# The universal gravitational constant (G)

 P: 54 A most common tool in engineering is "Dimensional Analysis": http://en.wikipedia.org/wiki/Dimensional_analysis This tool can provide you with the dependence and scale. For example, using Dimensional analysis one can easily derive how the Period of a pendulum, T, is dependent on string length and acceleration of gravity thus: $T \propto \sqrt{ \frac{l}{g} }$ The problem of course is that you need to know apriori that the Period is only dependent on string length and gravity, and not anything else (For example, if you don't neglect air viscosity the answer will be different). Now, I tried the method on planets and asked how is the Period of a planet circling the sun dependent on other parameters. I assumed that the Period is only dependent on distance from the sun and the sun's mass. Of course, no time unit can be derived from mass and distance. So I was stuck! I realized that I was missing something, and that something was the universal gravitational constant (G). But then it got me thinking what is G?? Can I say that this constant connects TIME to MASS and DISTANCE? I guess general relativity addresses this, so I post this here.