# Can the moon orbit be used as perpetual motion?

• B
I'll attach a picture of what I mean but essentially I wanted to know if it was possible to (under circumstances where the earth and moon do not change angle of rotation when orbiting the sun) create a perpetual motion machine using orbit as a means to turn a giant gear circling the earth in order to create energy?

If the answer is obvious im sorry but I was just wondering about it and I don't follow physics very well

thanks

for some reason the IMG isnt posting so here's the link
https://gyazo.com/47bea0dd5f475e34e7aedb43b658eac2

russ_watters
Mentor
I'll attach a picture of what I mean but essentially I wanted to know if it was possible to (under circumstances where the earth and moon do not change angle of rotation when orbiting the sun) create a perpetual motion machine using orbit as a means to turn a giant gear circling the earth in order to create energy?

If the answer is obvious im sorry but I was just wondering about it and I don't follow physics very well

thanks

for some reason the IMG isnt posting so here's the link
https://gyazo.com/47bea0dd5f475e34e7aedb43b658eac2
That looks really expensive and impractical. Fear not, though, because we already have something that does the same thing: tidal power.

But please note that tidal power is considered "renewable" which is commonly presumed to mean "perpetual", but really isn't.

gneill and anorlunda
gneill
Mentor
An orbit doesn't "propel" the orbiting object. The object is just continuously falling and missing the ground

Every orbit has a finite amount of mechanical energy associated with it. Steal the energy from an orbit and eventually you don't have an orbit anymore, you've got two colliding objects.

At least @russ_watters ' tidal power offering has limits on how much energy can be stolen (it's a very big amount, but still limited). Eventually the (Lunar) tides will cease when the Earth's rotation becomes tidally locked with the Moon's orbit. The Earth will then always present the same face to the Moon. I wonder which hemisphere will inherit the Moon? Don't bother placing bets, the outcome won't be known for some billions of years.

krater and davenn
Drakkith
Staff Emeritus
Even if we could design and build such a device, it would slowly pull the Moon towards the Earth as it saps gravitational potential energy from the Moon. So no perpetual motion here.

davenn
Merlin3189
Homework Helper
Gold Member
Apart from all the other problems, may I just object to the statement that, "the orbit propels moon."
An orbit is simply a description of where something goes. What causes it to go there are things like the force of gravity from the earth.
If you add other forces, such as pulling on a wire, then you will change the orbit. The orbit is a result, not a cause.

krater, davenn and gneill
rbelli1
Gold Member
The Earth will then always present the same face to the Moon.

Only if the Earth-Moon system survives the death of the Sun.

BoB