Energy expelled in orbit

[Opara]

Hi, this is just a general wondering on a slow day, i'm hoping this is what the forum is used for.

This was actually triggered by a post on perpetual motion the other day.What energy is used for the earth to orbit the sun.I'm sure it's gravitational but what stops the earth the earth being pulled directly towards the sun? The only thing I can think of is the gravitational pull from the moon combined with the pull from the sun gives a resultant force which produces the orbit pattern.

As a second point what keeps it moving, is it the law of inertia? If we were to tie a rope to the moon connected to a turbine, as this rotates the earth every 28 days this would turn a turbine creating energy, where would this energy be from and would it affect the moons orbit? Also is energy expelled during an orbit or does the lack of an atmosphere in space mean that there would be no resistance?

Thanks

[Eric Mc]

The earth's speed around the sun is sufficient to counter balance the direct gravitational pull of the sun.
This is what prevents the earth from falling in towards the sun

That is what an orbit actually is - the balance between the gravitational attraction of two bodies and the velocity of the orbiting body around the central body.

I know that others will now jump in and start saying that this is rather simplistic - and it is. But it gives a basic idea as to what is going on.

[The Black Flash]

Opara said:

Hi, this is just a general wondering on a slow day, i'm hoping this is what the forum is used for.

This was actually triggered by a post on perpetual motion the other day.What energy is used for the earth to orbit the sun.I'm sure it's gravitational but what stops the earth the earth being pulled directly towards the sun? The only thing I can think of is the gravitational pull from the moon combined with the pull from the sun gives a resultant force which produces the orbit pattern.

It's the speed it's moving at. Like a weight being swung around on a string, it's the speed which keeps it from falling inwards. Orbit is often described as "falling towards something, but constantly missing it".
Most (all?) orbits gradually decay, and the spin of orbiting bodies tends to equal out, which all absorbs energy. This is due to the interaction of gravity between the two bodies.

Opara said:

As a second point what keeps it moving, is it the law of inertia? If we were to tie a rope to the moon connected to a turbine, as this rotates the earth every 28 days this would turn a turbine creating energy, where would this energy be from and would it affect the moons orbit? Also is energy expelled during an orbit or does the lack of an atmosphere in space mean that there would be no resistance?

Thanks

The lack of any other force keeps it moving (or rather, means that it doesn't slow down). In your turbine example, the moons orbit and earth's spin would both gradually slow down due to the energy being removed. Eventually, when the moon's speed dropped low enough, it would indeed fall into the earth.

[Opara]

So where did this initial velocity come from, presumably if the earth was stopped then it would be pulled towards the sun rather than an orbit restarting?

What effect would it have if the moon was destroyed?

Thanks

[shakotan]

Opara said:

So where did this initial velocity come from, presumably if the earth was stopped then it would be pulled towards the sun rather than an orbit restarting?

What effect would it have if the moon was destroyed?

Thanks

You are a Bond villain AICMFP!

[Mr E]

Opara said:

So where did this initial velocity come from, presumably if the earth was stopped then it would be pulled towards the sun rather than an orbit restarting?

The spinning cloud of matter that clumped into the start and associated planets...

[mrmr96]

shakotan said:

Opara said:

So where did this initial velocity come from, presumably if the earth was stopped then it would be pulled towards the sun rather than an orbit restarting?

What effect would it have if the moon was destroyed?

Thanks

You are a Bond villain AICMFP!

Class!

But yeah, as others explained the earths speed came from the original dust cloud. I suppose like stuff circling a drain there's a pull on stuff already moving which causes it to spin as gravity pulls it in. Maybe? Ah well, spinny dust cloud results in spinny solar system. It's why all the planets orbit in (roughly) the same FLAT plane around the sun, and all in the same direction of orbit.

Destroying the moon, no matter how evil, wouldn't affect the earth's orbit around the sun. (much. without the moon our orbit would be slightly rounder.)

[Eric Mc]

Opara said:

So where did this initial velocity come from, presumably if the earth was stopped then it would be pulled towards the sun rather than an orbit restarting?

What effect would it have if the moon was destroyed?

Thanks

As has been stated, solar systems begin by dust clouds coallescing around a central proto-star. As the cloud condenses, it will develop a spin. Eventually there will be a vast spinning disk of gas, dust and rocky debris rotating around the new star at the centre.
Over a relatively short period of time (a few million years), the spinning dust cloud will condense into proto-planets, asteroids and comets - the vast bulk of which will continue to rotate/orbit around the central star in the general direction of the original cloud.

[Opara]

Cheers lads

[SMGB]

Eric Mc said:

As has been stated, solar systems begin by dust clouds coallescing around a central proto-star. As the cloud condenses, it will develop a spin. Eventually there will be a vast spinning disk of gas, dust and rocky debris rotating around the new star at the centre.
Over a relatively short period of time (a few million years), the spinning dust cloud will condense into proto-planets, asteroids and comets - the vast bulk of which will continue to rotate/orbit around the central star in the general direction of the original cloud.

Indeed and just as Newtons first law deals with inertia this is the conservation of angular momentum. That fast twirly thing figure skaters do is a very watchable example.

[annodomini2]

Opara said:

So where did this initial velocity come from, presumably if the earth was stopped then it would be pulled towards the sun rather than an orbit restarting?

What effect would it have if the moon was destroyed?

Thanks

The spin has been covered above.

If the moon completely disappeared, we would lose most of our tides and the weather would change dramatically.

The Earth would begin to shift it's tilt on it's axis (this is usually over millions of years) and is due to a gravitational interaction between the Sun and the outer heavy planets (mostly Jupiter)