Quick Question: Gravity and flying broomsticks

Interesting question. Here's some more to think about which might help you figure out an answer:

1) When things "float off", do they go straight up or are they affected by wind or other forces? Do they fall down eventually outside the gravity wrecked zone?

2) How strong is this gravity wrecking phenomenon? Does the effect weaken farther away from the ground? The atmosphere is pretty thick, so it would take a lot to have something fly all the way out.

3) What's causing objects to float? If normal rules of physics apply, an object in motion will stay in motion and an object at rest will stay at rest unless acted upon by an outside force. If something zapped gravity in an area, I would expect things which were sitting on the ground to stay on the ground until something moved them.

4) If the witch's broom has some sort of propulsive force which operates independently of gravity, I would expect her to be able to fly through the zone (even if she does it badly).

For the balloons, it again depends on physics. For the girl to fly, that means the upward pull of the balloons on her mass must be greater than the downward pull of gravity. The balloon's pull is because of air pressure. If I'm doing this right, I think it's the higher pressure of the air which is trying to push the lower pressure within the balloons to a higher elevation where the pressure inside the balloon equals the pressure outside. It might be feasible from a physics standpoint, but you might want to have someone more versed in physics weigh in about this (pun intended ).

From a story standpoint, I'd be willing to accept the girl floating on the balloons based on the tone of the story. If the story is humorous, I'd be much more willing to accept stretching the laws of physics than if the story is serious.

ETA: Where did the second question go? Keeping the reply in case it's helpful...

When I think of gravity getting monkeyed with, I used to think of what it's like to be swimming under water, because while you are in water you "weigh less." I used to imagine that we would be lighter than normal in the midst of all that air. Now when we're in water, the water currents can take hold of us and yank us around if they are stronger than us or stronger than whatever might be holding us or stronger than what might be propelling us. So I used to think we cuold maybe swim through the air the same way we swim through water.

I have since ammended that supposition.

I now instead believe that in such an off-kilter gravity situation as you have described, the air will NOT behave like water and allow us to be more bouyant. Instead, BOTH us AND the air will be made equally (propotionally) lighter. And both us and the air will get sucked upward. (As a side note, technically speaking, scientists regard the air we breath as a liquid because it behaves more like a liquid than like a gas). So I see two things happening:

1) As people start to float around, the air surrounding them will also start to "float" (because the air will also weigh less) and it will perhaps rise upward in tubular shafts of gusty upward winds. The width of each tube will be determined by how wide each low-gravity spot actually is. Maybe two or three feet wide, or maybe half a block wide (that's up to you).

2) With the sudden onset of all these tubular updrafts, I foresee mini-tornadoes resulting, but only within the width of each low-gravity spot. I also foresee them rotating like tornadoes do. (Water rotates as it travels down a drain, right? So these mini-tornadoes will also rotate as the air rushes up each shaft.) But I am undecided as to whether these tornadoes will travel or remain stationary. I guess it depends upon the nature of these gravity disruptions. (So again, that is up to you.)

F = GMm/R²

M is the mass of the earth.

m is the mass of your person.

R is the distance between them.

G is the gravitational constant, and your variable in this equation (G = 6.67*10-11 N-m²/kg²).

When F<<<, your person will float away

Moon Wolf said:

I dunno, Plot... if it was a spell, the same rules wouldn't apply, would it? If it affected the gravity on the objects and people only, and not the air, what would happen then?

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Here's my reasoning:

1) Helium rises because it is lighter than air.

2) Oil floats on top of water because it is lighter than the water.

Therefore, if you take a three-dimensional section of space (like maybe your entire back yard) and suddenly reduce the force of gravity in that entire piece of space, then EVERYTHING in that section of space will suddenly have less of a gravitational pull on it. The air molecules will be included in that phenomenon.

The air molecules in your back yard will then start to rise straight upward away from the grass. That will then cause a suction to happen --a suction focused way down at ground-level (and this suction is an entirely different force independent of the gravity shift)-- drawing more air molecules into your back yard from your neighbors' yards (and the gravity in your neighbors' yards is still normal, btw). And now an upward draft is underway: a tornado of sorts.

dempsey said:

F = GMm/R²

M is the mass of the earth.

m is the mass of your person.

R is the distance between them.

G is the gravitational constant, and your variable in this equation (G = 6.67*10-11 N-m²/kg²).

When F<<<, your person will float away

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I don't know what <<< means. I know < but not <<<. Can you explain <<<?

My reasoning: It's magic, so that's all the explanation you need. Have as much fun with it as you want. That's why it's a FANTASY novel.

I'm with DeleyanLee.

But here's some more info. I think the girl would be able to lift off the ground with less balloons than we would now if there was a moment of less gravity. With magic I guess it would depend on how the magic worked. If you want the magic to be effected by gravity it could, if not than it could be effected by something else. ^_^

On the balloons, Plot Device has the point. Even if we ignore any large scale turbulence caused by the air basically leaking into space, it will take exactly as many balloons to lift a person off the ground regardless of what the gravity is, assuming the gravity is not 0.

The lift of a helium balloon is a result of the relative weight of helium to air. If air and helium weigh less, then the balloon lifts less. Mystiphysically, you also weigh the same proportion less, so it takes the same number of balloons.

Mythbusters came up with 3500 helium balloons for one four-year-old, 44lb. child.

I've been thinking of a serious response (any area where gravity is reduced, especially if the effect went up several hundred miles into space, would indeed have the air rushing upward out into space as other air from the surronding areas rush in to replace it), but all I can think of is a girl saying "Fido, I've got a feeling we're not in Salem anymore!"

That and the odd character who says "There's no such thing as gravity. The Earth sucks."

