# Gravitational Constant?

I'm wondering when to use the Gravitational Constant G.

When to use these two equations and what are their differences:

Fgrav = m1*m2/d^2

and,

Fgrav = G*m1*m2/d^2

Not sure which equation to use especially when comparing two objects (ie. two ships docked next to each other)

### 1 Answer

- oldprofLv 76 years agoFavorite Answer
Fgrav = m1*m2/d^2 is not...not...a proper equation. It's a proportionality Fgrav ~ m1*m2/d^2 . To make this into an equation, one must provide a constant of proportionality so that the ratio Fgrav/(m1*m2/d^2) = k a constant.

And that makes the proportionality into the equation Fgrav = k m1*m2/d^2; where k is given the symbol G because the whole thing is about gravity. Fgrav = G*m1*m2/d^2 is what you use for two docked ships.

You can't use Fgrav ~ m1*m2/d^2 like an equation because the LHS does not equal the RHS; it's not an equation. In fact, if you do a units analysis, you'll quickly see that the units on either side are not the same. And they must be in a proper equation.

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