Need help understanding the constant G

[Miles-z-o]

I'm trying to learn to calculate orbital speed and I can't wrap my head around G, specifically the part with N m^2 kg^-2. Help!

[MechBFP]

G as in gravity? That is 9.81 m/s^2.

[Miles-z-o]

Originally posted by MechBFP:

G as in gravity? That is 9.81 m/s^2.

No no, I mean the gravitational constant G.

[edorward]

The units? G is a constant and constants usually have wonky units in order to cancel out the appropriate other units in the equation. In this case, I'm guessing you're worried about F = G(m1m2/r^2), right? In order for the right side of the equation to equal units of force (N), the units for G have to cancel out mass*mass/distance^2 and leave you with N. To do that, you need Nm^2/kg^2. There's no reason to try thinking what a Newton meter squared per kilogram squared means.

[Miles-z-o]

Originally posted by edorward:

The units? G is a constant and constants usually have wonky units in order to cancel out the appropriate other units in the equation. In this case, I'm guessing you're worried about F = G(m1m2/r^2), right? In order for the right side of the equation to equal units of force (N), the units for G have to cancel out mass*mass/distance^2 and leave you with N. To do that, you need Nm^2/kg^2. There's no reason to try thinking what a Newton meter squared per kilogram squared means.

I kind of see what you're saying but I don't see how the extra units would apply to calculating orbital velocity.

[Jupiter3927]

G as in Newton's Gravitational Constant is 6.67*10^-11.

Use that constant in your equations or use the Standard Gravitational Parameter, GM, the game gives you for each planet.
GM is G*Mass so it saves a bit of button pushing.

Units matter a lot when you're balancing equations.
G needs to have the funky units attached at the end to balance the equation.
If you're just plugging stuff in, they don't matter.

[edorward]

Originally posted by Miles-z-o:

I kind of see what you're saying but I don't see how the extra units would apply to calculating orbital velocity.

Because if G didn't have units, you wouldn't end up with units of force as your answer. Force is mass times distance over time squared. If G didn't have units, you'd end up with kg^2 per m^2, which is a nonsense answer, instead of Newtons (kg*m/s^2). Gotta solve for units, not just quantity.

[juanxlink]

Originally posted by Miles-z-o:

I kind of see what you're saying but I don't see how the extra units would apply to calculating orbital velocity.

We need a facepalm emote...

So you are trying to do the math, but dont see the importance in the math language.

Read the rest that has been posted to help you, then go and use google to get you up to speed...