Starship Asterisk*

by **neufer** » Tue Jul 17, 2012 9:58 pm

Click to play embedded YouTube video.

Giordano Bruno wrote:
Supposedly on the Moon , Scott drops the feather and the hammer while Irwin is seen moving around . The question is : who is holding and operating the movie camera ???

http://www.youtube.com/user/Return2Moon09 wrote:
<<Giordano Bruno is a 22 km lunar impact crater on the far side of the Moon. The Narrow Angle Camera (NAC) on the Lunar Reconnaissance Orbiter acquired a sequence of images, which were used to create this seamless mosaic. The mosaic was then overlaid on a digital elevation model (DEM) that was derived from NAC stereo observations. This allows us to create virtual perspective views of the crater wall.>>

by **rstevenson** » Wed Nov 09, 2011 12:24 am

Zloq, thank you. You have shown me a subtle point of physics about which I was unaware. However, you did so while seemingly missing the fact that the old moon-landing video did exactly what it set out to do. It showed the public that the hammer and feather fall together. That's all the weight that little video -- that little physics experiment with its large uncertainties -- can carry. But thank you, sincerely, for the physics lesson.

Rob

by **zloq** » Tue Nov 08, 2011 8:58 pm

neufer wrote: It's amazing that that legendary story of Galileo dropping those cannonballs off the Leaning Tower of Pisa didn't permanently ruin science (if not Pisa itself) for all time. (Apparently Galileo was totally unaware of Newton's third law of motion.)

I suggest that you petition the White House for NASA to send a manned mission to the moon with a really heavy hammer so that we can finally settle this issue.

Well - you can make light of this, but I do take it seriously - because I have met so many people whose minds were imprinted at an early age with these images, backed by an incomplete, or just plain wrong, explanation of what's going on. Galileo and the tower is the physics equivalent of George Washington and the cherry tree - except there is no violation of common sense in that fable, whereas someone who didn't go to school at all might confidently answer, "the heavy ball falls first - obviously" - and be better off. Now - please - note that I am not talking here about the subtleties of how things work on the moon in a vacuum - here I'm talking about on earth and in air. It's precisely because these images are so compelling that something will get burned into the brain - for life - and I would prefer it to be something that is correct and actually aids in understanding and appreciating the natural world, rather than something that cripples that understanding. Or stifles a young, inquisitive mind that asks, "what if...?"

Science is really in trouble in the u.s., and it appears to be spreading to Europe. I think there are many reasons for this, but I also think it's important to be proactive when there is empirical evidence that certain teaching devices have unanticipated problems. I think the general u.s. population would do pretty poorly on a basic science test - but if you asked them - What happened with Galileo and the tower? - a lot of people would say - He dropped heavy and light spheres off a tower and they hit at the same time - just like that video on the moon with the feather.

As for your claim about science eschewing details - Yes - you can be distracted by details and fuss with them and make a big deal out of them - and be told just to ignore them. As happened here, when someone got distracted by a "bit of scruff":

http://en.wikipedia.org/wiki/Jocelyn_Bell_Burnell

zloq

by **zloq** » Tue Nov 08, 2011 8:15 pm

Ann wrote:zloq, I need to apologize to you. I sounded highly critical of those who question the idea that a hammer and a feather would fall at equal speeds, but your reasoning is interesting and your questions valid.

Ann

Thanks Ann. I did notice your earlier tone was kinda harsh, but I thought maybe you mistook what I was saying. Thanks for reading my later notes, and I appreciate your response.

zloq

by **Ann** » Tue Nov 08, 2011 6:28 pm

Art wrote:

Which is heavier: a pound of feathers or a pound of gold?

Yeah, well. When I was ten years old or so, I was asked which was heavier, a kilogram of cotton or a kilogram of lead. I answered that the lead was heavier. But, hey, I was only ten!

(Although I just heard that when great scientist Gauss performed some extremely advanced mathematical calculations when he was seven. So, yes, I know, I know, a tender age is not all that much of an excuse.)

Boy, six years old, to his Mom: Mommy, can a thing have only two corners?

Mom: Why don't you think about it for a while?

