# ShakaUVM

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A little about me...

Member Since: April 11, 2007

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Gentleman Adventurer

Member Since: April 11, 2007

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# Comments to ShakaUVM

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## 15 Comments

siftbotsays...Happy anniversary! Today marks year number 9 since you first became a Sifter and the community is better for having you. Thanks for your contributions!

siftbotsays...Happy anniversary! Today marks year number 8 since you first became a Sifter and the community is better for having you. Thanks for your contributions!

You've ignored the fact that they did it inside, so there's also the pull of the bowling ball on the walls and ceiling, and vice versa! ;-)

I'm pretty sure we agree on all counts except how detailed one must be when teaching simple lessons.

It's not falsifying details. It's nonsensical to pose the problem so that surface irregularities are taken into consideration. Or the fact like in this video the feather started below the bowling ball.

It's a trivial 3-body problem. You can represent them by three balls, one of mass Heavy, one of mass Medium, one of mass Light. As long as Medium and Light start at the same distance from Heavy, Medium and Heavy will always hit first. You can run this test in an orbital simulator if you don't believe me.

It's not falsifying details. It's nonsensical to pose the problem so that surface irregularities are taken into consideration. Or the fact like in this video the feather started below the bowling ball.

It's a trivial 3-body problem. You can represent them by three balls, one of mass Heavy, one of mass Medium, one of mass Light. As long as Medium and Light start at the same distance from Heavy, Medium and Heavy will always hit first. You can run this test in an orbital simulator if you don't believe me.

Oh...so it's OK with you to simplify and 'falsify details' significantly by modeling the earth as a perfect sphere, but not ignore the mathematically insignificant and immeasurably small possible movement of the earth in some direction or another due to multiple immeasurably small gravities?! WHAT?!? ;-)

....Um...1 degree on earth is 111.2 KM, there's such a tiny difference (1 cm+-) they are in the same place for all possible measureable purposes, nothing like 1 deg apart. My scientific calculator won't give an answer, 1 deg * (1cm/111.2km) =0.00deg on it. (OK, it's not hard math...1/11120000 deg.) Because of this, yes, they WOULD cross the imaginary line, AND hit the earth at the same time by any possible measurement. If the smallest distance measureable is FAR larger than the distance they differ by, and the smallest time measureable is MUCH longer than the difference in time they hit, normal (and most abnormal) people say it's exactly the same.

And again...the experiment properly ignores any infinitely tiny immeasurable movement of the earth in ANY random direction for the obvious reasons already stated. There's far more difference based on the precise position of mercury than the position of the bowling ball and feather, especially when they are nearly touching...You know and understand this.

Oh...so it's OK with you to simplify and 'falsify details' significantly by modeling the earth as a perfect sphere, but not ignore the mathematically insignificant and immeasurably small possible movement of the earth in some direction or another due to multiple immeasurably small gravities?! WHAT?!? ;-)

....Um...1 degree on earth is 111.2 KM, there's such a tiny difference (1 cm+-) they are in the same place for all possible measureable purposes, nothing like 1 deg apart. My scientific calculator won't give an answer, 1 deg * (1cm/111.2km) =0.00deg on it. (OK, it's not hard math...1/11120000 deg.) Because of this, yes, they WOULD cross the imaginary line, AND hit the earth at the same time by any possible measurement. If the smallest distance measureable is FAR larger than the distance they differ by, and the smallest time measureable is MUCH longer than the difference in time they hit, normal (and most abnormal) people say it's exactly the same.

And again...the experiment properly ignores any infinitely tiny immeasurable movement of the earth in ANY random direction for the obvious reasons already stated. There's far more difference based on the precise position of mercury than the position of the bowling ball and feather, especially when they are nearly touching...You know and understand this.

There's no such thing as acceleration of just the ball. Everything is relative; there are no fixed bodies. We just ignore the movement of the earth in these things, because as far as approximations go, it makes no practical difference.

They would not cross an imaginary line at the same time, since if the earth is modelled as a perfect sphere, it will be pulled slightly toward the bowling ball (the actual vector being somewhere between them because the feather has a small moment). If there's a 1 degree difference in the drop between the feather and ball, which looks about right for this experiment, this will result in a 1.7% advantage for the bowling ball hitting the earth first from the very slight movement of the earth.

There's no such thing as acceleration of just the ball. Everything is relative; there are no fixed bodies. We just ignore the movement of the earth in these things, because as far as approximations go, it makes no practical difference.

They would not cross an imaginary line at the same time, since if the earth is modelled as a perfect sphere, it will be pulled slightly toward the bowling ball (the actual vector being somewhere between them because the feather has a small moment). If there's a 1 degree difference in the drop between the feather and ball, which looks about right for this experiment, this will result in a 1.7% advantage for the bowling ball hitting the earth first from the very slight movement of the earth.

yes, but again that's not the point of the experiment. it would cross an imaginary line at the same time.

I also agree about approximations, just admit it and it's fine.

