A part of gravity that doesn't make sense to me.

[NIN]

Ok. As many of us know, the larger an object is, the more powerful the gravitation field. Also, the closer two object's are, the stronger the gravitational attraction. So, here's what doesn't make sense to me.

 

Look at the planet Mercury. It's a reasonably large planet, and the closest one to the Sun. It's close enough to stay in orbit, yet far enough not to get sucked in by the Sun's gravity. Then, look at Pluto. It's like 10 times as far from the Sun as Mercury, and way smaller, and yet it doesn't drift off?

 

So, my question is, how is that the Sun's gravity is weak enough to not suck in Mercury, yet strong enough to keep Pluto in orbit?

[ydoaPs]

Mercury is moving faster perpendicular to the gravitational attraction.

[Martin]

Mercury is moving faster perpendicular to the gravitational attraction.

 

Yourdad is right, that's the reason it can stay in a roughly circular orbit. It is going the right speed.

 

At each orbit radius there is a right speed to go, so you don't fall in or drift away. Just to take an example, the approximate orbit radii for some planets listed in astronomical units (au) are:

 

0.39 au, 0.72 au, 1.0 au, 1.5, 5.2, and 9.5 for

Merc, Ven, Earth, Mars, Jupiter, Saturn

 

You can calculate for yourself what the speeds have to be. Just type this into Google search box and press return

sqrt(G*mass of sun/0.39 au)

that will tell you the speed that Mercury is traveling in its orbit around the sun.

 

If you type this into Google and press return

sqrt(G*mass of sun/0.72 au)

then it will tell you the speed that Venus is traveling.

 

What will it compute for you if you type this into Google and press return?

sqrt(G*mass of sun/5.2 au)

 

For every distance from the sun there is a correct speed. If you can find out how far Neptune is from the sun (how many astronomical units or au) then you can use this to find out approximately how fast Neptune is going.

 

If it was going much faster it would swing out away, if it was going much slower it would tend to fall in towards sun, either way, going the wrong speed it wouldn't stay in roughly circular orbit.

[NIN]

Well, that sums that up. Thanks to both of you guys. I understand it now

[Daecon]

So you could do it the other way, input a speed and it'll tell you the ideal distance away from the Sun for a body traveling at that speed?

 

Weird thought: What if it was light speed? Would that describe the radius of an event horizon if you know the mass of a black hole?

[Martin]

... Would that describe the radius of an event horizon if you know the mass of a black hole?

 

At densities where black holes are formed the formulas for orbits change slightly. There is a distance at which light itself can orbit a black hole. Inside that radius nothing can orbit without help from a booster (even going at the max, it would need a constant boost to avoid falling in.)

 

The formulas are different due to relativistic effects at these extreme condtions, but we can still calculate.

 

Google calculator knows the mass of the the earth, so it can tell you the radius of a black hole with earth mass.

 

Let's try this formula

radius = 2GM/c^2

 

Suppose I put in "mass of earth" for M...OK I type this into googlebox and press return:

2G*mass of earth/c^2

 

and I get out 8.9 millimeters. or about 9 millimeters.

 

That means that if the mass of the whole earth could be compressed down to where it would collapse to hole, then it would form a black hole with radius of about 9 millimeters.

 

 

Try this for the mass of the sun. Type this in and press return:

2G*mass of sun/c^2

 

(the formula is called the schwarzschild black hole radius)

Edited November 17, 2008 by Martin

[Mr Skeptic]

Ok. As many of us know, the larger an object is, the more powerful the gravitation field.

 

Just to be pedantic, the more massive an object is, the more powerful its gravitational field. If you have a giant block of foam or cloud of gas, it might not have much of a gravitational field

[Daecon]

So there'd be no call for a planet-sized bouncy castle in space?

[Banshii]

I thought this topic and discussion was pretty interesting; something I really never thought to think about really.

Not that I'm a fan of bringing back an old post or anything.. I just enjoy learning and discussing things that interest me.

 

So if I understand this correctly, it's the "moving" of the planets that are keeping them orbiting around the sun along with the gravitational pulls.

 

Like, if for example, mercury suddenly stopped moving or orbiting, it would just simply get sucked into the sun by the suns and mercury's gravity? Same with Pluto.

 

Maybe kind of like the slingshot effect, but not so much so because the planets aren't moving fast enough for that to happen. So because of their current speed, instead of sling shotting away altogether, their gravities and speed keep them in a constant orbit around the sun. However, if it slowed down or sped up, they'd be sucked in or break orbit and leave the solar system.

 

Did I basically hit it?

[dr.syntax]

I thought this topic and discussion was pretty interesting; something I really never thought to think about really.

Not that I'm a fan of bringing back an old post or anything.. I just enjoy learning and discussing things that interest me.

 

So if I understand this correctly, it's the "moving" of the planets that are keeping them orbiting around the sun along with the gravitational pulls.

 

Like, if for example, mercury suddenly stopped moving or orbiting, it would just simply get sucked into the sun by the suns and mercury's gravity? Same with Pluto.

 

Maybe kind of like the slingshot effect, but not so much so because the planets aren't moving fast enough for that to happen. So because of their current speed, instead of sling shotting away altogether, their gravities and speed keep them in a constant orbit around the sun. However, if it slowed down or sped up, they'd be sucked in or break orbit and leave the solar system.

 

Did I basically hit it?

 

Repy: The way I understand it is that the Earth or any other planet orbiting the Sun is falling toward it at a certain speed and is also falling away from it because the sun is round, at the same speed it is falling toward it. If the speed were to increase it would orbit at a further distance from the sun. If it were to move fast enough it would reach escape velocity and move in a straight line away from it. Does that make sense to you ? ...Dr.Syntax ... I did not mean to suggest anything you said was wrong. This is the way it`s been explained to me

[Banshii]

Yes that makes perfect sense to me. That's really the same thing that was going on in my head but you put it into words a lot better than I did. Like how I mentioned the slingshot thing, you simply said moving away and falling towards at the same time. That's what I was trying to explain but couldn't put it in the right words. Thanks though, that cleared it up

[Mr Skeptic]

dr. syntax, what does that have to do with the sun being round?

 

Basically, the force is perpendicular to the motion, so it causes a curvature in the motion, which is a type of acceleration.

[D H]

Repy: The way I understand it is that the Earth or any other planet orbiting the Sun is falling toward it at a certain speed and is also falling away from it because the sun is round, at the same speed it is falling toward it. ... This is the way it`s been explained to me

Whoever gave you that explanation did you wrong. That is nonsense.

 

Basically, the force is perpendicular to the motion, so it causes a curvature in the motion, which is a type of acceleration.

The force is perpendicular to the motion for a circular orbit only. While Earth and Venus have nearly circular orbits, Mercury's orbit is somewhat eccentric (e=0.206), and comets have highly eccentric orbits (e.g., 0.967 for Halley's comet).

 

A good place to start is Kepler's laws.

1. The orbit of a planet around the Sun is an ellipse, with the Sun at a focus of the ellipse.
   
2. For any given planet, the line between the Sun and the planet sweeps a constant area per unit time.
   
3. The square of the period of a planet's orbit is proportional to the cube of the semi-major axis of the orbit.

 

Kepler's laws answer the question raised in the original post in the sense that they say that planets do not fall into the Sun. They follow elliptical paths instead. The answer provided by Kepler's laws is not particularly satisfying. Why do planets follow ellipses?

 

The answer is because Newton's law of gravitation says that is what the must do. Newton's law of gravitation combined with Newton's laws of motion form a second-order differential equation. The solutions to this differential equation are in the form of conic sections: Circles, ellipses, parabolas, and hyperbolas.