Calculating a planet's mass & gravitational pull

[jimbob]

right, so 6.67300 × 10-11 m3 kg-1 s-2 could be written as:

6.673 x 10^-11 m^3/kg*s^2

or

0.00000000006673 m^3/kg*s^2

I like to put the breaks in every 1000 so I usually write this:

0.00 000 000 006 673

This, as IF said, is quite different than:

-0.00 000 000 006 673

I hope that this is helpful, and isn't construde as beating a dead horse

[ August 21, 2003, 04:31: Message edited by: jimbob ]

[dogscoff]

Thanks everyone, I think I have the spreadsheet working now. For Earth I get a gravity value of 9840866.19 (starting with a diameter of 12756km and density of 5520kg/m^3). For Mars I get 3736591.2 (diameter 6794.4, density 3935). That's exactly right in proportion to one another, but I have no idea whether or not the values themselves are accurate- all the websites that list such data measure surface gravity in comparison to Earth's.
Also, I haven't factored in rotation which (apparently) increases the effective gravitational pull.

I'm still a little fuzzy on the kg-1 s-2, although I now understand that it's just the unit by which gravity is measured. How would you pronounce it? What does that s stand for? Is it seconds?

=-=-=-=-==-=-=-=-=-=-==-=-=-

Any O&C fans in the audience (if I haven't killed them all off with the lack of updates) may be interested to know that I've now used the spreadsheet to come up with some values for Ceres IV (Plenty).
diameter: ~19000km.
Circumference: ~60,000 km.
Surface area: 2.2 times that of Earth, but I want at least 85 or 90% of it to be water, which leaves betwee 1.15 and 0.77 times Earth's land area. I'll probably go for 90%.
Density: Not sure yet. With an Earth-like density, it would be about 1.5 times Earth gravity (bloody hard work, but probably livable). Interestingly enough, with a Mars-like density, the number comes out only a little above Earth's gravity.

I always intended it to be high in mins and orgs, but very low in rads, so that would fit in well with low density and therefore only-slightly-higher-than-Earth gravity, and it would also help to explain (along with all the beaches) why the place is so damned popular.

Of course, all these numbers are muddied in the text by literary vagueness, so none of them have to be particularly precise.

[Asmala]

Originally posted by dogscoff:
For Earth I get a gravity value of 9840866.19 (starting with a diameter of 12756km and density of 5520kg/m^3).

Did you remember convert the diameter to meters? The answer (9840866.19) is right, but the unit would be μm/s^2.

I'm still a little fuzzy on the kg-1 s-2, although I now understand that it's just the unit by which gravity is measured. How would you pronounce it? What does that s stand for? Is it seconds?

Yes s is seconds. s-2 is equal to s^-2 which is equal to 1/s^2

[dogscoff]

Hmm... I hadn't done that. However if I divide the diameter by 1000 to get metres, my gravity figure comes out even bigger. Or is that right?

[Andrés]

Let's see what I remember from my physics classes.
Gravity on earht should be 9.8 m/s2 pronounced "meters per square second". Meaning the speed of an object in free fall will increase in 9,8 m/s every second.
So you seem to have the point shifted several spaces.

No, rotation should actually create a centrifugal force that will pull you away from the planet and tend to lower gravity close to the equator.

[Suicide Junkie]

You need to multiply by 1000 instead of divide, since there are more meters than kilometers.

[Slick]

DOH! That's pretty much how NASA missed Mars with the Last mission. Or maybe they hit Mars, but they certainly didn't get into orbit as desired. It was a unit conVersion error.

Slick.

[Fyron]

It helps if you set up "conVersion factors" (lame, yes; but still useful). An example:

code:

---

         / 1000 m \

12 km x  | ------ |  =  12000 m

         \  1 km  / 

---

(those are parenthesis)

[dogscoff]

Yeah, one of my planned features will be the ability to select different units for input/output. I'll just have to filter all calculations through a "conVersion" function.

[Suicide Junkie]

Why not just output it in all units simultaneously.
B5 = meters
B7 = B5 * feet/meter
...etc...