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

[neufer]

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?

A pound of feathers is ≈21.53% heavier than a pound of gold.

<<Troy ounce is a traditional unit of gold weight. Troy weight is a system of units of mass customarily used for precious metals, gemstones, and black powder. Named after Troyes, France, the troy system of weights was known to exist in medieval times, at the celebrated fair at Troyes in northeastern France. There are 12 troy ounces per troy pound, rather than the 16 ounces per pound found in the more common avoirdupois system. The avoirdupois pound is 147⁄12 (≈ 14.583) troy ounces.

The troy pound (troy) is 5,760 grains (≈ 373.24 g, 12 ozt), while an avoirdupois pound is ≈21.53% heavier at 7,000 grains (≈ 453.59 g).

[Beyond]

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

[Chris Peterson]

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.

[Ann]

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

[Ann]

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

[zloq]

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

[zloq]

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

[rstevenson]

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

[neufer]

Click to play embedded YouTube video.

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 ???

<<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.>>