About planetary gravity

Discussion in 'Science and Technology' started by RB_Kandy, Sep 14, 2012.

  1. RB_Kandy

    RB_Kandy Commander Red Shirt

    Joined:
    Aug 7, 2012
    Location:
    RB_Kandy
    I recently found out that gravity on any given planet is caused by the mass of the planet.

    I had always assumed that gravity on a planet was caused by the speed of rotation, orbit, proximity to the sun, and size of the sun.

    I began exploring why gravity didn't get stronger during its ellipse when it was closest to the sun, and less when it was far. It was googling that which lead me to realize the size of the planet was the cause of gravity.

    But I am wondering, does speed of orbit and rotation play any part of planetary gravity?
     
  2. RobertVA

    RobertVA Fleet Captain Fleet Captain

    Joined:
    Jun 6, 2005
    Location:
    Virginia USA
    There would be a minor weight decrease and change of net angle at low latitudes due to a planet's rotation, which would be in inverse proportion to the rotational period. Those forces would also tend to make the planet bulge a little around its equator.

    There's also a daily tidal fluctuation, although for Earth the moon's proximity produces much more noticeable tidal effects. Tides produced by the system's star would be more powerful with a more massive star or a smaller distance between the star and the planet (as would be the case with Mercury)
     
  3. Tiberius

    Tiberius Commodore Commodore

    Joined:
    Sep 28, 2005
    No. Technically speaking, gravity is caused by the density of a planet. If you had a planet with the same mass as Earth (the same amount of material), but spread out to the size of Jupiter, the gravity would be a great deal less, because it is less dense. However, if the planet had the same mass as Earth but was only the size of the moon, it would be denser, and therefore have a higher gravity.
     
  4. RB_Kandy

    RB_Kandy Commander Red Shirt

    Joined:
    Aug 7, 2012
    Location:
    RB_Kandy
    I can't say that you're wrong, but to me, it just sounds wrong, it goes against my understanding of physics in general (though it is easily argued my understanding of physics is lacking).
    Here on earth, an object's weight is the total mass being pulled by earth's gravity. But you're telling me earth's gravity is not established by mass; but by density? It just doesn't make sense.
     
  5. Alidar Jarok

    Alidar Jarok Everything in moderation but moderation Moderator

    Joined:
    Apr 14, 2003
    Location:
    Norfolk, VA
    Gravity is a strange force. It's powerful enough to keep a giant chunk of rock rotating around the earth, but it's not powerful enough to prevent me from jumping into the air.

    It's clear gravity isn't caused by the Sun. Otherwise, we'd get sucked to the sun instead of staying on Earth. Likewise, Jupiter wouldn't have more gravity than us because their farther away from the sun and travel slower around it. However, their gravity is strong enough to attract dozens of moons and would be crushing if we were to enter Jupiter's atmosphere.
     
  6. Maurice

    Maurice Vice Admiral Admiral

    Joined:
    Oct 17, 2005
    Location:
    Walking distance from Starfleet HQ
    Think of it this way: an Earth-sized body made of the stuff the Earth is has 1g of gravity. An identically sized body made of collapsed star material is a white dwarf and about 200,000 as dense. So two objects of the same size only have the same mass if they are are made up of material of the same density.

    Or try this...
    The Sun has ~333,000 times the Earth's mass

    • On the surface of an object with the Sun's mass that is the size of the Sun, you'd weigh ~28 times what you do on Earth
    • But on the surface of an object with the Sun's mass that is the size of the Earth, you'd weigh 333,000 times what you do on Earth.

    Why?

    Density.

    Explained here in pretty simple terms (link).


    That's because you're forgetting to factor a burst of acceleration, which gravity quickly overcomes and pulls you back down.

    At Jupiter's "surface" (where gas becomes liquid) gravity is only 2.54 time Earth's. Ergo, we wouldn't be crushed by Jupiter's gravity. We'd be crushed by the insane atmospheric pressures there.
     
  7. Trekker4747

    Trekker4747 Boldly going... Premium Member

    Joined:
    Jul 16, 2001
    Location:
    Kansas City
    Think of it this way. Imagine space as a semi-stretchy fabric spread across an open area until taut.

    Imagine planets, stars, moons, etc. as different sized and weights of balls and such being placed on this fabric.

    So, say, you had two balls that weighed the same but where different sizes. One the size of a baseball, one the size of a basketball and placed them on this stretchy fabric.

    The baseball-sized object is going to cause a deeper "hole" in the fabric because it's more dense (takes up less space) than it's same-weighted counterpart, the basketball-sized object.

