Who Wants To Be A Millionaire with Jeremy Clarkson

[MojoJojo]

Exactly, the two forces cancel out, resulting in the observed force, the weight, between the Earth and Moon being zero. Not the force on the Moon, not the force on the Earth, the force measured between the two, that's the distinction.

What two forces? There's only one.

The Moon itself does feel the pull of the Earth (and vice versa), but that's not what weighing something is. Weight is the force measured between two objects, and when they're in a stable orbit that force has to be zero or else the two objects would have to be moving towards each other (or apart)

No this is just wrong. The moon, and anything in orbit, is constantly accelerating. Because it has a net force on it.

In other words, the gravitational force is cancelled out by the inertial force of the orbit, meaning zero weight. Stop the orbital motion and you'd start seeing weight as you'd expect, but then you'd get an eventual collision.

Uh, no such thing as inertial force. Whether the moon is in orbit or not does not change the forces on it. You'll be talking about centrifugal force soon.

I think the confusion here is caused by the fact that we describe being in free fall on the vomit comet or whatever as being weightless - but your weight doesn't actually change, it's just a good approximation that is possible while still being in earth's gravitational field.

[Replies From View]

Could somebody please explain how a creature that has only just hatched from the moon is able to immediately lay a new moon exactly the same size as the first?

[samadriel]

Probably the same way the creature started weighing more as it got closer to hatching.

[Blumf]

The Moon itself does feel the pull of the Earth (and vice versa),

No this is just wrong. The moon, and anything in orbit, is constantly accelerating. Because it has a net force on it.

Um... as you can see I said that.

You need to counter this bit

but that's not what weighing something is. Weight is the force measured between two objects

Put simply, attach a newton metre between the Earth and the Moon. What will it say? Give that answer. Until you do, you can't really move forward because you're not talking about weighing something.

[Isnt Anything]

a newton metre

autocorrect trying to be too clever ? :)

[MojoJojo]

No this is just wrong. The moon, and anything in orbit, is constantly accelerating. Because it has a net force on it.


Um... as you can see I said that.

You need to counter this bit

Put simply, attach a newton metre between the Earth and the Moon. What will it say? Give that answer. Until you do, you can't really move forward because you're not talking about weighing something.

If the newton meter actually measured force, it would show the weight of the moon, which is non-zero. The fact that newton meters don't actually measure force, but the extension of springs under the action of a force from which we can calculate the force, is neither here nor-there.

We don't actually need a Newton meter, anyway, since we can observe that the moon is moving in a circular path, so if we can measure the diameter of that path and the period we can calculate the force acting at that point.

You seem to be trying to redefine what weight is.

[Blumf]

...stuff...

Oh dear.

So basically you realise you can't claim the Earth and Moon are approaching, and in a rage deny force measurement is possible. Not much help I can give you there.

[Endicott]

One or both of you should burn your dinner, pronto.

[Isnt Anything]

:D

[MojoJojo]

Oh dear.

So basically you realise you can't claim the Earth and Moon are approaching, and in a rage deny force measurement is possible. Not much help I can give you there.

No, I explained how you can measure the force without any force measurement device. If the moon had no weight it would fly off in a straight line.

[Blumf]

If the moon had no weight it would fly off in a straight line.

So now you're denying light bends with gravity.

Oh dear.

[MojoJojo]

That would be true if I thought light had no mass.

It has no resting mass, but it has energy and E=mc²

[Replies From View]

No, I explained how you can measure the force without any force measurement device. If the moon had no weight it would fly off in a straight line.

OOOOOH AN HOOP

[Blumf]

If the moon had no weight it would fly off in a straight line.

That would be true if I thought light had no mass.

It has no resting mass, but it has energy and E=mc²

So is this hypothetical Moon, flying off in a straight line, moving or at rest? Would help if you could keep your arguments straight in your head.

...and F= ma, is the Moon accelerating towards the Earth? Why hasn't crashed into it yet? Looks to me like  measuring the force BETWEEN the Earth and Moon is close to zero. Even if the force on the Moon is greater than zero.

[St_Eddie]

...is the Moon accelerating towards the Earth? Why hasn't crashed into it yet?

The moon, yesterday.

[Endicott]

...and F= ma, is the Moon accelerating towards the Earth? Why hasn't crashed into it yet? Looks to me like  measuring the force BETWEEN the Earth and Moon is close to zero. Even if the force on the Moon is greater than zero.

My problem is, I don't really know what the force that exists BETWEEN the Earth and Moon has got to do with anything. It's not the gravitational force, because we established what that was in a previous post. As far as I can see, you just made it up. If I may refer you to my previous reply:

I really, really don't like this way of explaining weight. On the other hand I'm not going to say it is wrong, I think I may be being a bit pedantic about terms. But I will say I think weight is a very poor concept when it comes to talking about planets and moons.

