Wikijunior talk:Solar System/About gravity, mass, and weight

Weight: does it mean mass or force?
Please also see other discussions on this topic at and Weight and User:Kernigh/pound.

Renewing perspective
I was going to step back for a while to see what came of people's contributions other than mine or from Metric1000, but then thought something I wrote elsewhere might help:

"'I'd say that as most of us aren't practising scientists we don't use the word 'mass' that much in our everyday life on Earth...'"

If so, I reckon young people must use the word 'mass' even less - I probably didn't use it until I started studying science as physics, chemistry and biology - so why don't we write this module from the point of view of your 'weight' - i.e. that thing you measure when you step onto the bathroom scale - changing from one planet etc to the next, whilst you don't (unless, say, you go on a diet or start bingeing)? Then say the "stuff" that goes to make up you (or anything else) is what you may've heard people refer to as 'mass' (especially scientists) so although your or something else's 'mass' remains the same from planet to planet or the like (so long as you don't diet or binge etc) your 'weight' changes. Why does your 'weight' change? Enter a little about gravity. Well, something along those lines and not necessarily in that order.

Potentially much simpler and I don't think it need compromise the situation we have here on Earth where most (adult) people use the words 'weight' and 'mass' interchangeably.

David Kernow 19:59, 24 October 2005 (UTC)


 * How is this a weight point-of-view if you still mention mass and gravity? --Kernigh 20:41, 24 October 2005 (UTC)
 * Sorry not to be more clear; I was writing in more of a stream-of-consciousness manner than usual. What I mean is start the module from something young people are likely to've experienced, viz. that measurement called 'weight' given to you by a bathroom scale. This is certainly a measurement that would produce a different result when tried on the Moon, Mars, etc. Then move on to contrast that with what doesn't change, viz. your mass. Then touch on gravity to indicate why there's a difference. David Kernow 23:05, 24 October 2005 (UTC)


 * Suppose you have a fishing line that will break if a force of 50 newtons is applied to it. With that, you should be able to lift a 5 kilogram catfish out of the river on Earth.
 * Now, suppose you put your boat out on the Sea of Tranquility. That line will still break if a force of 50 newtons is applied to it.  But now you should be able to lift a 31 kg catfish out of the "Sea" with that line.
 * Then, cut out a couple of steaks, throw them on the fire, and toss on a handful of green cheese as it gets warm! "Oooohh, sooo good!"  Or maybe that belongs in the Cookbook namespace? Metric1000 15:45, 25 October 2005 (UTC)


 * That doesn't work at all. "Your weight" DOES NOT CHANGE as that term is used by professionals in the medical sciences, nor even as it is used in sports such as the Olympic weight classes (63 kg, etc.) for judoka, boxers, wrestlers, and the like.  Do you suppose you could calculate for us how much difference there was in weightlifting in the mass of a 20 kg "weight" at the Helsinki Olympics and the mass of a 20 kg "weight" at the Mexico City Olympics?


 * I agree, we do not often use the word "mass" in our everyday lives. But that doesn't mean we are unfamiliar with the concept.  We do indeed very often use the quantity which physicists or astronomers call by the name mass in their own jargon.  We simply call it by the name we had already been using for hundreds of years before anybody ever used the word mass with that meaning:  we call it weight.


 * Like I said before, you 'can choose not to call this quantity weight. But that is your only option.


 * It is not a legitimate, viable alternative to continue to call it weight, but to misapply a definition of that ambiguous word which is inappropriate to the context.


 * People have long known that it is harder to push a "heavy" boat away from the dock than it is to push a "light" boat away from the dock. That is one everyday way in which people have experienced the effects of mass in a way mostly unrelated to the pull of gravity, something we as a people have been familiar with for thousands of years. Metric1000 20:51, 24 October 2005 (UTC)

Please, Metric1000, this is a module in a book meant for young people. I hope you'd agree that 'weighing' yourself on a bathroom scale is something young people are likely to've encountered (unlike the notion of Olympic weight classes, changes in 'weight' due to altitude, etc) and your 'weight' as given by such a scale would change if you 'weighed' yourself using it on the Moon, Mars, etc. Perhaps you'd prefer to say something like "the reading has changed" as if something else hasn't, but what other than 'mass' would that "something else" be?