    That "hole" or impression in the fabric is, essentially, what gravity is, and the fabric is the "material" of space. You just have to extrapolate our demonstration into all three dimensions at once.

    Take a look at our sun, obviously it creates a lot of gravity. (28g's on the surface IIRC). But if we compressed the sun's mass into the size of Earth it'd be a miniature black hole. Because it's be a lot of mass in a relatively small space to the point it crated a very deep depression in the fabric of space.

    (And counter to what Sci-Fi tells us, black holes do not "suck" things into them. They just have strong gravity. If the sun became a black hole the Earth (gravitationally speaking, at least) would remain un impacted. Though it might move closer, but not be "pulled in."

    [​IMG]
     
  8. Deckerd

    Deckerd Fleet Arse Premium Member

    Joined:
    Oct 27, 2005
    Location:
    the Frozen Wastes
    Here you go: this cork and penny have the same mass (they weigh the same)

    [​IMG]

    which has the higher density?
     
  9. Maurice

    Maurice Vice Admiral Admiral

    Joined:
    Oct 17, 2005
    Location:
    Walking distance from Starfleet HQ
    Actually, the Sun's mass the size of the Earth would be a white dwarf, and crunched down to the diameter of Brooklyn would be a black hole.
     
  10. Tiberius

    Tiberius Commodore Commodore

    Joined:
    Sep 28, 2005
    Think of it like this. If you make the Earth less dense, you have to give it a larger volume - it will get bigger. But gravity pulls everything towards the center of the mass, which is why you don't get mountains pulling you sideways. This less-dense Earth is still pulling you towards its center, but because it is larger (in order to be less dense), you are now further from the center of gravity's pull, and gravity is pulling on you less.
     
  11. Methos

    Methos Fleet Captain Fleet Captain

    Joined:
    Jan 10, 2012
    Location:
    Hiding under Gaila's bed...
    Very simple way of explaining density...

    Cannonball... very heavy chunk of metal right? So what happens when you drop it in a pool of mercury... mercury is a liquid, and cannonballs are pretty damn heavy...

    [yt]https://www.youtube.com/watch?v=Rm5D47nG9k4[/yt]

    easiest practical explanation of density i've ever seen :)

    M
     
  12. Chemahkuu

    Chemahkuu Admiral Admiral

    Joined:
    Dec 23, 2004
    Location:
    United Kingdom
    :wtf:

    I don't know what they teach you over there but back here we knew that as soon as we were old enough to spell gravity.

    Really?
     
  13. Crazy Eddie

    Crazy Eddie Vice Admiral Admiral

    Joined:
    Apr 12, 2006
    Location:
    I'm in your ___, ___ing your ___
    It's both, actually. Two things to understand here:
    1) Gravity always pulls towards the exact center of a mass. So if you are in orbit around, say, an enormous gas cloud half a light year in diameter, you will be pulled towards the exact center of that cloud. If you then collapse that cloud into a point the size of a baseball, you are still being pulled towards the exact center of the ball.

    2) Over short ranges at least, gravity is subject to the inverse square law. That means the farther you get from the center, the less its gravity affects you. From the nebula/ball example above, this means that the force of gravity half a light year away from the center is considerably less than it would be half a mile from the center. Just as important to understand, however, is that you can only be pulled in by the gravity of something BELOW you, so if you are INSIDE the gas cloud, you will not be affected by any mass that is farther away from the center than you are.

    The second point is why density matters. If Earth 1 and Earth 2 have the same mass, but Earth 2 is half as wide, then Earth 2's surface gravity is greater than Earth 1. On the other hand, if you're 7000 miles from the center of Earth one (low orbit) you're subject to the same gravity force as you would be 7000 miles from the center of Earth 2 (slightly higher orbit).
     
  14. Christopher

    Christopher Writer Admiral

    Joined:
    Mar 15, 2001
    Density doesn't "cause" gravitation, it's simply a factor in determining the intensity of a planet's surface gravity.

    Gravitation is a fundamental force felt between all masses. Any mass, even a single particle, exerts an attraction on every other mass around it, and vice-versa. But this is a very, very weak attraction, the weakest by far of the four fundamental interactions in nature, so only a really, really large concentration of mass, such as a star, planet, moon, or asteroid, exerts enough gravitational pull to be significant.