Objects have mass, gravity causes forces (lets not complicate this with relativity), and those forces cause motion. An object only has 'weight' when some other force opposes that motion. The Moon has no weight, it is weightless, because nothing is opposing its motion. If I jump off of the Empire State, nothing is opposing my motion so I am weightless. If I stand on the floor, my motion caused by gravity is being opposed. I can measure how big that opposing force needs to be with some scales, and call it 'weight'.

If you could get the Moon to sit on the surface of the Earth, then it would have weight. But it's not a very realistic proposition and I don't think it transfers at all well when discussing an object in orbit.

It doesn't help of course, that jojo's idea of weight also seems to be all over the place. If the Moon had no mass it would fly off. If you're going to tell me the Moon has weight, you're going to have to explain how I get it on some scales to measure it.

[Endicott]

I think the confusion here is caused by the fact that we describe being in free fall on the vomit comet or whatever as being weightless - but your weight doesn't actually change, it's just a good approximation that is possible while still being in earth's gravitational field.

I mean, this is complete bull shit. Of course your weight changes! That's the whole point of it. It's your mass that doesn't change. Try these two thought experiments:

You're on a space ship. You're light years from any star so any gravity can be ignored. Your engine is off. You're floating around, and so is everything else in your spaceship because you're all weightless. You turn your engine on to give an acceleration of 1G. Mind you, you strap everything down first because all the objects floating around are about to get heavy, because once you turn on the engine they have weight.

Engine off, objects have no weight. Engine on, objects have lots of weight.

Now instead of a spaceship, you're in a lift on the Earth that can go down at 1G. Lift stops, you have weight. Lift goes down, now you have no weight. And that's the vomit comet. It's exactly the same.

[MojoJojo]

The force you would will feel on a ship accelerating is not gravity.

On the vomit comet or your hypothetical lift, the occupants are accelerating towards the earth at 1g. F=ma. The force is gravity.

The vomit comet simulates weightlessness.

[Replies From View]

NO MORE BUTTERED SCONES FOR ME, MATER; I'M ORF TO PLAY THE GRARND PIARNO.

[Endicott]

The force you would will feel on a ship accelerating is not gravity.

General Relativity says that you can't tell the difference.

[Replies From View]

General Relativity says that you can't tell the difference.

He's only a Lieutenant General and he is a fuckwit.

Tell him that he is a fuckwit and that he is banned from joining in with this discussion.

[Kelvin]

Fairly sure you guys wouldn't have this long to answer on the show.

[Isnt Anything]

General Relativity says that you can't tell the difference.

Yup. Well in fact it says that there IS no difference and the rest follows from there.

For others -

Gravitational mass (the quantity that determines how much a body attracts or is attracted by something else) ≡ inertial mass (the quantity that determines how much force is needed to accelerate something).

sorry for possibly insulting website title but seems better than wikis although still not the best ive seen in some dead tree somewhere
http://www.dummies.com/education/science/physics/einsteins-general-relativity-theory-gravity-as-acceleration/
the original has cute cartoons so may be more helpful than this extract

Einstein came to realize the principle of equivalence, and it states that an accelerated system is completely physically equivalent to a system inside a gravitational field.

As Einstein later related the discovery, he was sitting in a chair thinking about the problem when he realized that if someone fell from the roof of a house, he wouldn't feel his own weight. This suddenly gave him an understanding of the equivalence principle.

As with most of Einstein's major insights, he introduced the idea as a thought experiment. If a group of scientists were in an accelerating spaceship and performed a series of experiments, they would get exactly the same results as if sitting still on a planet whose gravity provided that same acceleration.

Einstein's brilliance was that after he realized an idea applied to reality, he applied it uniformly to every physics situation he could think of.

For example, if a beam of light entered an accelerating spaceship (from the side), then the beam would appear to curve slightly. The beam is trying to go straight, but the ship is accelerating, so the path, as viewed inside the ship, would be a curve.

Both acceleration and gravity bend a beam of light.
By the principle of equivalence, this meant that gravity should also bend light. When Einstein first realized this in 1907, he had no way to calculate the effect, other than to predict that it would probably be very small.

Ultimately, though, this exact effect (being measured experimentally) would be the one used to give general relativity its strongest support.

[Replies From View]

Okay so we have explained why fatties can't stop stuffing their faces.  Next question please Jez.

[Isnt Anything]

nice

[Darles Chickens]

KenM knows the answer.