How does gravity work?

[Eric Mc]

That's a shame - because his guests are very informative.

Listening as I write.

[TwigtheWonderkid]

deckster said:

or, to put it another way, if you set off from Pluto to the sun, then you won't pass Jupiter until you're 80% of the way there. So fundamentally the sun is not 'much further away' from Pluto than Jupiter is; the distance is, to a degree of approximation, pretty much the same. As the sun is about 1000x the mass of Jupiter, the small difference in separation is vastly outweighed (geddit) by the difference in mass.

Wow, just looked up the figures and you're right. Going from Pluto to the sun, when you get to Jupiter you're 87% of the way there!

Mind blown.

[Boring_Chris]

TwigtheWonderkid said:

deckster said:

or, to put it another way, if you set off from Pluto to the sun, then you won't pass Jupiter until you're 80% of the way there. So fundamentally the sun is not 'much further away' from Pluto than Jupiter is; the distance is, to a degree of approximation, pretty much the same. As the sun is about 1000x the mass of Jupiter, the small difference in separation is vastly outweighed (geddit) by the difference in mass.

Wow, just looked up the figures and you're right. Going from Pluto to the sun, when you get to Jupiter you're 87% of the way there!

Mind blown.

Are you talking about distances across the solar system?!

http://joshworth.com/dev/pixelspace/pixelspace_sol...

^ try that

[TwigtheWonderkid]

Boring_Chris said:

Are you talking about distances across the solar system?!

http://joshworth.com/dev/pixelspace/pixelspace_sol...

^ try that

Yes, it's mental. I knew it was big, and that charts of the solar system are ridiculously out of scale, but for some reason (probably because I'm a bit thick) I thought that Jupiter was about 60% of the way from the sun to Pluto.

[RobM77]

Boring_Chris said:

Are you talking about distances across the solar system?!

http://joshworth.com/dev/pixelspace/pixelspace_sol...

^ try that

I like that!

[Hugo a Gogo]

I've seen a good demonstration for kids across school fields

it was done half the size of this scale, with a football for the sun, earth is 75m away, pluto ends up nearly 3km away
http://www.lpi.usra.edu/education/explore/solar_sy...

[Nimby]

Hugo a Gogo said:

I've seen a good demonstration for kids across school fields

it was done half the size of this scale, with a football for the sun, earth is 75m away, pluto ends up nearly 3km away
http://www.lpi.usra.edu/education/explore/solar_sy...

There's a list of scale model solar systems here.
You either need very tiny Sun and planets, or a lot of room.

[Halmyre]

If London is the sun, and Edinburgh via the A1/M1 is Pluto, then Earth is somewhere around Edgware. Mars is where you meet the M25. Jupiter is Milton Keynes.

[Moonhawk]

ash73 said:

Is there still one in the grounds at Jodrell Bank? I remember pacing it out as a kid.

Yes - I believe it is still there (was the last time I visited).

[RobM77]

There's quite a good one where we often go walking in the Austrian Tirol. At ground level there's a big shiny ball for the sun, and then as you walk up the mountain you pass scale models of each of the planets. Because it's such a steep mountain it gives you a really good sense of relative distance.

[TwigtheWonderkid]

Halmyre said:

If London is the sun, and Edinburgh via the A1/M1 is Pluto, then Earth is somewhere around Edgware.

If the Earth is Edgware, then Burnt Oak is the end of the Earth. It's all beginning to make sense.

[ATG]

deckster said:

TwigtheWonderkid said:

If gravity didn't exist and the sun and Pluto were both magnets, would they move towards each other, given the initial distance between them (3.67 billion miles)? If not, then isn't gravity stronger than magnetism, given the hold the sun exerts on Pluto?

Yes they would attract, but not very strongly compared to gravity at that range. We believe that electromagnetic fields, like gravitational fields, have potentially infinite range however the force exerted by electromagnetism follows an inverse-cube curve, whereas gravity falls off as an inverse square - so although electromagnetism is much the stronger of the two forces at small distances, gravity quickly comes to dominate as the separation between objects increases.

I'm not sure what you mean?

If the Sun and Pluto were oppositely electrically charged then they would attract each other and the force, exactly as with gravity, is proprtional to the inverse square of the distance between them (not the cube). In the gravitational force equation GMm/r^2 the mass terms are a measure of "gravitational charge" exactly analagous to the force eqyation for electric charged kQq/r^2

These are inverse SQUARE laws because all that is happening is that a fixed amount of "something" is getting stretched over a bigger surface area as it spreads out from a point and expands outwards into a 3D space. So long as the amount of "something" stays fixed (sound, light, enclosed electric charge, enclosed mass, or whatever) then the 1/r^2 arises purely from geometry.