    This gravitational force follows an inverse square law -- if you're twice as far away from something, you feel 1/4 the pull, and if you're 10 times as close, you feel 100 times the pull. Also, the mass of a given object behaves as if it's all concentrated at the object's center of mass. The pulls of parts of it in opposite directions from the center of mass cancel out, so the vector of attraction you feel toward an object is straight toward its center of mass, and you feel an acceleration equivalent to what you'd feel if all its mass were concentrated in a single point at the center. The reason you feel weaker gravity on a planet with lower density, therefore, is because that planet has a larger volume relative to its mass, and thus a larger radius. In short, you feel less gravity because you're farther away from the center of mass.

    So no, gravity isn't "caused by density." That's confusing the concepts. It's caused by the presence of mass, and the magnitude of attraction between two masses is determined by the distance between them. Density is just a relationship between the mass and size of an object.

    A planet's rotation does not generate its gravity. This is a surprisingly common misconception among the general public, and I think it's based on confusion with what we've seen in science fiction movies like 2001, showing space stations and ships generating "gravity" with rotation or spinning centrifuges -- an effect we experience in everyday life watching clothes spin in the dryer or riding certain carnival rides. But that's not literally gravity. It's an acceleration that creates a sensation of weight, equivalently to how the acceleration caused by the force of gravitation creates weight, but not generated in the same way. And rotational "gravity" pulls outward from the center, not in toward it. The Earth's rotation actually creates a slight outward (centrifugal) acceleration that cancels a tiny bit of the inward gravitational pull of its mass.

    The size and proximity of the Sun has no effect on the amount of gravity/weight you'd feel on the surface of the Earth or Mars or any other planet. This is because planets are in orbit of the Sun, and orbit is free fall. That means that the planet and everything on it is literally falling toward the Sun, being pulled inward by its gravity -- yet is falling sideways fast enough that it curves around the Sun and never gets closer. Effectively, when you're in orbit, you're spinning fast enough that the outward, centrifugal acceleration is equal to the inward, gravitational acceleration, and they cancel out to zero -- so you don't fall into the thing you're orbiting and you don't feel any weight while you're in orbit.

    So it doesn't matter how close you are to the Sun or how big it is; you don't feel its gravity when you orbit it. The only mass whose pull noticeably affects you is the Earth itself.
     
  15. gturner

    gturner Admiral

    Joined:
    Nov 9, 2005
    Location:
    Kentucky
    Perhaps we should keep things simple. Gravity is the force that attracts liquids to your keyboard.
     
  16. JarodRussell

    JarodRussell Vice Admiral Admiral

    Joined:
    Jul 2, 2009
    Just why did you think that the SUN was causing us getting pulled towards the Earth? Or that the rotation of Earth would pull us towards Earth? Have you ever been on a carousel, and where you pulled inward or outward by the rotation?

    Glad you recently found out that gravity is caused by mass.

    Now, orbital speed, the planet's rotational speed, the gravity of the sun, the gravity of the Moon, they all affect the gravity of the Earth.

    If Earth wouldn't spin, gravitational pull would naturally be a bit stronger. The Moon's gravity pulls on Earth and anything: it causes tides (because the whole body of water is pulled towards the Moon, and the Earth rotates through that water "hill"), earthquakes, and the gravitational friction constantly reduces the Earth's rotation speed. The gravity of the sun affects us more when Earth is closer to the sun, and less when it's farer away from the sun, naturally. But it's a pretty minor difference. And yeah, even the sun causes tides, just like the Moon.
     
  17. Trekker4747

    Trekker4747 Boldly going... Premium Member

    Joined:
    Jul 16, 2001
    Location:
    Kansas City
    And anyone who's ever ridden on a merry-go-round or other spinny-type ride knows that the rotation tries to throw you OFF of it not pull you more into it. If Earth's spin were to have any real impact on us it'd be more likely to throw us into space than it would to pulls us into it. Thankfully, gravity is stronger than centripetal force.
     
  18. Crazy Eddie

    Crazy Eddie Vice Admiral Admiral

    Joined:
    Apr 12, 2006
    Location:
    I'm in your ___, ___ing your ___
    Which is why you should turn your chair sideways while watching internet porn.:evil:
     
  19. RB_Kandy

    RB_Kandy Commander Red Shirt

    Joined:
    Aug 7, 2012
    Location:
    RB_Kandy
    First, I would like to start off by explaining that my school was a very stupid school that made no effort to educate. In middle school and early high school my subjects were History, Social Studies (history and political correctness), and English (part history part social studies), PE, Art & Music, Math, Science, and Lunch. Damn I did good in Lunch, it was my best subject.
    Science class in middle school was pretty cool, and super easy. Thus I am guessing we weren't really learning much. I remember there being lots of props that did odd things, and then we would be explained why it worked like that. Example, a device with a metal dome that send electricity through your body, made your hair stand on edge, and you had to stand on a crate.


