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Newton's Law of Gravity

written by: Kathy Foust•edited by: SForsyth•updated: 1/31/2013

This study guide explains Newton's Law of Gravity so that it can be easily understood and applied to motion lessons plans and other aspects of physical science.

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    Newton's Law of Gravity

    Newton's Law of Gravity states that every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

    While this sounds very technical and confusing, it's really pretty basic. Always keep in mind that half the battle in understanding scientific equations and laws lies in understanding the terminology. Let's look at some of the terms.

    • Gravity: Gravity is a fundamental quantity, which is to say that it is in terms that we can understand, but the exact cause of the existence of gravity is unknown.
    • Directly proportional: adjusted to each other based on the values of each other.
    • Inverse: Contrary in direction or effect.

    Still confused? That's okay because in the next section we are going to walk through a formula based on Newton's Law of Gravity.

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    Applying Newton's Law of Gravitation

    When using Newton's Law of Gravitation, keep in mind that gravity, or G, always = 6.67 x 10-11 N • m 2 /kg 2

    force=( a constant x mass1 x mass2 ) / (separation distance) or F= Gm1m2 / r2

    So if two objects of 1.0 kg and 2.0 kg are 1.0 m apart, then the formula works like this.

    F = (6.67 x 10-11 N • m2 / kg2) (1.0 kg) (2.0 kg) / (1.0 m)2

    Cancel out the like terms that cancel each other out. (1.0 kg x 2.0 kg= 2.0 kg2 ), which cancels out the kg2 on the bottom of the fraction as well as the to the top of it. The m2 on the top and bottom also cancel each other out. So, you do the actual math and come with the answer of 1.3 x 10-10N

    If the portion of the equation that has to do with powers-of-ten or significant terms confuses you, see this study guide on significant numbers and this one on scientific notation. Also, be sure to consult your teacher for clarification on any portion that you are having difficulty with. Even the teachers who seem the most rigid are usually willing to lend some time to further explanation for those that are having problems understanding their studies.