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# Kostas

03/19/2009 5:33 PM

If a man holds in place, with his hand, a rotating object soon he gets 'tired', i.e. he expands energy to keep the rotating object in its 'orbit' although, according to the theory, he is doing no 'work'!!! (Assume that there is no 'friction' of any kind and once the object is given the initial push it keep rotating ... )

Now, the earth is 'holding' the moon in its orbit by the gravitational force: does the earth 'get tired', i.e. does it expand energy? If no, what's the diefference between earth and man? If yes, where does the energy come from?

Pathfinder Tags: Energy Gravity orbit work
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#1

### Re: Kostas

03/19/2009 8:01 PM

If you consider someone keeping an orbitating object with a line, you'll need to put energy in to compensate for atrict with air, internal attrict in the line deformation, etc. If you invent a line with no power loss, once initialized the movement, it'll be kept indefinitely without energy addition, what is called angular uniform movement. If energy is added, a change in the equilibrium is observed. If you loose the line, the object will just keep its instantaneous speed vector, going out in a straight orbit with constant speed, what is called straight uniform movement - with zero acceleration or actually in the case, it will be a parabola with the gravity acceleration.

There's a number of concepts and some formulas that are used to describe it (kinetic energy, gravitational potential energy, angular movement energy quantity), but I'll avoid it.

Now, in the case of the earth and the moon, both have speed. The same gravitational potential attraction hold them in close orbit, in which the tendency of the moon of getting out in straight uniform movement is balanced.

If no attrict is considered (almost there...), the speed will not be reduced, so, the same path will be followed. You'll need to spend energy to CHANGE the speed or orbit, not to MANTAIN it.

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#3
In reply to #1

### Re: Kostas

03/20/2009 9:40 AM

Thanks bhrescobar for the try but actually my question has to do with some ideas 'outside' the 'customary' theory. Simply put, a man gets 'tired', i.e. spends energy, for keeping an object in 'orbit'. Logically, the earth should get equally 'tired', i.e. spend some energy, for keeping the moon in orbit but according to the 'customary' theory the earth does not spend energy! Why so?

Looking at it differently: Why 'Work' was defined in such a way so that a man holding a weight does no work and yet he does spend a lot of energy, we know that! he is sweating, i.e. heating up; he gets 'hungry' needs to replenish his 'energy'...

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#2

### Re: Kostas

03/20/2009 4:22 AM

Actually, the Moon is getting further away by a few mm per orbit. The energy it requires to do this is coming from a gradual slowing down of the rotation speed of the Earth/Moon system.

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#4
In reply to #2

### Re: Kostas

03/20/2009 10:14 AM

Hello PWSlack and thanks for your input.

If the moon is getting away it means that the force keeping them 'together' is not big enough. In this case the moon accelerates 'outwards' which is equivalent to saying that it is accelerating 'inwards' less, following its natural inclination to go 'straight'.

To me it seems/feels that you need energy to keep an object rotating in orbit and not when you let it 'fly away' as it happens when the 'string' holding a bucket in 'orbit' breaks; i.e. the 'energy' needed to keep the moon in circular orbit around the earth is greater than if the moon gradually gets away from the earth.

My question was/is : where does the earth find all that energy to keep the moon rotating around it?

I guess my question comes down to : why is there 'gravity', i.e. gravitational force... and i guess noone really knows yet...

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#5
In reply to #4

### Re: Kostas

03/21/2009 9:19 AM

I see...

It does not take energy from anywhere. It's still the same concept: the moon and the earth, both, still "hold" energy from its creation in form of speed. Of course, I am partidary to the big bang theory.

Regarding your comment about "work", remember, work, as stated, is a physical concept, a definition, to help describe physical phenomen in mathematics. When someone holds a brick, it doesnt mean that he is not spending energy or getting tired just because the calculated physical-defined work is zero, it just mean that the brick is not being displaced from its position in the same direction the forces that are in equilibrium holding it in place. Like, W = F . d . In this example, the work of the mans hand force over the brick is zero, as well as the work of the weight force of the brick. The work of the attractive force of the moon over the earth is zero, or a little negative, because its displacing slowly in the same way of the force, opposite diraction. The same for the attractive force of the earth over the moon, and the force of the sun over the earth, and so on.

Everything is in an equilibrium resulting from millions of years of universe expansion.

Work, only in "human" feelings, mean something you have to maje some effort to do.

Confusing? No... just basic physics.

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