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Gravity Doesn't Make Any Sense

  1. Aug 26, 2010 #1
    Hi all,

    First of all nice forum. Secondly, I'm new here and I wasn't sure where
    to start this thread, but seeing that it's mostly about gravity, I guess
    this section is suitable. I apologize if it's not.

    Anyway, I have this question that's been screwing up my mind.

    Imagine that there were aliens that lived on a planet very much like
    ours. Only difference is that their planet is much bigger
    than ours and has much more gravity. Let's say that their
    gravity is 10 times that of earth. So their physical
    strength will obviously be much greater since their
    bodies have to resist all that gravity.

    Now let's say one of them comes here...

    Since a human acts 'weird' (floats a little etc...) when he's on
    the moon, will the alien act the same way on our planet, since
    his homeplanet has much stronger gravity?

    According to physics that's what will happen I suppose.

    But that doesn't make any sense because that would mean
    that physical strength will effect the gravity that's pulling you,

    So in other words, if (hypothetically speaking) a human reaches
    the same strength of that alien, he will start to jump more, float
    a little on our planet as well!?

    Because that's sounds unbelievable to me.

    To continue with this little argument, gravity pulls every bit of matter
    in your body, so it's like a force holding you. When you put a human
    in a place where gravity is very little or non existent, that force is
    gone, so how come he isn't able to move faster?? how come we don't
    move faster on the moon?

    To make it even more clear, that alien we were talking about earlier,
    wouldn't he be able to move 10 times faster on our planet since the
    gravity is 10 times less than the gravity of his planet?

    Looking forward to your answers.

  2. jcsd
  3. Aug 26, 2010 #2


    User Avatar

    Staff: Mentor

    Welcome to the PF. Think of gravity as an acceleration. The acceleration here on Earth is 1/10 of that on your alien friend's world, but it is still the same when he's here and doing things. The acceleration is the same for you and for him, so if you both jump a meter off the ground, you will both have the same motion. He may be able to jump 10 meters high, but his motion will look natural, the same as if you threw something 10 meters high and it fell back down.

    The acceleration of gravity is much less on the moon, so that's why the astronauts looked so funny bouncing around. You and the alien and the Apollo astronauts all would bounce around the same on the moon. And you can't move faster horizontally on the moon even though it has a smaller acceleration due to gravity -- gravity acts in the vertical direction. You can jumpp higher, but that doesn't help you run faster, for the most part.

    Does that make more sense?
  4. Aug 26, 2010 #3
    Hi man,

    So his physical capabilities will be much better than ours right?

    (on earth)

    If on average humans can jump aprox 2 feet high, he could jump aprox 20 feet?
    If the average deadlift is 200 kg, he would lift aprox 2000 kg?

    BUT could he (the alien) move faster on earth? or will he float and bounce around on earth
    just like we do on the moon? (since the earth in terms of gravity
    is like the moon compared to his planet

    EDIT: In other words, we move funny on the moon because it has 1/6th of earth's gravity, so will he move funny
    on our world which has 1/10th of his planet's gravity? Or will he move normally but much faster?

    That's the main question that I was trying to get
    an answer to.
    Last edited: Aug 26, 2010
  5. Aug 26, 2010 #4


    User Avatar

    Staff: Mentor

    No, he most likely would not be faster. In fact, he'd probably look like a turtle -- short, squat, strong muscles. He would not bounce around -- his motion would look like all the rest of the stuff here living with 1g.
  6. Aug 26, 2010 #5

    Pls note that I was picturing this alien to move just as fast as we do
    on earth in his world.

    You're thinking about how he probably would be if he was raised
    on a planet like that.

    Imagine he was like us. Walking upright, moving the same way we
    do, etc...

    What I can't understand is...

    If he moves fast as us on his world then why wouldn't he be
    able to move faster and easier on our planet that has
    much less gravity?

    If we went to mars for example, will our sprint records increase
    since the gravity is weaker?

    Cheers. ;)
  7. Aug 26, 2010 #6


    User Avatar

    Staff: Mentor

    When we run, for the most part we are not working against gravity. If you could bound more like a gazelle, then you might be able to go faster, but that would take a different anatomy, more than a smaller gravity.

    The extra strength of your 10x alien would mostly be to support them vertically. So yes, they would be able to lift more than us here on Earth, but that doesn't translate into horizontal speed necessarily. If they could bound like a gazelle, then maybe they would be a bit faster than us here.
  8. Aug 26, 2010 #7
    I see. Thanks Berkeman ;)
  9. Aug 26, 2010 #8
    Since he's a normal human but 10x stronger, I'm sure he would be able to move faster.

    He wouldn't be 10x faster, though. Not even close. A limitation of how fast we run is not so much our strength, but the actual geometry of our legs. We're just not "geared" to run fast.

    As berkeman said though, the alien would not be built like us. He would be built to work on his planet, not ours, and hence he probably wouldn't be faster than us. If we went to Mars, I doubt we would be faster than on earth. Maybe we would, but certainly we would not be as fast as a human who's had some time (many generations) to evolve for Martian conditions.

    Again, we're "geared" for our planet, for our environment. What works here won't work as well somewhere else.
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