I have a test tomorrow, my teacher won't be there, so I can't go in early, but I cannot get this problem to work. If anyone can help, I'd appreciate it greatly:

On average, the Earth and Moon have their centers 3.84E5km away. What is the mass of the Moon if the force of gravity between them is 1.99E20N?
Earth's mass = 5.97E24kg
Earth's radius = 6.37E6m

Answer: 7.35E22kg


The formulas are:
F = G (M1M2)/d^2
g = G(m/d^2)


Any ideas? Thanks

 

Sure. Use a bit of Algebra and solve for the missing quantity, the mass of the moon.

If you divede F by d^2 then you have G times M1 times M2. Continue the process until you have the expression the way you need it with the unknown quantity all by itself on one side of the equal sign and everything else, whose values you know, on the other side. When you get the answer, remember to substitute it back into the original equation to check your arithmetic or I guess calculator work. Lot easier then doing it with a slide rule or pencil and paper.

 

Thanks, it was a calculator mistake, though. Not enough parenthesis.

Oops.

 

An RPN calculator will eliminate that as a source of error.

__________________
Ron (aka Rube)

 

LOL! That looks REALLY complicated! Anyway, I got a 97% on the test!

Thanks guys (I may need to buy an RPN calculator on eBay, just for fun!)

 

It's like Marmite, you either love it or hate it!.

It might cure the brackets problems, but it introduces plenty of other ones! - essentially it's FAR easier to write the software for an RPN calculator than a conventional one, which is why the first scientific calculators tended to be RPN.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

I prefer RPN, I find it easier than the algebraic types.

My first cal was algebraic but then I was given a RPN and found it much easier to use.

__________________
Len