Boy (after thinking for a while): No, Mommy, it must have three corners. At least.

Mom: That's right. Good.

Boy: Mommy, how many corners can a thing have?

Mom: Why don't you think about it for a while?

Boy (after thinking for a while): Oh. That's gonna be a circle.

Ann

by **Ann** » Tue Nov 08, 2011 6:09 pm

zloq, I need to apologize to you. I sounded highly critical of those who question the idea that a hammer and a feather would fall at equal speeds, but your reasoning is interesting and your questions valid.

Ann

by **Chris Peterson** » Tue Nov 08, 2011 3:05 pm

zloq wrote:My point was that you said "exactly equal" twice in a single note - and all along I have been saying the difference in fall time is nonzero, strictly based on Newtonian mechanics.

I said it in two examples, both correct. One case is where the fall time is exactly equal is when the difference is less than 10^-43 second. The other is when the objects are falling under equal gravitational acceleration. (Newtonian mechanics itself is only an approximation of reality, so if you're going to complain about an experiment that deviates from theory by immeasurable differences, you really shouldn't complain about relativistic or quantum effects being brought up, since those things affect the object velocities more than Newtonian effects!)

My hope is that kids in grades 6-8 are given a correct explanation for how things work - especially if they ask probing questions such as, "What if the falling object is really massive?"

When I teach, I consider the point I'm trying to teach. What the video in this APOD is trying to demonstrate is NOT that two objects of different mass dropped on the Moon land at the same time. If you walk away with that, you've missed the point completely. What it is trying to teach is an important concept, that two bodies of different mass subject to the same gravitational acceleration move at the same speed (which goes against most people's intuition). In a typical middle school classroom, if I then confounded this concept by introducing a totally different one- Newton's law of universal gravitation- many of the kids would become confused and fail to understand the key concept completely. It would be a mistake to first explain that bodies falling in a uniform gravitational field land at EXACTLY the same time (which is true), and then try to modify this by discussing an entirely different concept that they aren't ready for yet. For the purposes of demonstrating that bodies subject to equal gravitational forces experience identical motion, the Moon-feather system is a close enough approximation to make for an excellent test, without considering (in a middle school classroom) where that approximation fails.

If a kid asks "what if the object is really massive" I'd answer correctly: it makes absolutely no difference. A body of the smallest conceivable mass falling next to a body of the largest conceivable mass, in the same gravitational field, will fall at exactly the same velocity. If there's some brilliant kid in the class to asks about mutual gravitational effects and recognizes that using a large mass to produce the "fixed" gravitational acceleration isn't perfect... well, great. I'll tutor him outside the classroom. That's not an issue most of the students need to be concerned with.

by **Beyond** » Tue Nov 08, 2011 2:40 pm

ha-ha, I'm not going anywhere near THAT spoiler

We all know that all that glitters is not gold, so there fore you may have a mixed bag of material in your pound of gold to throw us off

by **neufer** » Tue Nov 08, 2011 1:57 pm

zloq wrote:
My hope is that kids in grades 6-8 are given a correct explanation for how things work - especially if they ask probing questions such as, "What if the falling object is really massive?" More likely, they will be silenced and told that mass has no effect on how things fall on the moon, due to its lack of atmosphere and the equivalence principle. "Just look at the video!" Not to mention - how Galileo dropped wooden and iron cannonballs off the Tower of Pisa - and they landed at "exactly" the same time, as proof of the same point. A sketch of this historic moment may well be in their textbooks.

It's amazing that that legendary story of Galileo dropping those cannonballs off the Leaning Tower of Pisa didn't permanently ruin science (if not Pisa itself) for all time. (Apparently Galileo was totally unaware of Newton's third law of motion.)

I suggest that you petition the White House for NASA to send a manned mission to the moon with a really heavy hammer so that we can finally settle this issue.

My own personal feeling is that these inquisitive 6-8 graders might be better suited to becoming lawyers rather than scientists. (Perhaps one day they could even get Peterson off from having to pay for all those Texas speeding violations.)

