this post was submitted on 03 Nov 2024
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Tap for spoilerThe bowling ball isn’t falling to the earth faster. The higher perceived acceleration is due to the earth falling toward the bowling ball.

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[–] [email protected] 15 points 1 day ago* (last edited 1 day ago) (2 children)

If anyone's wondering, I used to be a physicist and gravity was essentially my area of study, OP is right assuming an ideal system, and some of the counter arguments I've seen here are bizarre.

If this wasn't true, then gravity would be a constant acceleration all the time and everything would take the same amount of time to fall towards everything else (assuming constant starting distance).

You can introduce all the technicalities you want about how negligible the difference is between a bowling ball and a feather, and while you'd be right (well actually still wrong, this is an idealised case after all, you can still do the calculation and prove it to be true) you'd be missing the more interesting fact that OP has decided to share with you.

If you do the maths correctly, you should get a=G(m+M)/r^2 for the acceleration between the two, if m is the mass of the bowling ball or feather, you can see why increasing it would result in a larger acceleration. From there it's just a little integration to get the flight time. For the argument where the effect of the bowling ball/feather is negligible, that's apparent by making the approximation m+M≈M, but it is an approximation.

I could probably go ahead and work out what the corrections are under GR but I don't want to and they'd be pretty damn tiny.

[–] [email protected] 8 points 1 day ago (2 children)

Physics books always say to assume the objects are points in doing calculations. Does the fact that the ball is thicker then the feather make a difference?

[–] [email protected] 1 points 17 hours ago

It would, similar to how the mass of each object does have an effect, even if negligible. But the question is if the radius of the bowling ball vs feather has a greater effect than the mass of the bowling ball vs the feather.

You can adjust the value r in the universal gravitational equation by the radius of the bowling ball and compare the extremes (both plus and minus the radius) and the middle point to see the tidal effects.

If the feather starts at the middle height of the bowling ball, the tidal effects would help the bowling ball. If it starts at the lowest point of the bowling ball, the tidal effects would hinder the bowling ball.

But the magnitude of that effect depends on the distance from the center of the other mass.

I think the main thing would be the ratio of the small mass vs big mass compared to the ratio of the small radius vs the big radius.

Though, thinking of it more, since the bowling ball is a sphere (ignoring finger holes), the greater pull on the close side would be balanced by the lesser pull on the far side (assuming the difference between those two forces isn't greater than the force holding the ball together), so now I think it doesn't matter (up to that structural force and with the assumption that the finger holes aren't significant).

If they are falling into a small black hole, then it does become relevant because the bowling ball will get stringified more than the feather once the forces are extreme enough to break the structural bonds, but the math gets too complicated to wrap my mind around right now. If I had to guess, the bowling ball would start crossing the event horizon first, but the feather would finish crossing it first. And an outside observer would see even more stretched out images of both of them for a while after that, which would make actually measuring the sequence of events impossible.

And who knows what happens inside, maybe each would become a galaxy in a nested universe.

[–] [email protected] 4 points 1 day ago* (last edited 1 day ago) (1 children)

Possibly?

A bowling ball is more dense than a feather (I assume) and that's probably going to matter more than just the size. Things get messy when you start considering the actual mass distributions, and honestly the easiest way to do any calculations like that is to just break each object up into tiny point like masses that are all rigidly connected, and then calculate all the forces between all of those points on a computer.

I full expect it just won't matter as much as the difference in masses.

[–] [email protected] 2 points 18 hours ago (1 children)

For the bowling ball, Newton’s shell theorem applies, right?

[–] [email protected] 1 points 11 hours ago* (last edited 9 hours ago)

Yeah it would fair point, I'll be honest I haven't touched Newtonian gravity in a long time now so I'd forgotten that was a thing. You'd still need to do a finite element calculation for the feather though.

There's a similar phenomenon in general relativity, but it doesn't apply when you've got multiple sources because it's non-linear.

[–] [email protected] 2 points 19 hours ago* (last edited 18 hours ago)

Quick intuition boost for the non-believers: What do things look like if you're standing on the surface of the bowling ball? Are feather and earth falling towards you at the same speed, or is there a difference?