And "Looks like a witch has cast a spell to make that area suck less than the rest of the Earth..." There's a logical (and wong, but don't let that stop you!) explanation for everything.

Moon Wolf said:

if they were having gravity problems AND the MC wasn't very good at flying (assuming broomsticks can fly, of course ^^)... is there any possibilty that she could accidentally fly right out of the atmosphere? With the weakened gravity (or lack of) holding her down and the fact she can't fly very well anyway, if she wasn't paying attention could she float up? Or would she just crash?

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How do broomsticks fly? Here's three ways, and three possible answers:

1. Broomsticks fly because they already look a lot like horses, and an enchantment makes them more horse-like. (Not that horses can fly either, but if you can run around without any legs, then you're flying and the broomsticks aren't smart enough to realise that they don't have legs).
   
   Because broomsticks are made into wooden horses, they're not silly enough to just go up and up and UP unless they're very frightened or you're not a good rider. So, don't worry about it.
   
2. Broomsticks fly because they're lifted by demons or invisible air spirits which are compelled/seduced/appeased into flying you around.
   
   If gravity is weakened, then the world is getting more chaotic/airlike, so darn TOOTIN they're going to mess you around. They might lift you so high into the sky that you can't see the earth at ALL. If they're air spirits then you can probably still breathe - though the air might be thin and smell of snow or wherever else they've been playing lately. If they're demons, it will smell of sulfur or baboon armpits or whatever other stinky thing they can offend you with.
   
3. Broomsticks have lift because the wood has been made very light. They have thrust because the broomsticks emit flames/gas/propulsion rays behind them.
   
   You should always keep your broomstick tethered, because otherwise it can blow away in a high wind, and you'll need another broomstick to go and get it back. You should never stand behind a broomstick when someone is on it. You'll get burned, blown over or squashed against a wall. If gravity is suddenly light then your broomstick will tend to oversteer going up and understeer coming down. You have to rely more on your thrust to descend, and less on thrust to ascend.
   
   That means shallow take-offs unless you have an advanced broom with a swivel mount to let you change the angle of your thrust... and it also means steeper descents which is scary - especially when you have shopping in bags.

If the city was having one of its weak-gravity moments... would the helium in the balloons combined with the weakened gravity enough to get lifted off the ground and into the sky?

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In our world, gravity doesn't change much from place to place, and (so far) it's never changed spontaneously and locally. But if that did happen, then the air would immediately get thinner and harder to breathe because it's gravity that causes the air pressure that we normally have.

But the air around the city would be having none of that. Jostled by its high-pressure friends, it would rush in to take the space of the more upwardly-mobile air in the city, only to find itself suddenly rising too. So you'd have a massive updraft over the city, surrounded by a convection cell of air rushing into the city. It'd be very windy.

Some side-effects of lower air pressure are that balloons will expand, water in open containers will evaporate faster, gas-mains and gas bottles may leak more, food will dry out quickly, lips and eyes may get chapped quickly, people may suffer altitude sickness, and incautious scuba-divers returning to the city after a hard day of abalone-hunting or the like, may get the bends.

Little toddlers to whom families have incautiously tethered their half-dozen helium balloons may experience a range of interesting phenomena:

1. If the gravity reduces, balloons may get big and burst
2. Tethered to things bigger than themselves in a windy environment, they'll be blown about the landscape like Democrats at a primary.
3. All things considered, with updrafts and lightness and big balloons and all, and lots of sky above them and only ground below them, and readers expecting entertainment, they'll tend to go up.

If this question sounds completely stupid, does anybody have any ideas on how she MIGHT have gotten into the sky? Witches and wizards aren't very common anymore, especially not in big cities.

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You can have some of ours. We can't move for them here.

Well, between backblasts from broomsticks, loose air spirits and demons, spooked broomsticks, a half-dozen helium balloons, a severe updraft and the sheer ignorance of her family needing a cautionary lesson it's hard to imagine how the poor little tyke can stay down. Assuming that her family are typical, I would expect the stratosphere to be littered with children summersalting from backblasts, holding Mummy's umbrella, brother Johnny's kite, blowing their bubblegum just a shade too much, strapped into experimental rocket-chairs, having jumped on the trampoline alone when they shouldn't, or having fed their broom too much sugar that day.

Really, somebody should do something.

And remember, don't tell how it works, show how it works.

But to show you have to know how it works. A.k.a. don't write the explanations down in your book, but write them down for yourself so you can look them up later for consistency.

Anyway, things to remember:
Gravity doesn't stop us from flying off into space, gravity makes us stick to the planet. If gravity just stops you don't just start flying upwards, you only start flying upwards if there is something pushing (or pulling) you upwards and gravity does not pull you down.
Now, if you're on a rotating planet, there is of course the centrifugal effect, so everything really will fly off. Everything not nailed down. Probably including most buildings, or at least parts of buildings, since they're rarely built to withstand someone pulling them upwards.
The same goes, by the way, for anything flying. They're not on the ground, but still follow a planets rotation, so they will also get pulled upwards when gravity stops. A magic broom might not be affected, but planes will be in trouble.
As for the balloons, without gravity, they will not pull the girl upwards at all. Since there is no gravity, there is nothing that pulls the heavier gases downwards, so there's nothing that pulls the lighter gases upwards. If the gravity is weakened there'll be no effect. The lifting force of the balloons gets smaller, but the girl gets lighter, so she won't be suddenly lifted upwards. Of course, if she's on a rotating planet and the gravity stops alltogether she'll go flying off anyway.
But since basically the whole planet will get torn to pieces anyway i think she'd have bigger problems.

And gravity is the weakest of all the forces. Any wizard should figure this out pretty fast.