"'I agree, we do not often use the word 'mass' in our everyday lives. But that doesn't mean we are unfamiliar with the concept...'"

My point at the start of this thread is to suggest that most young people are unlikely to've linked the word 'mass' with the concept of 'amount of matter'. I'm sorry if you don't think that's clear.

David Kernow 23:05, 24 October 2005 (UTC)


 * Yes, of course, it works the other way as well. You don't have to call it mass; you can use the familiar synonym weight instead.
 * But you still can't apply the wrong meaning to it, and say or even imply that the students' weight would change.
 * Your bathroom spring scale just won't quite work right on another planet. Maybe you could paste on a new dial with the readings we want.  If your tool is broken, fix it.
 * It's much like astronauts weighing themselves in a "weightless" environment. None of our normal scales work at all.  So they had to find a different tool.  (What they do is set a spring in motion, and time its oscillations as they ride on it.)
 * Note also another problem with those spring scales, even if what you want to measure is force. Suppose you have one whose dial makes a full circle with 300 lb on it on Earth.  Then our 86 lb student will deflect this scale by about 103 degrees of arc.  But on the moon, that deflection will only be 17 degrees of arc.  The springs are too stiff to get much deflection at all.  On Pluto it would be much worse; only about 6 degrees of arc.  The springs are just too stiff to work very well for this purpose in those environments.
 * So if you have to change springs anyway, you might as well fix the dial at the same time, to get a good reading of mass in the location in which it will be used. Metric1000 15:30, 25 October 2005 (UTC)


 * Or, just choose the new springs with a spring constant that will work with the old dial in the new environment. Metric1000 15:35, 25 October 2005 (UTC)

"'Yes, of course, it works the other way as well. You don't have to call it mass; you can use the familiar synonym weight instead. But you still can't apply the wrong meaning to it, and say or even imply that the students' weight would change...'"

You hang something from a spring on Earth and the spring extends. I say that this extension indicates weight. Do you? You hang the same thing from the same spring on the Moon and the spring extends, but not to the same length as when on Earth. Do you agree? I say that this extension indicates weight. It is not the same length as the extension when on Earth, so I say the weight it indicates is not the same as that on Earth. In other words, I say the weight has changed.

Weight changes when gravity changes. Mass doesn't change when gravity changes.

We need to come to some agreement on this. Anything else is and has been distraction.

David Kernow 20:42, 25 October 2005 (UTC)

Just don't use units like kilograms as units of weight. That is the main thing Metric1000 and I have been saying. The implication is that most science museums that have you step on a scale and see what you would weigh on the Moon or Jupiter (I've seen it) is completely unrealistic and attempts to suggest that your mass changes when you go to different planets. That doesn't happen at all and is the whole point of this article and the "myth buster" that we are trying to cover. The fine point that is a source of contention is that pounds-force is sometimes used to describe weight and used as a demonstration for how much your weight would change in a different gravitational field. Pounds are used for both mass and force calculations. This is most painfully demonstrated with the calculation of units for specific impulse of a rocket, which is measured in pound-seconds/pound, and usually just reported as seconds of specific impulse. Unfortunately, this is actually pound(force)-seconds/pound(mass), so this isn't really proper reduction of units. SI units for the same concept is N-s/kg, which shows more clearly how that is a false concept for seconds of impulse. Specific impulse, BTW, is the measurement of the effeciency of a rocket engine and a way to compare how powerful one engine type is compared to another rocket design. Rocket scientists know about this unit screwup, but most scientific literature uses this corrupted unit of measurement anyway, even outside the USA.--Rob Horning 17:33, 26 October 2005 (UTC)


