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Homework Help: Law of Gravity

  1. Sep 23, 2007 #1
    1. The problem statement, all variables and given/known data

    Calculate, using Newton's law of gravity, the size of the force of attraction between the Earth and a mass of 3.0 kg on the Earth. Data: Distance to the center of Earth from the surface = 6370 km. Mass of Earth = 5.98 1024 kg. Gravitational constant (G = 6.67 times 10-11 Nm2/kg2).

    2. Relevant equations

    F = G(m1 * m2)/ r^2

    3. The attempt at a solution

    I plugged the numbers into the equation and converted km to meters and kg to g ...

    6.67E-11 * (3000)(5.98E27) / (6370000)^2
    and I got 2.949E-7, but it is wrong I think
  2. jcsd
  3. Sep 23, 2007 #2
    The answer is quite a way off.

    1.Why did you convert Kg to g? The SI unit of mass is Kg. When you substitute 6.67e-11 for G, it means G = 6.67e-11 Nm2/Kg2. So make sure the units are uniform throughout.

    2.You can make a rough estimate of the answer. If you've come across the term acceleration due to Earth's gravity(g), you may know that it is roughly equal to 10ms-2. The weight of an object on the Earth, which is nothing but the gravitational force due to the Earth, is mg, where m is the mass of the object.
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