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Thread: Earthly Items On The Lunar Surface Questions.

  1. #1
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    Earthly Items On The Lunar Surface Questions.

    How long would it take a well tuned ten minute hour glass to empty?

    Would an Earthly sundial be of any use at all?

    How would you design a lunar sundial to keep Earth time? (That's probably a hard one.)

    Would Newton balls travel their arcs the same speed or slower? Or would they start slow then speed up?

    Indoors:

    Would a top spin longer in lunar gravity than on Earth, or is that governed by another principle?

    In a kitchen pressurized to STP are cooking times affected/effected by lower gravity? (Since humans can survive long periods of microgravity, I pressume not a lot of chemestry has a gravity function.)

    To be fair, would you have to stand six times farther away from a dart board? Though I can't see lunar gravity giving someone much of an advantage in a regulation dart game. ( I played A LOT of darts thirty years ago.)

    Ping Pong, how much of the velocity is gravity based and how much is from smacking the ball?

    Has it ever been decided one way or the other if humans can develope and come to term safely in micro or lunar gravity?
    Time wasted having fun is not time wasted - Lennon
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    Nice questions! I can speculate about a few...

    Quote Originally Posted by BigDon View Post
    How long would it take a well tuned ten minute hour glass to empty?
    I imagine that differing gravity would have a small effect on how fast the sand gets through the bottle-neck, so I'd guess somewhere about 10 minutes and 15 seconds... we could test this by putting an hour glass in a centrifuge and seeing how long it takes at 6gs.

    Quote Originally Posted by BigDon View Post
    Would an Earthly sundial be of any use at all?
    Yes. The shadows progress on the moon as the Sun crosses the sky.

    Quote Originally Posted by BigDon View Post
    How would you design a lunar sundial to keep Earth time? (That's probably a hard one.)
    I'd have to make it very large, and have lots of labeled curves on it.

    Quote Originally Posted by BigDon View Post
    Would Newton balls travel their arcs the same speed or slower? Or would they start slow then speed up?
    They'd go slower, just as the speed of a seconds-pendulum would go slower.

    Quote Originally Posted by BigDon View Post
    Indoors:

    Would a top spin longer in lunar gravity than on Earth, or is that governed by another principle?
    The top loses its rotation through friction and wind resistance. Assuming one-sixth less force downward, and spinning in a vacuum, I'm guessing it would go lots longer on the Moon.

    Quote Originally Posted by BigDon View Post
    In a kitchen pressurized to STP are cooking times affected/effected by lower gravity? (Since humans can survive long periods of microgravity, I pressume not a lot of chemestry has a gravity function.)
    I don't yet see why gravity would have an effect here.

    Quote Originally Posted by BigDon View Post
    To be fair, would you have to stand six times farther away from a dart board? Though I can't see lunar gravity giving someone much of an advantage in a regulation dart game. ( I played A LOT of darts thirty years ago.)
    I don't think the arc of the dart is a large fraction of the uncertainty in the repeatability of a dart throw... so no, I'm guessing no.

    Quote Originally Posted by BigDon View Post
    Ping Pong, how much of the velocity is gravity based and how much is from smacking the ball?
    gravity affects things a little in ping pong. Air resistance is a bigger effect.

    Quote Originally Posted by BigDon View Post
    Has it ever been decided one way or the other if humans can develop and come to term safely in micro or lunar gravity?
    I don't think we know completely. Microgravity has some obstacles we don't know how to fix yet... you've seen the thread about the male astronaut's eyes.
    Forming opinions as we speak

  3. #3
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    Just responding to a few of these where I might be suggesting something a little different from antoniseb's post.

    Quote Originally Posted by BigDon View Post
    How long would it take a well tuned ten minute hour glass to empty?
    I don't know, but one possibility is: never. There might be enough friction in low gravity to stop the flow.

    Also, if this isn't in atmosphere, and it's not well sealed, that would affect it too.


    Would an Earthly sundial be of any use at all?
    Yes, but don't expect to easily see short time graduations (a bigger one would be better).

    In a kitchen pressurized to STP are cooking times affected/effected by lower gravity? (Since humans can survive long periods of microgravity, I pressume not a lot of chemestry has a gravity function.)
    It might be, in some cases, or at least require some different procedures. I'm thinking of things like a soup mix in a pot, where slower convection might get bits overcooked at the bottom, undercooked at top, so might require more stirring or cooking at a lower temperature, and giving it more time.

    By the way, boiling liquids could be a real nightmare to deal with. It wouldn't take much for liquids to get all over the place. You'd also need to be careful about stirring too hard. Pressure cookers might be pretty popular, maybe with an internal stirrer.

    Has it ever been decided one way or the other if humans can develope and come to term safely in micro or lunar gravity?
    No, but best guess is not, considering the problems there are with bone density and other issues.

