# Universal Gravitation and Local G

1. Jan 21, 2010

### physicalx

K, so I dont quite understand how I find a persons weight on a different planet using the said person's weight, the mass of the other planet, and the radius of the planet?

So, for example:

Max weighs 1000 N on Earth. What would his weight be on a planet with twice the mass of Earth, but with the same radius?

x

2. Jan 22, 2010

### Pythagorean

Weight is a force, F:

$$F = G \frac{m_1 m_2}{r^2}$$

If you have a constant, known m1 and a constant r (like Earth) the equation can be simplified down to:

$$F = m_2 g$$

where:

$$g = \frac{G m_1}{r^2}$$ (basically all the other values in the equation besides m2. m1 was the mass of Earth, so m2 is the so-called "test mass" (you or me, or a ball, for instance).

So look at the first equation. What happens to F when we double m?