 * Your "myth buster" is completely out of place in a Solar System book. It's way way off topic. This is not the place to be arguing over units for mass and weight, even if the subject wasn't unitless in the first place. AlbertCahalan 03:19, 27 October 2005 (UTC)


 * It is not off topic but indeed is the very topic that is the subject of this section. This section was added to this Wikibook because it was felt that there were some "special topics" that covered all of the planets and would be endlessly repeated over and over again for each seperate planet.  This is indeed done with other Wikijunior books as well, so it is not a unique circumstance.  I will agree that this Wikimodule is a train wreck right now in terms of having it flow throughout the whole article.  It needs to change and that is apparent.  All I'm trying to do is to make a compromise that would be acceptable to everybody, and to point out what effect a change in gravity can have.  This is something hard to grasp, which is precisely why we seem to be having all of this argument right now.  I don't think a "unitless" relative comparison of each planet's acceleration at the surface is going to work out for kids.  --Rob Horning 08:48, 27 October 2005 (UTC)


 * Heh. It's on-topic for a section that is off-topic for the book. In other words, the subject matter matches the page but the page does not belong in this book. This isn't a physics book. It doesn't cover the math of orbiting bodies in space, does it? (I sure hope not) I think the proper place for this page is here. AlbertCahalan 23:42, 27 October 2005 (UTC)

ditch the units
This is silly. The use of unnecessary units for unitless values is bad, even if we didn't have to deal with poundforce and such. Probably this page should just be deleted.

Do like this:

People on Endor are pulled down with 1.23456 times as much force as they would be on Earth.

That's the number you need if you want to take your weight, in any system of units, and multiply it by something to get a feel for the difference. It works for any loose idea of stones, pounds, newtons, grams, slugs, etc.

If you decide to get all scientific, you can argue about units for acceleration, not force. Acceleration of gravity on Earth is 20/7 rods/microfortnight/microfortnight.

AlbertCahalan 23:49, 24 October 2005 (UTC)


 * This is exactly what I've done, if you look at the section "How does gravity change what happens?", I put relative gravitational pull that is merely a comparison to that of the Earth. BTW, I love both rods as length and microfortnights as a unit of time.  I even worked on a computer operating system (VMS) that used microfortnights as the internal clock measurement for system accounting purposes.  Only later did I catch the joke there, however.  Windows NT also used fortnights as a unit of measurement because after a fortnight of running continuously the OS would crash from an internal overflow bug in the system clock. (actually about 39.5 days of running, but that is close enough to a fortnight for this purpose.) --Rob Horning 04:15, 25 October 2005 (UTC)


 * Which is correct?
 * a. The Earth's atmosphere is 75.5% nitrogen.
 * b. The Earth's atmosphere is 78.1% nitrogen.
 * The answer, of course, is both. It depends on what is being compared to get these dimensionless ratios.  It is 78.1% by volume (and probably the same,  or very close to it, on an amount of substance basis).  It is 75.5% by weight (the terminology often used in science as well as in our everyday lives, though some scientists will insist on using the equivalent phrase "by mass").
 * The dimensionless ratio for "my weight" on Mars compared to "my weight" on Earth is 1.00.
 * AlbertCahalan is presumably well aware that if his phrasing is used, a good number of teachers are going to be telling their students that his 1.23456 number is the ratio for their "weight".  Metric1000 13:33, 27 October 2005 (UTC)


 * Great, so you agree with me? If not, well, I'm not seeing a complaint. We can perfectly well say "78.1% by volume", "75.5% by weight", or "75.5% by mass". Probably the "by X" part belongs in a footnote though. We can perfectly well say that the 1.23456 number is for weight. While we could equivalently say that it is for acceleration, we'd best not make things needlessly complicated in the Wikijunior Solar System book. Never lose sight of the subject matter of this book. This is not the Wikijunior Newtonian Physics or Wikijunior SI Measurement book. AlbertCahalan 23:52, 27 October 2005 (UTC)

the gravitational force of me on the Earth
The module currently claims

''For instance, the Earth is very massive and pulls on you very strongly. When you jump up, it quickly pulls you back down to the ground. You are also pulling on the Earth — but your mass is tiny compared to the Earth, so your pull on the Earth is tiny too.''

