w.r.t. EARTH + P.E. w.r.t. MOON ? if it is,then practically we can say that a body in outer space has unlimited P.E. right or wrong? pls help me.
 Mar 28th 2009, 01:33 AM #7 Physics Team   Join Date: Feb 2009 Posts: 1,425 If there are only two heavenly bodies, what you say is right. However P.E. is not unlimited in outer space. As you go further away R increases so the magnitude of this energy decreases.

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