# Law of Gravity

1. Sep 23, 2007

### sheri1987

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. Sep 23, 2007

### neutrino

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.