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Thread: Gravity

  1. #1 Gravity 
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    Hi All,

    Is the gravitational pull is same for all the materials. I mean does it depends on the surface area and weight of the material? If not then why does a leaf takes longer time than a stone to fall down!?


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    Quote Originally Posted by Sujithganguly View Post
    Hi All,

    Is the gravitational pull is same for all the materials.
    All matter has gravity. So, in your question, it must be clarified that you're referring to the gravitational pull of the Earth.
    Quote Originally Posted by Sujithganguly View Post
    I mean does it depends on the surface area and weight of the material?
    The dependence of weight, here on Earth, is very slight. If you take a feather and a hammer, the hammer has more mass and will pull on the Earth more than the feather would. This difference, in relation to the large and massive Earth is so slight, that for all intents and purposes we can say the attraction between the Earth and the hammer and the attraction between the Earth and the feather is the same. It isn't quite the same in actuality but the difference between is infinitesimal.
    Quote Originally Posted by Sujithganguly View Post
    If not then why does a leaf takes longer time than a stone to fall down!?
    Air resistance. You and I are not the only things held down by gravity. Air is held down by gravity. We are at the bottom of a massive ocean of air.
    It gets somewhat thick down here, relatively speaking.
    An object that falls must push its way through the air. A feather will have greater difficulty pushing air out of the way on its journey to Earth than the hammer.

    Apollo astronauts on the Moon took a large feather and a hammer with them and dropped both in the absence of atmosphere on the Moon. They struck the ground at the same time (Again, infinitesimal difference).


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    Quote Originally Posted by Sujithganguly View Post
    Hi All,

    Is the gravitational pull is same for all the materials. I mean does it depends on the surface area and weight of the material? If not then why does a leaf takes longer time than a stone to fall down!?
    Leaves have a natural inclination to attempt to fly away like avians.
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    Brassica oleracea Strange's Avatar
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    This is a great question because it introduces so many interesting things. But where to start ...

    Let's start with Newton and his universal law of gravity. This says that the gravitational pull (force) between two objects, for example the earth and your stone, depends on three things: the mass of the two objects and the distance between them.

    Mathematically, this is:

    Where M is the mass of the Earth (in this case), m is the mass of your stone (or leaf), r is the radius of the Earth, f is the force on the object and G is just some constant that relates all these.

    What this shows is that the "pull" on an object on Earth is just dependent on its mass. And, on Earth, this pull is what we call weight. So a big stone weighs more (has more gravitational pull) than a small one or a leaf.

    If you went to the Moon, that same stone would weigh a lot less because M for the moon is much less (and the r is smaller too).

    Now for the really neat bit. When you drop your stone it accelerates towards the ground. The rate at which it accelerates is determined by two things: the pull (force) and the mass of the stone. In other words: .

    What this means is that a larger stone has a stronger gravitational pull BUT it accelerates more slowly because it is more massive. This exactly cancels out so that all weights fall to earth at the same speed (as demonstrated by Galileo dropping things from the leaning tower of Pisa).

    Finally, we get to the leaf versus stone thing. This is where surface area comes in. A leaf falls more slowly (and drifts around) because of air resistance. On the moon (no air) the leaf and the stone would fall at exactly the same rate (but slower than on Earth). You can find videos online of astronauts on the moon dropping a feather and a hammer to show they fall at the same rate.

    Hope that helps (and I really hope it wasn't a homework question! )
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    Quote Originally Posted by Strange View Post
    This is a great question because it introduces so many interesting things. But where to start ...

    Let's start with Newton and his universal law of gravity. This says that the gravitational pull (force) between two objects, for example the earth and your stone, depends on three things: the mass of the two objects and the distance between them.

    Mathematically, this is:

    Where M is the mass of the Earth (in this case), m is the mass of your stone (or leaf), r is the radius of the Earth, f is the force on the object and G is just some constant that relates all these.

    What this shows is that the "pull" on an object on Earth is just dependent on its mass. And, on Earth, this pull is what we call weight. So a big stone weighs more (has more gravitational pull) than a small one or a leaf.

    If you went to the Moon, that same stone would weigh a lot less because M for the moon is much less (and the r is smaller too).

    Now for the really neat bit. When you drop your stone it accelerates towards the ground. The rate at which it accelerates is determined by two things: the pull (force) and the mass of the stone. In other words: .

    What this means is that a larger stone has a stronger gravitational pull BUT it accelerates more slowly because it is more massive. This exactly cancels out so that all weights fall to earth at the same speed (as demonstrated by Galileo dropping things from the leaning tower of Pisa).

    Finally, we get to the leaf versus stone thing. This is where surface area comes in. A leaf falls more slowly (and drifts around) because of air resistance. On the moon (no air) the leaf and the stone would fall at exactly the same rate (but slower than on Earth). You can find videos online of astronauts on the moon dropping a feather and a hammer to show they fall at the same rate.

    Hope that helps (and I really hope it wasn't a homework question! )


    Ha ha . . Not at all . . BDW thank you so much for the detailed explanation.
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