This reminds me of that old question:

Which is heavier: a pound of feathers or a pound of gold?

by **zloq** » Tue Nov 08, 2011 8:40 am

Yes - keep it simple by bringing in quantum effects.

My point was that you said "exactly equal" twice in a single note - and all along I have been saying the difference in fall time is nonzero, strictly based on Newtonian mechanics.

My hope is that kids in grades 6-8 are given a correct explanation for how things work - especially if they ask probing questions such as, "What if the falling object is really massive?" More likely, they will be silenced and told that mass has no effect on how things fall on the moon, due to its lack of atmosphere and the equivalence principle. "Just look at the video!" Not to mention - how Galileo dropped wooden and iron cannonballs off the Tower of Pisa - and they landed at "exactly" the same time, as proof of the same point. A sketch of this historic moment may well be in their textbooks.

zloq

by **Chris Peterson** » Mon Nov 07, 2011 10:57 pm

zloq wrote:The second I think refers to the fact that the actual demo video involves both objects dropped at the same time, and again uses the phrase, "exactly the same time." This is a more complex 3-body problem (than dropping each mass separately) and whichever one hits first depends on the masses, geometry, surface curvature if present, etc. I don't see how they would land at exactly the same time unless the surface was specifically contoured to make this happen based on the trajectories of the masses involved...

I think you're taking a simple concept, well demonstrated by the video, and turning it into something complicated, and confusing for most people. I certainly wouldn't suggest in my 6-8 grade classroom that the objects don't land at the same time, since to do so would not be constructive to the point being made.

by **zloq** » Mon Nov 07, 2011 10:32 pm

Chris Peterson wrote: And if you do that, you need to consider that there will be a point where the mass difference between the dropped objects and the large body is so large that the difference in velocity is less than 10^-43 seconds, which means that the two land at exactly the same time.

Or, you can just recognize that the fundamental question here has to do with objects of different masses falling under identical gravitational acceleration. Under that condition, which is extremely closely approximated by a feather and hammer falling on the Moon, both objects land at exactly the same time.

Here are two conclusions where two different masses will land at *exactly* the same time. The first brings in quantum effects way outside the realm of Newtonian mechanics - and says they land at exactly the same time. The word "exact" does not apply and is inappropriate in a complex macroscopic quantum context - and dragging in quantum concepts is outside the scope of the demo anyway - as is relativity.

The second I think refers to the fact that the actual demo video involves both objects dropped at the same time, and again uses the phrase, "exactly the same time." This is a more complex 3-body problem (than dropping each mass separately) and whichever one hits first depends on the masses, geometry, surface curvature if present, etc. I don't see how they would land at exactly the same time unless the surface was specifically contoured to make this happen based on the trajectories of the masses involved.

zloq

by **Chris Peterson** » Mon Nov 07, 2011 8:03 pm

zloq wrote:Anyone who says the difference in fall time is zero, regardless of mass, is espousing a theory that violates multiple physical laws.

Fortunately, nobody seems to have made that claim.

by **zloq** » Mon Nov 07, 2011 7:50 pm

I clearly have failed for you - but I am not concerned about that. I don't know how to make this more clear for others: a videocamera on the moon recording a falling hammer would see it fall fairly slowly. A videocamera on the moon would see the earth fall much faster. There is nothing subtle about the distinction. The demo video would look very different, and show very different rates, if a very heavy object replaced the hammer. Of course - in the case of the earth, the camera would be noticeably accelerating upwards while attached to the surface of the moon - but it accelerates upwards, based on theory, for the hammer also. Either way - the video would show a different speed for the process, and the resulting time to impact would be different.

And none of this violates the equivalence principle.

And this answers some of the questions that came up earlier - correctly.

Anyone who says the difference in fall time is zero, regardless of mass, is espousing a theory that violates multiple physical laws. You can build perpetual motion machines with similar concepts, and I thought such crazy ideas weren't allowed here. I'm just going by Newton in this scenario. Whether or not the small motions are easily measurable is an irrelevant detail since I am describing the fall time based on theory - and we all know that good science abhors details.

zloq

by **Chris Peterson** » Mon Nov 07, 2011 6:35 pm

zloq wrote:My point has been to provide an accurate theoretical description of what happens when an object falls, and I have heard no criticisms of the theory - in fact an early response was that it was "obvious."