[–] [email protected] 9 points 22 hours ago (1 children)

Obviously the bowling ball because it's more MASSIVE.

[–] [email protected] 0 points 20 hours ago
[–] [email protected] 57 points 1 day ago (1 children)

Stupid question, bowling balls don't fit through the vacuum's hose.

[–] [email protected] 22 points 1 day ago

Ur mom could suck it through

[–] [email protected] 22 points 1 day ago* (last edited 1 day ago) (1 children)

This would make a good "What if?" for XKCD. In a frictionless vacuum with two spheres the mass of the earth and a bowling ball how far away do they need to start before the force acting on the earth sized mass contributes 1 Planck length to their closure before they come together? And the same question for a sphere with the mass of a feather.

[–] [email protected] 3 points 1 day ago* (last edited 23 hours ago)

I actually thought the answer might be never, but a quick back of the envelope calculation suggests you can do this by dropping a ~1kg bowling ball from a height of 10^-11^m. (Above the surface of the earth ofc)

This is an extremely rough calculation, I'm basically just looking at how big a bunch of numbers are and pushing all that through some approximate formulae. I could easily be off by a few orders of magnitude and frankly I didn't take care to check I was even doing any of it correctly.

10^-11^m seems wrong, and it probably is. But that's still 1,000,000,000,000,000,000,000,000 times further than the earth moves in this situation. Which hey, fun What If style fact for you: that's about the same ratio of 1kg to the mass of the Earth at ~10^24^kg.

That makes perfect sense because the approximations I made are linear in mass, so the distance ratio should be given by the mass ratio.

[–] [email protected] 12 points 1 day ago (1 children)

“In our limited language that tries to describe reality and does so very poorly, how would you describe this situation that would literally never happen?”

[–] [email protected] 4 points 1 day ago (1 children)

I'm pretty sure bowling balls and feathers fall all the time

[–] [email protected] 6 points 1 day ago (2 children)

I think they mean the vacuum part.

To which I'd add that we had astronauts perform this experimentally on the surface of the moon.

[–] [email protected] 2 points 20 hours ago

True fair enough, but since I'm here, being an internet clown, I might as well double down...

Obviously heavy and light objects never experience gravitational attraction in a vacuum throughout the vastness of the universe. Clearly F = G(m1m2)/R^2 only applies to objects in earths atmosphere.

[–] [email protected] 2 points 1 day ago

Also, I've seen a video of an experiment done in a vacuum chamber. (Although they kind of botched the point of the video by showing lots of slow-mo and junk like that.)

[–] [email protected] 14 points 1 day ago (1 children)

Reading that spoiler, I hate scientists sometimes.

[–] [email protected] 13 points 1 day ago (1 children)

For some reason on my client, it can't remove the spoiler (gives a network error). I'm assuming it says that since the ball has more mass, it has a higher attraction rate of its own gravity to Earth's, so does fall faster in a vacuum but so miniscule it would be hard to measure?

[–] [email protected] 12 points 1 day ago

"The bowling ball isn't falling to the earth faster. The higher perceived acceleration is due to the earth falling toward the bowling ball." is what the spoiler says

[–] [email protected] 20 points 1 day ago (1 children)

Depends on the color of the feather and the ball.

There's a simple explanation.

[–] [email protected] 8 points 1 day ago (1 children)

Exactly, red has way more up-quarks than blue

[–] [email protected] 7 points 1 day ago (1 children)

Because light-blue weighs less than blue.

[–] [email protected] 3 points 1 day ago (1 children)

and Diet light-blue weighs less than light-blue

[–] [email protected] 4 points 1 day ago

Please stop by the office and pick up your combo Nobel Prize in Physics and Chemistry.

[–] [email protected] 30 points 2 days ago (1 children)

Brian Cox shows ball and feathers falling together in vacuum: https://youtu.be/E43-CfukEgs

[–] [email protected] 7 points 1 day ago

I love it when scientists who know something to be true in theory get to see practical experiments like this. The jubilation on thier faces.

[–] [email protected] 25 points 1 day ago (1 children)

There’s too many words in this meme that’s making me dizzy from all your fancy science leechcraft, wizard.