    In high school science class consisted of, two years in a row, this science teacher that just lectured people on the utmost irrelevant things. If you've seen Mr. Garrison on South Park, and the way he teaches class, you have to understand, that's almost exactly how this teacher was.


    I failed math because when I changed schools, I some how went from 6th to 7th grade, because I got held back in 2nd grade. When I got into 8th grade math, it was super hard, and I couldn't understand it. My teacher just kept giving me an F and lecturing me on how smart I am, and how I CAN do this if I just apply myself. I honest to god couldn't do it because I skipped certain mathematical knowledge. It's like learning HTML, skipping the year they teach you Java, and going right to PHP and MySQL, and it is assumed you know Java. You'd never pass, because every time they'd ask you to write a Java script to defer to the MySQL data base, you'd just sit there like WTF!?


    Now I do remember my Social Studies teacher teaching us all about Science... in social studies class.
    It started out by learning about all the black and Hispanic scientists that have contributed to science. And some how moved in to her teaching science.
    She freakin told me the earth's rotation is what made gravity and pulled us inward.


    I recall saying "wouldn't the spinning of the earth push us outward? Like, if you dip a basketball in water, and then spin it on your finger, the water goes outward and splashes you, because spinning pushes objects away from the surface" she says "no, because we're also orbiting the sun."


    I just nodded my head, not understanding it, and assuming I was too stupid to understand it.


    And the thing is, a week later I am talking about it to a friend (who has the same class) and he's explaining how the teacher is totally right.


    I recall from other history, social studies, and English teachers, that Edison invented the light bulb. George Washington Carver invented peanut butter. Some black guy in world war one invented the radio. Benjamin Franklin accidentally discovered electricity when flying a kite in a storm. Christopher Columbus discovered America. Leonardo Da Vinci invented the first air plane.


    I was taught a lot of stuff that wasn't true.


    But in science classes, throughout elementary school, it was all about home made volcanoes, and interesting "tricks" you could do at home.


    In middle school (because 6th and 7th grade over lapped for me) I only had two science teachers, and only my 8th grade science teacher, Mr. Little, actually attempted to teach us about science.
    I look back at school, and besides the students, and the social environment, all I can remember about the lessons were Rosa Parks, Martin Luther King, Malcolm X, Susan B Anthony, Second Wave Feminism, and reading The Grapes Of Wrath, Of Mice And Men, Greasers, and a lot of books that remind me of The Fox And The Hound. And learning about historical people who we were told invented or discovered things they didn't.
    To this day, I still remember my 9th and 10th grade science teacher the day he had to take his dog to the vet, and that got us learning about the miracles of Vitamin B. And for the rest of the month we learned about Vitamins... In Earth Science class! I remember he made a speech about the laws of the road, and how at 60 MPH some special inertia takes place. I remember him briefly touching on the theory of relativity. We learned about global warming, and recycling, I remember that because he looked at me and my best friend and said "You two could at least recycle your beer bottles". And when he caught his son coming home drunk, and made him drink more until he puked his guts out, to teach him a lesson. All of which is interesting, because 2 years after I dropped out of high school, and worked at my family's liquor store (18 to sell it as long as there is no open container in the establishment) he used to come in there every day, by a huge fifth of brandy, and he never recognized me as a former student. Apparently he lost his job a year after I quit school, and he died two years after that. He literally drank himself to death.
    Oh yeah, in one of the years he was teaching us about whales for a couple of months. And he did mention Star Trek The Voyage Home.
    You know, to this day, I don't know what the hell Earth Science is.
    To this day I get a lump in my throat when I hear "Earth Science" because all I can think about is whales, alcoholism, global warming, vitamin B, and the fact that boys can hold it in longer than girls, so boys can't leave the class room, and neither can the girls because they only go there to do their makeup. The teacher was off the charts crazy.
     
  20. gturner

    gturner Admiral

    Joined:
    Nov 9, 2005
    Location:
    Kentucky
    Wow. That sounds horrible.

    I had an idiot science teacher in 7th grade who insisted that atomic bombs are more powerful than hydrogen bombs because atomic bombs are fission, which throws things outwards, and hydrogen bombs are fusion, which is an implosion that sucks things inwards. I sat there thinking, "That's so stupid it's not even wrong!"

    Gravity is an attraction between two masses, with the formula Force = G * M1 * M2 / R^2, where M1 and M2 are the masses of the two objects, G is a constant, and R is the radius between their centers.