Then you have failed, because you have not described, in general, the behavior of a falling object. You have described the behavior of a pair of objects undergoing mutual gravitational attraction- at best, a subset of what it means to "fall". You have not described the theory which is actually being demonstrated: that objects of different masses fall at the same speed when subjected to identical gravitational acceleration.

by **zloq** » Mon Nov 07, 2011 6:26 pm

The video is meant to demonstrate a *theory* in action. I am talking about the *theory*, quite separate from the experimental difficulties of measuring small, but real, effects. Just because an effect is difficult to measure, or swamped by noise *in a measurement*, it doesn't mean it isn't playing a role.

If I can't measure the change in the velocity of jupiter as a spacecraft goes by, does that mean I should say it doesn't change, or that it is of no consequence? No - I say what theoretical assumptions I am making and I describe the expected effect as delta V even if I don't measure it. You are saying that if something is hard to measure because it is small, it should be regarded as zero - and should be taught to students that it is zero (ugh!). I am saying that some things truly are zero, according to theory, and other things are not. The difference in acceleration of two different masses toward another is expected, by the equivalence principle, to be ZERO. The difference in time to collision, based on Newtonian mechanics, is NONZERO. The difference in velocity of jupiter due to a small spacecraft slingshot is NONZERO.

I am not interested in debating this since I think my description is sound, but if others gained anything from the detail I presented, I welcome postings here. My point has been to provide an accurate theoretical description of what happens when an object falls, and I have heard no criticisms of the theory - in fact an early response was that it was "obvious." I am not lecturing on how science works or something - I am describing the details of how mass does play a role in a falling object in the specific context of this thread and its Newtonian assumptions - in hopes the detail is of interest to some readers here, including those who early in the thread expressed an inkling of puzzlement that the answer, ZERO, didn't seem right.

zloq

by **Chris Peterson** » Mon Nov 07, 2011 5:22 pm

zloq wrote:So - when someone asks - does a hammer hit the moon faster than a feather in a vacuum - the only correct answer is a resounding yes...

No, that isn't the case at all. On the Moon, there are many other forces at play which alter the speed objects fall, and those forces massively dominate over Newton's law of universal gravitation. As previously noted, on the Moon the feather probably falls faster. So if you want to frame the question, you need to consider the abstract case of isolated masses with no other physical forces present. And if you do that, you need to consider that there will be a point where the mass difference between the dropped objects and the large body is so large that the difference in velocity is less than 10^-43 seconds, which means that the two land at exactly the same time.

Or, you can just recognize that the fundamental question here has to do with objects of different masses falling under identical gravitational acceleration. Under that condition, which is extremely closely approximated by a feather and hammer falling on the Moon, both objects land at exactly the same time.

by **zloq** » Mon Nov 07, 2011 5:07 pm

I am discussing correct theory vs. incorrect theory. The video on the moon is meant as a demonstration of theory in action, and I think it should be explained as follows:

The equivalence principle says that one mass in the gravitational field of another mass will feel a force proportional to the product of the two masses, but its acceleration will be inversely proportional to its own mass, based on second law. As a result, its own mass cancels and its acceleration toward the other mass is independent of its own mass.

For a small mass suspended over a larger mass and released, each mass will accelerate toward the other based solely on the "other" mass. As a result, small changes in either mass will change the time to collision. The hammer "sees" the big moon and accelerates rapidly toward it; the moon "sees" the small hammer and accelerates slowly toward it. The interaction is symmetric, but the rates are different, and the acceleration of each mass depends only on the other mass.

So - when someone asks - does a hammer hit the moon faster than a feather in a vacuum - the only correct answer is a resounding yes, with the above explanation and a clarification that it's all based on the equivalence principle and Newtonian mechanics - and the effect would be very, very small for objects like hammers, but dramatic if you "dropped" the earth onto the moon. The key benefit of explaining this is it conveys the underlying symmetry of the interaction, and nails down the role of the equivalence principle. And it is correct rather than incorrect.