In this instance however, because it's ONLY about the acceleration of the bowling ball vs acceleration of the feather, there's no difference at all. It's only when you change what you're looking at to include the movement of the 'gravity well' and RELATIVE distances that you change which hits the gravity well first, but still not how fast each is accelerated...which was the only point.

EDIT: Shall I guess that you've never found the area/circumference of a circle? It seems, with your insistence on being 100% technically correct to the last decimal, and never rounding off numbers, that trying to multiply by PI would leave you stuck in an infinite loop writing PI forever, unable to ever do the calculation because you can't finish PI. ;-)

Even with a small degree difference, you can do the trig to show the bowling ball would hit first.

I don't mind approximations. At all. It just should be presented in the full form, with the note you can drop the mass of one object from the equation when it is dominated by the other mass. This way people won't learn bad physics.

First....nice, nice.

Second. I get your point. They should have been more clear that they are intentionally ignoring any other forces, such as the force exerted by the objects on the planet and each other, and the pull of the observer, and the pull of the milky way, the sun, the moon, Venus, etc. Because those forces are completely inobservable, even with top notch equipment, it's simpler for most to not mention them at all. They have no bearing on what they're teaching, and the smart children who see farther into the details are smart enough to know what this experiment is designed to show, and what it ignores....or at least smart enough to ask the right questions, while the less science/math minded would only be confused by the mention of them while also ignoring them. it's not exactly the same thing as teaching that 5/0=0, when it's really infinity, the exact opposite of 0.

This experiment was about what's observable, not what's mathematically provable at the tiniest detail level. Those details are for higher level physics. I will agree, it's a disservice to not mention that clearly, but I think it's implied by the parameters and the intent (teaching that acceleration due to gravity is independent of mass).

EDIT: Also, please remember that for all intents and purposes, they are releasing the objects from the same point, so they still 'hit' at 'exactly' the same time because their forces are in line, off by what, perhaps <.0000000001deg?. As you said, all solved by equivocating 'exactly' to 'nearly exactly' or 'approximately the same' or even 'observably exactly the same time'.

First....nice, nice.

Second. I get your point. They should have been more clear that they are intentionally ignoring any other forces, such as the force exerted by the objects on the planet and each other, and the pull of the observer, and the pull of the milky way, the sun, the moon, Venus, etc. Because those forces are completely inobservable, even with top notch equipment, it's simpler for most to not mention them at all. They have no bearing on what they're teaching, and the smart children who see farther into the details are smart enough to know what this experiment is designed to show, and what it ignores....or at least smart enough to ask the right questions, while the less science/math minded would only be confused by the mention of them while also ignoring them. it's not exactly the same thing as teaching that 5/0=0, when it's really infinity, the exact opposite of 0.

This experiment was about what's observable, not what's mathematically provable at the tiniest detail level. Those details are for higher level physics. I will agree, it's a disservice to not mention that clearly, but I think it's implied by the parameters and the intent (teaching that acceleration due to gravity is independent of mass).

EDIT: Also, please remember that for all intents and purposes, they are releasing the objects from the same point, so they still 'hit' at 'exactly' the same time because their forces are in line, off by what, perhaps <.0000000001deg?. As you said, all solved by equivocating 'exactly' to 'nearly exactly' or 'approximately the same' or even 'observably exactly the same time'.

Technically correct is the best kind of correct.

The trouble with teaching people that the bowling ball and feather will hit at, quoting the physicist in this clip, "exactly the same time", is that (relativity issues aside making the statement a joke anyway) it leads people to have a faulty understanding of how gravity actually works.

It's fine to teach that bowling balls and feathers will hit at *approximately* the same time, due to one mass in the equation being much higher than the other (allowing us to approximate it out), but it seems to never be taught this way. So these students end up with all sorts of wrong ideas about gravity when they get to me to work on n-body solvers.

It's the same problem, for example, as teaching elementary school kids that 5 divided by 0 is 0. It might make that teacher's life a little easier, but causes problems downstream.

Technically correct is the best kind of correct.

The trouble with teaching people that the bowling ball and feather will hit at, quoting the physicist in this clip, "exactly the same time", is that (relativity issues aside making the statement a joke anyway) it leads people to have a faulty understanding of how gravity actually works.

It's fine to teach that bowling balls and feathers will hit at *approximately* the same time, due to one mass in the equation being much higher than the other (allowing us to approximate it out), but it seems to never be taught this way. So these students end up with all sorts of wrong ideas about gravity when they get to me to work on n-body solvers.

It's the same problem, for example, as teaching elementary school kids that 5 divided by 0 is 0. It might make that teacher's life a little easier, but causes problems downstream.

Now I'm starting to think you just want to argue. If you're smart enough to make those technical assertions, you're smart enough to know that's not what the experiment was about, and that you're just adding data to confuse the lesson.

The experiment is about the effect of gravity on the moving objects under 2 conditions, and how their mass means nothing when determining THEIR accelerations/speeds in a vacuum. Period.

You want to introduce other, completely unobservable forces and movements to say 'nope'. Technically, you may be correct, but you must completely ignore the purpose and parameters of the experiment and assume inobservably small movements to make your point...a point that does not actually change the experimental findings or the lesson, but does confuse it thoroughly.