That seems incorrect. Aren't these 2 forces exactly equal and opposite?

Rather than fixing this example to make it true, but confusing, perhaps we could pick a more appropriate example -- perhaps compare the pull of the earth on you to the pull of a bowling ball on you ? Or the pull of the earth on a bowling ball to the pull of you on a bowling ball? Or the pull of, say, the International Space Station on an astronaut clambering on its outside to the pull of the earth on that same astronaut at the same time? (Even though that astronaut is nearly "weightless", his mass is the same as when he is walking on earth, and the force due to Earth's gravity is nearly the same as when he is walking on earth). --DavidCary 02:11, 28 October 2005 (UTC)


 * It is interesting that the term "weightless" is being depreciated for internal use by NASA and other space science researchers. The new term that is more commonly used is microgravity, to suggest that gravitational fields do exist in orbital environments, but that the acceleration effects are essentially insignificant in term of the calculations in the experiments.  The ISS does indeed have a gravitational pull on objects around it, but only equivalent of a small asteroid of the same size, roughly, being about 100 meters from one end to the other.  The ISS has high mass density components (like titanium and steel) but also has some interesting structure that would make for some interesting gravitational effects.  This is also supposedly one reason (the huge size of the ISS) why many science researchers are claming that the ISS is useless for some critical experiements:  It is so large that the gravity from the station itself starts to become a factor.  I am not kidding here either.


 * BTW, in regards to your question above, when you jump up on the Earth, you pull the Earth back toward you as well as pulling you toward the Earth. I guess it is a matter of perspective, but the whole Earth does indeed move just a little bit toward you when you come back down.  Maybe the distance is the diameter of a proton, but it is something that is technically measureable.  This is also just a matter of point of view, where both are valid points of view in trying to determine which is correct.  I'm talking scientific point of view, not political point of view here.  Relativity vs. discussions of relatives. --Rob Horning 03:07, 31 October 2005 (UTC)

Alternate approach to this module (About gravity and weight)
This module is indeed a "train wreck" and I am beginning to wonder if it is or will cause more trouble than it's worth. As it stands currently I feel it is out of kilter with the rest of the book, is too long, over-tabulated and introduces more words and concepts than it explains - some of which surely must be inappropriate for the intended audience (e.g. "Astronomerese").

However, in the spirit of trying to be constructive, I've created an alternate version of this module which:


 * 1) Tries to stay simple and succinct (although I realise there's room for improvement, e.g. shorter sentence lengths, fewer subclauses, etc);
 * 2) Does not refer to mass;
 * 3) Does not refer to units;
 * 4) Does not state that weight changes, but rather that (underline emphases added):
 * "You may've read or heard people sa y that your weight would be different on the Moon...";
 * "[the bathroom scale] would sa y you weighed less than when you weighed yourself with it on the Earth. So people sa y your weight has changed..."
 * "it's as if ... you have no weight..."

If people think this approach is a starter then I think a couple of illustrations or diagrams and perhaps a table of what a bathroom scale would say you weighed on other planets would add interest to the page.

I hope this helps.

David Kernow 02:30, 28 October 2005 (UTC)

Changed approach to this module
I have edited this module. Unlike with the alternate module, I intend to use the word mass. (Notice that "mass" is in the title of this module.) The intention of my edits was to improve the introduction of kilograms and newtons as mass and force (of gravity).

I did remove the references to scientists thinking that "weight" should be force, and astronomers thinking that they should always use the metric/SI system. I also reworded the introduction in a way so that it introduces SI kilograms and newtons, but never introduces pounds; this might need to be changed. (There was never any introduction to stones.)