Apparently you want to say, "The moon is so super big, it doesn't move at all as the hammer approaches. Don't worry your little head about it." That is pre-Newtonian thinking in the year 2011, in an astronomy forum - and ignores the symmetric role of gravitational interaction, along with conservation of momentum.

For some reason you both want to maintain a certain level of dumbing down of this discussion, and I think it's better to describe the actual theory involved. I'm not sure why the two versions - dumbed down and theoretically accurate - can't both be presented and either absorbed or ignored by readers. You have presented yours: Times are "effectively" (?) identical "due to the equivalence principle" (?) and good science never sweats the details. I have presented mine: Times can be very different depending on mass, and symmetry helps the understand the dynamics.

Either way - thanks for the two lectures on how Good Science works.

zloq

by **Chris Peterson** » Mon Nov 07, 2011 2:55 pm

zloq wrote:I think it's important to separate an accepted principle from the complexities that result when that principle is applied to a dynamic process. As a principle, it is regarded as an absolute. When applied to a time-dependent two-body problem, the results are not absolute at all. The principle is absolute, the resulting timings are not. Nothing to do with potential experimental error or compounding factors, since it is all Newtonian theory. You seem to view it as an engineering problem where little errors don't matter to the customer, but I'm talking about a fundamental principle and its scientific application.

I honestly don't know what you're getting worked up about. Theoretical physics has long recognized the difference between two bodies undergoing gravitational attraction and a single body moving under fixed gravitational acceleration. Practical applications of that physics recognize that there are cases where the two can be considered equivalent- such as a feather and hammer on the Moon- because the deviation between them is orders of magnitude smaller than our best ability to measure that deviation, and cases where they cannot- because the two masses are not radically different.

Good science has a tradition of not caring about details? Wow.

Good science recognizes which details are important or relevant to an experiment, and which are not. In the case of the hammer/feather experiment, Newton's law of universal gravitation is not relevant, and the equivalence principle is.

by **neufer** » Mon Nov 07, 2011 2:46 pm

zloq wrote:
Good science has a tradition of not caring about details? Wow.

Absolutely!

by **zloq** » Mon Nov 07, 2011 2:26 pm

I think it's important to separate an accepted principle from the complexities that result when that principle is applied to a dynamic process. As a principle, it is regarded as an absolute. When applied to a time-dependent two-body problem, the results are not absolute at all. The principle is absolute, the resulting timings are not. Nothing to do with potential experimental error or compounding factors, since it is all Newtonian theory. You seem to view it as an engineering problem where little errors don't matter to the customer, but I'm talking about a fundamental principle and its scientific application.

This distinction is not subtle at all if you consider the paradox of the moon falling onto the earth, vs. the earth falling onto the moon. The principle of equivalence still applies - but if you think all objects fall to the surface of the moon at 1.6 m/s^2, while at the same time all objects fall to the surface of the earth at 9.8 m/s^2, then something has got to "give." If you think 1.6 m/s^2 "always works pretty good" for objects falling on the moon - then I have provided a counter-example fairly close to home where a better guess is around 11.4 m/s^2, and your approximation is off by an order of magnitude.

It's clear that some people are not interested in this distinction - but again I refer to earlier comments by others that were alluding to it in their questions, and to the simple fact that the distinction I make here is not well known, and I think should have been mentioned in this discussion - assuming people did in fact know about it before I brought it up.

Good science has a tradition of not caring about details? Wow.

zloq

by **neufer** » Mon Nov 07, 2011 12:46 pm

zloq wrote:
For people who don't care for these details - as usual they don't need to bother with them. For others - this is a description of the actual dynamics, and how they relate to the equivalence principle - that was missing in this thread - and that a few people seemed correctly puzzled about.

Science is nothing more than a hierarchy of different approximations that seem to work quite well at their own individual levels.

It is fine to mention a higher level of approximation if you like but you shouldn't dismissively jump over intermediate levels of approximation as if they didn't exist.

For people who do care about details we have tried to enumerate numerous considerations that are much more important to the actual dynamics than the miniscule movement of the moon.

