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Thread: Question about gravity.

  1. #1

    Question about gravity.

    If I am on a planet that has exactly half the gravity of Earth, does that mean if I drop a pebble, it will take exactly twice as long to hit the floor? Only the gravity is different. Air type, etc., is all the same.

  2. #2
    https://en.wikipedia.org/wiki/Equati...a_falling_body

    Time t taken for an object to fall distance d :

    time = sqrt ( 2 x distance / acceleration due to gravity)

    So yes, twice as long (plug in example numbers, compare).

  3. #3
    Originally Posted by Un1k0rn View Post (Source)
    If I am on a planet that has exactly half the gravity of Earth, does that mean if I drop a pebble, it will take exactly twice as long to hit the floor? Only the gravity is different. Air type, etc., is all the same.
    All you ever need to know:

    https://en.wikipedia.org/wiki/Equati...a_falling_body

    (... well, about the effects of gravity on a falling body. )


    ninja'd

  4. #4
    The relationship
    You are looking at
    Is quadratical in time.

  5. #5
    Thanks guys. This was bugging me.

  6. #6
    Originally Posted by Un1k0rn View Post (Source)
    If I am on a planet that has exactly half the gravity of Earth, does that mean if I drop a pebble, it will take exactly twice as long to hit the floor? Only the gravity is different. Air type, etc., is all the same.
    I would think that a little more needs to be factored in than simply the amount of G. The effects of gravity will increase or decrease depending on how close to the center of the massive body you are. So, for example, the gravity of the planet may be half that of Earth, but it may be twice the size of Earth. You, being on the surface, would be further away from the planets center and the effects of gravity may be lessened.

    I'm no physics expert, so I may very well be wrong. We need Neil deGrasse Tyson in here to answer this one, or maybe David Braben?

  7. #7
    Isaac ! Come back !


  8. #8
    Originally Posted by DanGriswold View Post (Source)
    I would think that a little more needs to be factored in than simply the amount of G. The effects of gravity will increase or decrease depending on how close to the center of the massive body you are. So, for example, the gravity of the planet may be half that of Earth, but it may be twice the size of Earth. You, being on the surface, would be further away from the planets center and the effects of gravity may be lessened.

    I'm no physics expert, so I may very well be wrong. We need Neil deGrasse Tyson in here to answer this one, or maybe David Braben?
    As said, only the gravity is different in my hypothetical'. Literally everything else is the same. Basically, imagine Earth with half the gravity. I'm keeping the situation simple so I have a chance of understanding the core mathematics!

  9. #9
    Originally Posted by Un1k0rn View Post (Source)
    As said, only the gravity is different in my hypothetical'. Literally everything else is the same. Basically, imagine Earth with half the gravity. I'm keeping the situation simple so I have a chance of understanding the core mathematics!
    What he's getting at is this: gravity is determined by mass and distance. When you say "half the gravity", you could have Earth at exactly the same mass but but above it enough to experience half the gravity as on the land-surface. So Earth wouldn't have to change at all, just your location.

    So for your interest, your question could kinda be reworded as: if you dropped the pebble at a height where G (gravity) is .5 of surface G on Earth, would it take 2 x time to land? Then you get a different answer. It's kinda your next step from a half-mass Earth with distances being the same!

  10. #10
    Originally Posted by Thargon View Post (Source)
    Time t taken for an object to fall distance d:
    time = sqrt ( 2 x distance / acceleration due to gravity)
    So yes, twice as long (plug in example numbers, compare).
    Aehem, NO! It takes sqrt(2) as long not twice as long. It will take twice as long if the gravity is a fourth.