I also placed the sections about inertia into comments. (The sections did not use the word "inertia", but referred to difficulty in starting and stopping objects.) Inertia is probably too much for this module. Remember that this is a module that ignores all forces other than gravity; it never introduces the idea, for example, of a force for a person to lift a heavy object. --Kernigh 05:07, 3 December 2005 (UTC)

Pluto
Pluto is a really bad example of forces. For one thing, we don't really know all that much about Pluto. It's better for this purpose, to use something more familiar, perhaps the Moon. Or maybe Mars or Venus. The Moon is probably best, because we see it so often, it's close. And people have actually walked on it

I've edited much of the article before the section that brings up Pluto. I've made the language use "we" and "us" instead of "you." It's more effective, speaking with kids and giving examples of things that humans experience, to not speak to them as if they or we were aliens. "You may fall when you jump, but I only fall if I want to."

I'm only guessing, I haven't looked yet, and don't have time right now, but I suspect that this may be an issue through the whole book. --Abd (talk) 17:26, 10 September 2010 (UTC)

Checking out that Pluto thing, I found out what happened. May 1, 2008, the page was hit by a vandal, 216.180.4.34. It wasn't noticed. The next day, the page was again vandalized. That was noticed, and it was reverted. Back to the vandalized version, which was more subtle vandalism. The same editor had been frequently reverting vandalism to the article, but may never have read the whole article. Happens. Anyway, there used to be a full table, which was removed that day in 2008. I'll restore it. --Abd (talk) 01:12, 11 September 2010 (UTC)

The table
The table claims that you could kick a ball fast enough to escape the gravity of Phobos. However, several calculations there may have been off for Phobos. Or not. It gets complicated because Phobos is small, and, in general, for small values of how high you could toss a ball, compared to the radius of the planet or moon, if the gravity is cut in half, you can toss it twice as high. But as the speed with which you can toss it increases, and you approach escape velocity, the ball will go higher than just that ratio, until when you reach escape velocity, it would never fall back. I did some searching on this and found sources that confirmed should be easy to throw a baseball with more than escape velocity on Phobos, so it would indeed go into orbit around Mars. But I'm not sure at all about the statements of how high one could jump. It's not a simple calculation from the force of gravity, and I don't have time to do the math and make sure I get it right. So where I left the page may be quite imperfect. --Abd (talk) 02:52, 11 September 2010 (UTC)

Page strategy, and gravity on the moon
There's a misleading passage in the current revision of the page (as of this writing). I'll try my hand at fixing it if someone else doesn't beat me to it (which, let's face it, they might).

We want, I think, a clear and engaging explanation of the major elements of the law of universal gravitation, by going unhurriedly through the sequence If there's a reason why that wouldn't work, it's probably that the details get too esoteric, at about the time one has to qualify the second point by saying that it's at the same distance; but that can probably be finessed successfully.
 * mass is how much material;
 * gravity at a given distance is proportional to the mass of both objects, so at equal distances from the Moon and the Earth, the Moon's gravity is only 1/81 as much as the Earth's; and
 * gravity is much stronger (inverse square) when you're closer, so since the moon is much smaller than the Earth, putting you almost four (three and two thirds) times closer to it when you're on its surface than when you're on the Earth's surface, that makes up for a lot of the smaller mass of the Earth.