Most people, however, don't care about details and that also is in the tradition of good science.

by **zloq** » Mon Nov 07, 2011 11:54 am

Actually no - I don't get how this is somehow confused with the moon landing hoax, for example. People are bringing in other complicating effects that would make it hard to measure this effect - but they are outside the Newtonian realm of the demo.

Deepstar1 showed insight that - surely - something else is going on here that would affect the fall - and his point was glossed over. Somehow the combined masses would make a heavier object hit the surface sooner? Aren't they pulling on each other more so they would collide faster? Well - yes - that's exactly right, but it's a very small effect if one mass is a lot smaller than the other.

Early on, some people requested really good empirical measurements to confirm the times are the same, and were told - there is no need. The fall time is the same due to the equivalence principle. Well - it isn't, and careful measurement would in fact reveal this effect.

As for the impracticality of the point I'm making - ironically if there is anything impractical it is the concept that heavy objects fall as fast as light objects. On earth, with friction and air resistance, this is never true. Yet - somehow - people who see this demo, and who hear the fable of Galileo dropping spheres off towers, believe that a wooden ball would hit the ground at exactly the same time as an iron one, on earth, in air. I think a very large percentage of people have that image in their heads - even though it is completely counter to what would actually happen, and to common sense.

For people who don't care for these details - as usual they don't need to bother with them. For others - this is a description of the actual dynamics, and how they relate to the equivalence principle - that was missing in this thread - and that a few people seemed correctly puzzled about.

zloq

by **neufer** » Mon Nov 07, 2011 10:44 am

zloq wrote:
I was very careful to talk about dropping one object at a time because the two-body dynamics are easier to understand than more complicated 3-body dynamics of a feather, hammer and moon. If you replace the hammer with a small object that has the mass of the sun (no need to worry about black holes since this is Newtonian - it is just a small, very dense object) then the mass won't "fall" much at all and instead the moon will shoot straight up while the feather slams sideways into the object.

We all understand your point, zloq.

Do you understand our points?

by **zloq** » Mon Nov 07, 2011 7:54 am

I'm describing everything in the specific context of Newtonian physics and the equivalence principle - which is also the context of the demonstration. This thread, and the demonstration, imply in very absolute terms that a fundamental principle of nature will cause the falling of one body onto another to be independent of the mass of the smaller body, and that isn't true. If you are standing on the moon and you drop a heavy object - sure - the moon will rise up to meet it, just as the object will fall down to meet the moon. There is nothing wrong in my language. It's the symmetry of the interaction of the moon to the object and the object to the moon that is part of the Newtonian lesson here - and it is as fundamental as the conservation of momentum - and shouldn't be overlooked.

I was very careful to talk about dropping one object at a time because the two-body dynamics are easier to understand than more complicated 3-body dynamics of a feather, hammer and moon. If you replace the hammer with a small object that has the mass of the sun (no need to worry about black holes since this is Newtonian - it is just a small, very dense object) then the mass won't "fall" much at all and instead the moon will shoot straight up while the feather slams sideways into the object.

So- when people talk about how fundamental this stuff is and the mass doesn't matter at all - conservation of momentum is also fundamental in Newtonian physics, as are equal and opposite reaction forces. These two things were ignored in this thread but shouldn't be forgotten. Anyone who quietly pondered, either in this thread or in a classroom, "What if the hammer was REALLY heavy?", might think it doesn't matter due to "the equivalence principle." What never changes is the acceleration the hammer, of any mass, experiences toward the fixed center of mass of the moon/hammer - but both the moon and the hammer accelerate toward, and "fall", to that center of mass. How much the moon accelerates and moves before they make contact depends on the mass of the hammer.

If people think this stuff is so small that it gets lost in the noise somehow - humans often aim probes near to other planets so they can experience this symmetric dynamic, resulting in a tiny perturbation of the planet's position and orbit around the sun, that is transferred to the probe. That's another 3-body problem, but the point is - planets do respond to human-made objects in their gravitational fields.

zloq

Starship Asterisk*

APOD: Hammer Versus Feather on the Moon (2011 Nov 01)

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