I reject your reality and substitute my own: the feather falls faster. It’s more streamlined than the bowling ball, and thus it slips through the vacuum much faster and does hit the ground and stay on the ground, I think. The ball will bounce at least once, maybe even three times. On each bounce, parts of it probably break off, which change the weight. Thankfully those broken pieces won’t hurt anyone because they’re sucked up by the vacuum. Thus, rendering your dungeon wizard spells ineffective against me.

[–] [email protected] 9 points 1 day ago

This person sciences good

[–] [email protected] 47 points 2 days ago (1 children)

So will the bowling ball gravitationally attract the earth to itself there by reach the earth an infinitesimally small amount?

[–] [email protected] 43 points 2 days ago (5 children)

Yes, the earth accelerates toward the ball faster than it does toward the feather.

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[–] [email protected] 34 points 2 days ago* (last edited 2 days ago) (1 children)

This argument is deeply flawed when applying classical Newtonian physics. You have two issues:

  1. Acceleration of a system is caused by a sum of forces or a net force, not individual forces. To claim that the Earth accelerates differently due to two different forces is an incorrect application of Newton's second law. If you drop a bowling and feather in a vacuum, then both the feather and the bowling ball will be pulling on the Earth simultaneously. The Earth's acceleration would be the same towards both the bowling ball and the feather, because we would consider both the force of the feather on the Earth and the force of the bowling ball on the Earth when calculating the acceleration of the Earth.
  2. You present this notion that two different systems can accelerate at 9.81 m/s/s towards Earth according to an observer standing on the surface of Earth; but when you place an observer on either surface of the two systems, Earth is accelerating at a different rate. This is classically impossible. If two systems are accelerating at 9.81 m/s/s towards Earth, then Earth must be accelerating 9.81 m/s/s towards both systems too.
[–] [email protected] 22 points 2 days ago* (last edited 2 days ago) (3 children)

Re your first point: I was imagining doing the two experiments separately. But even if you do them at the same time, as long as you don’t put the two objects right on top of each other, the earth’s acceleration would still be slanted toward the ball, making the ball hit the ground very very slightly sooner.

Re your second point: The object would be accelerating in the direction of earth. The 9.81m/s/s is with respect to an inertial reference frame (say the center of mass frame). The earth is also accelerating in the direction of the object at some acceleration with respect to the inertial reference frame.

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[–] [email protected] 26 points 2 days ago (11 children)

But what weighs more:

A ton of bowling balls or a ton of feathers? 🤔

[–] [email protected] 64 points 1 day ago (1 children)

When you carry a ton of feathers, you also have to carry the weight of what you did to those poor birds...

[–] [email protected] 3 points 1 day ago

What about all the bowling cattle you had to castrate for those balls?

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[–] [email protected] 25 points 2 days ago (21 children)

Why your spoiler is wrong:

The gravitational force between two objects is G(m1 m2)/r²

G = ~6.67 • 10^-11 Nm²/kg²

m1 = Mass of the earth = ~5.972 • 10^24 kg

m2 = Mass of the second object, I'll use M to refer to this from now on

r = ~6378 • 10^3 m

Fg = 6.67 • 10^-11^ Nm²/kg² • 5.972 • 10^24^ kg • M / (6378 • 10^3 m)² = ~9.81 • M N/kg = 9.81 • M m kg / s² / kg = 9.81 • M m/s² = g • M

Since this is the acceleration that works between both masses, it already includes the mass of an iron ball having a stronger gravitational field than that of a feather.

So yes, they are, in fact, taking the same time to fall.

[–] [email protected] 2 points 1 day ago

This is not correct, the force on the objects is the same sure, but the accelerations aren't so you can't calculate them both in one go like this.

[–] [email protected] 13 points 1 day ago (7 children)

Uh... That's not how that works. The distance between two objects changes with acceleration a1-a2 where object 1 moves with acceleration a1 and object 2 a2 (numbers interchangeable). In the bowling ball's case a2 is the same but a1 is bigger in the negative direction so the result is that the bowling ball falls faster.

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[–] [email protected] 13 points 2 days ago* (last edited 2 days ago)

But ... Steel is heavier than feathers ...

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