Right now, the progression from point two to point three seems to be a bit rushed, and the part about equal distance isn't clear when point two is made, so that it comes out sounding as if you'd weigh 1/81 as much on the surface of the moon, instead of three and two thirds squared times that. --Pi zero (talk) 15:32, 11 September 2010 (UTC)


 * Thanks. I think I fixed it. I wasn't trying to generate a mathematical calculation, except for two relationships: the force of gravity between two objects varies directly with the mass of either object, i.e., double either and the force doubles. Half either and the force halves. And then I added to the article the inverse square relationship, that may be within reach of the target age, at least some of the kids. If this were a physics page, I'd go into a lot more detail, it is actually not difficult to explain why you get an inverse square law with various phenomena. See how you like what I did. --Abd (talk) 00:58, 13 September 2010 (UTC)


 * That does seem to fix the problem, yes.
 * I appreciate that getting drawn into details on WJ can be a slippery slope. --Pi zero (talk) 14:19, 13 September 2010 (UTC)

Yeah. The trick is to write things that are correct when seen from a more informed perspective, but that also convey simple truths without unnecessary complications. Looking above, there has been a huge flap over "weight." The problem is that the word "weight," in ordinary usage, means two different things. It is used to refer to the force applied, and it is used to refer to mass. Good pedagogy starts from what kids already know, and it leads them from that into wider understanding. So demands above that units of weight not be used had completely lost the purpose.
 * Kids know that it is harder to throw a heavier object than a lighter one. This isn't an effect of gravity, it's an effect of mass. It is just as hard to throw a baseball on the moon (and we completely neglected the weight or clumsiness of a space suit, perhaps the baseball player is inside a habitat big enough to play baseball in) as it is on the Earth. That is, you should be able to throw the baseball with about the same speed, particularly if you throw it horizontally. It will go further because it will not fall as quickly, that's all. Not because it is easier to throw.
 * But "newtons"? We can introduce the unit of force, but we shouldn't depend on it. What we can do is to make clear that when we say "weight," we are talking about the force of attraction, which depends on two things: the gravity and the mass (as well as the distance from the main source of gravity). But we need to keep in mind that "weight" is used commonly to mean "mass" as well, which is obvious, you know exactly what I mean when I say that a "heavier" ball is harder to throw. We mean that a 20 lb. ball, maybe a bowling ball? is harder to throw than an 8 oz. ball. Those are all units of weight, and we'd measure them with a scale. I measure mass with a scale, all the time. What is actually being measured, of course, is the force of gravity, so I'm assuming, in my weight measurements, a gravitational field of 1 g.
 * I wanted to explain what "g" means better, and I wanted to use units of milli-gs, one-thousandth g, in the table. The reason is that it's much easier to read and grasp the whole numbers. We could use centi-g, and the table could express the gravitational field value in percentage of Earth gravitation. So the Earth would be 100% and the Moon would be 16.7%. And Phobos would be 0.5%. It's okay that a value of less than 1 is used for Phobos.... I think I'll do this.
 * I think it's inspiring for kids to think of what it's like when there is no apparent gravity. It's an error, though, to say that there is any place with no gravity. Gravity is everywhere, but when there is nothing to resist it, we are in "free fall." There is gravity acting on satellites in orbit and on the astronauts appearing to float in their space-suits, and on everything inside the International Space Station. But because it is all "falling" together, it seems like there is no gravity, that nothing has weight.
 * I remember getting it, I was very young, that a satellite was "falling." It's just that (thinking of a circular orbit) it falls "around" the Earth. The gravity pulls it down just enough to make it move in a circle around the earth. Obviously, it has to be going pretty fast! And, obviously, if it were moving this fast near the surface, where there is air, air resistance would quickly slow it down. (It would, in fact, get immediately very hot like a meteor, and it would then quickly fall to the ground). But on the moon, an object could be very low and still remain in orbit. (But there is, I believe, a very thin lunar atmosphere that would slow it down, don't know how long that would take.)
 * I think most people imagine (without thinking about it) that there is no gravity in space. So we want to educate to dispel this idea. Kids can get that, I believe, easily.
 * I think we also tend to imagine that it is dark in space, but maybe kids now, having seen so many bright photos of the space shuttle and ISS, don't have that mis-impression. It's very, very bright, at least in the vicinity of the Earth. Way, way, out, it gets dim as we get very far from the Sun. --Abd (talk) 15:43, 13 September 2010 (UTC)