Magnetic suns and planets doesn't work because a magnet always has both a north and a south pole, so magnetic fields don't just spread out radially into space in all directions. They form loops that go from one end of the magnet to the other.

[Nimby]

Table of relative strengths of the fundamental forces here: https://en.wikipedia.org/wiki/Fundamental_interact....

[deckster]

ATG said:

I'm not sure what you mean?

If the Sun and Pluto were oppositely electrically charged then they would attract each other and the force, exactly as with gravity, is proprtional to the inverse square of the distance between them (not the cube). In the gravitational force equation GMm/r^2 the mass terms are a measure of "gravitational charge" exactly analagous to the force eqyation for electric charged kQq/r^2

These are inverse SQUARE laws because all that is happening is that a fixed amount of "something" is getting stretched over a bigger surface area as it spreads out from a point and expands outwards into a 3D space. So long as the amount of "something" stays fixed (sound, light, enclosed electric charge, enclosed mass, or whatever) then the 1/r^2 arises purely from geometry.

Magnetic suns and planets doesn't work because a magnet always has both a north and a south pole, so magnetic fields don't just spread out radially into space in all directions. They form loops that go from one end of the magnet to the other.

Y'see, this is the risk of spouting off half-cocked with something vaguely remembered from physics lessons 25 years ago. Somebody who actually knows what they're talking about comes along and exposes you for the fraud that you are. Of course you are quite correct on all counts.

I submit myself guilty of both gross inaccuracy and excessive oversimplification, and throw myself on the mercy of the forum.

[Jinx]

Gravity is a lie - the Earth sucks!

[TwigtheWonderkid]

deckster said:

Y'see, this is the risk of spouting off half-cocked with something vaguely remembered from physics lessons 25 years ago. Somebody who actually knows what they're talking about comes along and exposes you for the fraud that you are. Of course you are quite correct on all counts.

I submit myself guilty of both gross inaccuracy and excessive oversimplification, and throw myself on the mercy of the forum.

I may be a dummy on magnetism and gravity, but I know about the fundamental laws of PH. Never apologise, just keep arguing your case even when it's completely obvious you are wrong. Hand in your membership card now!

[Engineer792]

TwigtheWonderkid said:

I get where the op is coming from. It seems odd that gravity is strong enough for the sun to hold Pluto in an orbit, yet at the same time weak enough for me to be able to pick up my mug of tea.

Yes, it does seem incredible that a huge (comparatively) chunk of rock like Pluto can be held in orbit over such a vast distance.

But imagine a single molecule orbiting the sun at the same distance.

That molecule is so vanishingly tiny and light that it doesn't seem in the least bit strange that it could be attracted over such a vast distance.

Now imagine two molecules floating around out there. Each one still undergoes the same attraction.

Now three, four, five, everything's still the same, and this doesn't change regardless of the number.

And it still doesn't change when you have enough molecules to form a planet, and they all come together.

But for each one of that vast number of molecules, the attraction remains the same.

Edited by Engineer792 on Friday 3rd March 20:28

[Moonhawk]

Jinx said:

Gravity is a lie - the Earth sucks!

Intelligent falling FTW.

https://en.wikipedia.org/wiki/Intelligent_falling

[pingu393]

Engineer792 said:

TwigtheWonderkid said:

I get where the op is coming from. It seems odd that gravity is strong enough for the sun to hold Pluto in an orbit, yet at the same time weak enough for me to be able to pick up my mug of tea.

Yes, it does seem incredible that a huge (comparatively) chunk of rock like Pluto can be held in orbit over such a vast distance.

But imagine a single molecule orbiting the sun at the same distance.

That molecule is so vanishingly tiny and light that it doesn't seem in the least bit strange that it could be attracted over such a vast distance.

Now imagine two molecules floating around out there. Each one still undergoes the same attraction.

Now three, four, five, everything's still the same, and this doesn't change regardless of the number.

And it still doesn't change when you have enough molecules to form a planet, and they all come together.

But for each one of that vast number of molecules, the attraction remains the same.

Edited by Engineer792 on Friday 3rd March 20:28

The attraction of each of the molecules to the sun remains the same, but they are also attracted to each other. The attraction of stuff to each other is why irregularly shaped rocks tend towards being spherical when they become around 100 miles in diameter.

[cymtriks]

Physics From The Edge, Mike McCulloch's blog regarding his theory, has several arguments against Dark Matter.

His theory is interesting, it does away with filling the universe with invisible "stuff" that just happens to have exactly the right properties to make your pet theory work and replaces it with a theory that actually matches the observations.