Now I'm starting to think you just want to argue. If you're smart enough to make those technical assertions, you're smart enough to know that's not what the experiment was about, and that you're just adding data to confuse the lesson.

The experiment is about the effect of gravity on the moving objects under 2 conditions, and how their mass means nothing when determining THEIR accelerations/speeds in a vacuum. Period.

You want to introduce other, completely unobservable (and irrelevant to the lesson) forces and movements to say 'nope'. Technically, you may be correct in a way, but you must completely ignore the purpose and parameters of the experiment and assume inobservably small movements to make your point...a point that does not actually change the experimental findings or the lesson, but does confuse it thoroughly.

EDIT: I would note that, by your standards, exactly where the observer is positioned makes MORE difference than the different masses of the bowling ball and feather, as do the exact positions of the two...if dropped from exactly the same position, they hit at exactly the same time because their gravitational forces are in line.

I would also note that if you change it to say, unimpeded, they would cross any imaginary line in space at the same time (essentially what they mean), again your point becomes moot.

If a planet would hit before a feather, then a bowling ball would hit before a feather. The only difference is the effect size.

If a planet would hit before a feather, then a bowling ball would hit before a feather. The only difference is the effect size.

No...I'm talking in reference to earth (or any 'stationary' gravity source), as is the video. You are talking about relative speeds of moveable, nearly equal gravitational forces. That's why we disagree, we're talking about completely different things.

Yes. You are correct. If you put 3 objects in space, 2 of them being planet masses (not sized, size has nothing to do with it) and the third being the mass of a feather, the planet masses will come together first because they attract each other.

But that's not what this video, or the discussion were about. They are about how mass is irrelevant when discussing/calculating acceleration due to gravity (in the absence of other opposing forces, like wind resistance), and that's still true in your example. It gets MUCH harder to understand when you change the experimental parameters to have the (now multiple) gravitational force(s) also acted upon and moving, but the same rules all still apply. It's just much simpler to use masses of magnitudes so different where you can consider the gravity well a stationary object not attracted towards or effected by either moving object.

No...I'm talking in reference to earth (or any 'stationary' gravity source), as is the video. You are talking about relative speeds of moveable, nearly equal gravitational forces. That's why we disagree, we're talking about completely different things.

Yes. You are correct. If you put 3 objects in space, 2 of them being planet masses (not sized, size has nothing to do with it) and the third being the mass of a feather, the planet masses will come together first because they attract each other.

But that's not what this video, or the discussion were about. They are about how mass is irrelevant when discussing/calculating acceleration due to gravity (in the absence of other opposing forces, like wind resistance), and that's still true in your example. It gets MUCH harder to understand when you change the experimental parameters to have the (now multiple) gravitational force(s) also acted upon and moving, but the same rules all still apply. It's just much simpler to use masses of magnitudes so different where you can consider the gravity well a stationary object not attracted towards or effected by either moving object.

You're starting to get it. Except we're not talking about acceleration in reference to earth, we're talking about which would hit first, a massive object or a less massive object.

So if you drop a planet on the earth, it will hit before a feather would from the same distance.

You're starting to get it. Except we're not talking about acceleration in reference to earth, we're talking about which would hit first, a massive object or a less massive object.

So if you drop a planet on the earth, it will hit before a feather would from the same distance.

No. The acceleration is exactly as fast, there are simply two of them in opposite directions, so the RELATIVE speed achieved is twice as fast, but actual speed AND the acceleration is the same.

You are confusing 'closing speed' with acceleration, and confusing what was being discussed in the first place, which was that objects are attracted by gravitational acceleration completely independent of their mass or density. You are adding a second gravitational acceleration and trying to say 'see, it's not the same', but of course it's not the same, because it's not the same question.

No. The acceleration is exactly as fast, there are simply two of them in opposite directions, so the RELATIVE speed achieved is twice as fast, but actual speed AND the acceleration is the same.

You are confusing 'closing speed' with acceleration, and confusing what was being discussed in the first place, which was that objects are attracted by gravitational acceleration completely independent of their mass or density. You are adding a second gravitational acceleration and trying to say 'see, it's not the same', but of course it's not the same, because it's not the same question.

Right. They will both accelerate towards each other at 9.8 m/s^2. So the total acceleration is twice as fast, and they hit in less time than if you drop a feather.

Right. They will both accelerate towards each other at 9.8 m/s^2. So the total acceleration is twice as fast, and they hit in less time than if you drop a feather.

I've run the math in physics class. My teacher showed me clearly how mass cancels out of the acceleration due to gravity equation, as does density in a vacuum.

I have no idea whatsoever what you mean about two earth sized objects. In that case, they accelerate towards each other at the same rate (even if one is made completely out of feathers!). What are you talking about? Now you're confusing things with 2 separate accelerations.

EDIT: This is why gravitational acceleration is labeled as 9.8m per sec per sec....period, and does not include 'per gram' or any other mention of mass/density. Mass cancels out. (I needed my teacher to prove this to me, and he did)

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