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    Just a comment about one: using a sundial to keep earth time. I suppose it's not impossible, but wouldn't it be unfeasible? Since the moon only rotates once every month, somehow you'd have to calibrate it so that the one-month rotation is translated into 24-hour time, so that you could figure out universal standard time, and then make another calculation based on the time zone on the earth you want to know about. Wouldn't it be quite difficult in practice?
    As above, so below

  5. #5
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    Hmmm.

    On the hourglass issue.

    Most of us thought in one sixth gravity a ten minute hour glass would take close to an hour to empty, or only be effected slighty like Anton stated.

    Mr. Rijn, Jen-san,

    Sundials aren't defined by size. A thirty foot diameter sundial with fifteen odd days worth of hours inscribed on it should work, now that I've had some answers to stir the imagination process.

    Huh oh. More questions.

    How hot a thirty foot diameter, six inch thick bronze sundial get by the end of the lunar day? (roughly would do)

    Would it even cool down by the next morning?

    Could you make a heat engine out of that for night use?
    Time wasted having fun is not time wasted - Lennon
    (John, not the other one.)

  6. #6
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    Hopefully this isn't considered a thread-hijack, but I have two questions in the same vein as BigDon's, close enough to not justify a new thread. Actually, I'd like to know if I'm on the right track with my own conclusions about gravity:

    1.) I know mass and weight are different metrics, and that while weight would of course be less on the Moon, the mass (and hence, inertia) would be the same. Does it follow that someone pushing, say, 60 kg of groceries* horizontally across the same kind of level surface would feel the same resistance on both Earth and the Moon, even though the weights are different?

    2.) Asimov wrote something in one of his introductory physics works that kind of blew my mind, even though I should've seen it coming. He stated that, while you could jump six times higher with a given effort on the Moon compared to the Earth, you would land with the *same* acceleration as you would on Earth, even though the Moon's gravitational acceleration is--of course--one-sixth that of Earth. Am I right in thinking that this is because the energy it takes to rise against any gravitational field is identical because of mass, and must be conserved on descent?

    *Answers / fact checks would be much appreciated.

    Now, to do my part:

    How hot a thirty foot diameter, six inch thick bronze sundial get by the end of the lunar day? (roughly would do)

    Would it even cool down by the next morning?

    Could you make a heat engine out of that for night use?
    I don't know exactly how how it would get, but I'm guessing it wouldn't be nearly as hot as the lunar surface itself; assuming it's polished bronze, its high albedo would reflect a great deal of light/heat. On top of that, while the metal would lose heat even more efficiently than the surface, the diurnal cycle is slow enough on the Moon that the lag between the surface and the sundial would be negligible. By the time the Sun set, it would lose its remaining heat very quickly.


    *When and if we colonize the Moon, it'll have to become pedestrian eventually.

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    Concerning the groceries, it would depend on the various coefficients of friction in the wheels on your shopping cart.

    Concerning the second one, basically it is simpler than conservation of energy, but you could work it out with CoE if you want to do more work. Basically your are able to get up to a certain speed when you extend your legs and ankles to jump, and that is the same speed you land at. It's slightly more complicated than Asimov let on, only because your muscles will have different efficiencies for different loads, and it is possible that you would actually take off and land faster on the Moon than terrestrially.
    Forming opinions as we speak

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    Tanks.

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    Quote Originally Posted by antoniseb View Post
    Concerning the groceries, it would depend on the various coefficients of friction in the wheels on your shopping cart.

    Concerning the second one, basically it is simpler than conservation of energy, but you could work it out with CoE if you want to do more work. Basically your are able to get up to a certain speed when you extend your legs and ankles to jump, and that is the same speed you land at. It's slightly more complicated than Asimov let on, only because your muscles will have different efficiencies for different loads, and it is possible that you would actually take off and land faster on the Moon than terrestrially.
    antoniseb. On the groceries. Nope. Frictional force is linear to mu times F N. FN is equal in magnitude but opposite in direction to Fg. Fg is reduced by a factor of ~ 6 on the moon....so the frictional force is reduced by ~ 6 also. It's why you put sand in the back of a pickup truck in snow/ice....your traction is much better with the increased normal force from the weight. pete

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    Quote Originally Posted by trinitree88 View Post
    antoniseb. On the groceries. Nope. Frictional force is linear to mu times F N. FN is equal in magnitude ...
    I disagree. I agree that it usually approximately true, but the systems in shopping cart wheels are complex, and rolling friction, static friction, and sliding friction all have different curves... especially when the wheel bearings have threads and other bits of debris rolled into the grease. ... hence my answer.
    Forming opinions as we speak

  11. #11
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    In one Earth gravity, the shopping cart rolls smoothly over the
    floor until one of the wheels runs into some little speck of stuff
    on the floor that you can hardly see. The wheel stops turning
    and instead pushes the speck across the floor, making the cart
    very hard to move at all. Wildly nonlinear. Lunar gravity might
    allow the wheel to ride up over the speck.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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