Question

Calculate the force of Earth's gravity on a spacecraft 12,800 km (2 Earth radii) above the Earth's surface if its mass is 1400 kg.

Answer 1

i know the equation is :

since the mass is the same, one of the masses cancel out.
but i'm confused on what to use for the radius because it says 12800 km (2 earth radii) so do i use 12800 km + 2 earth radii?

Answer 2

Well actually you're using one right equation and one wrong equation.

F=GM1M2/r^2 is the equation you want to use here, and actually we derive the equation F=mg from this by plugging in the mass of the earth and the radius of the earth and combine all that together with the gravitational constant to make the acceleration due to gravity at approximately the earth's surface.

So using F=mg is actually only useful for the earth's surface as an approximation, but once you get higher than a mountain, the approximation isn't useful anymore.

So I hope that clears it up slightly, and anyways you wouldn't want to equate the forces because then you'd have no force in your equation to solve for, which is what you want, right?

Ok now then... Draw a picture of the scenario and make sure you draw it relatively to scale so you have the earth's radius along with the two radii of distance away from the space craft.

Answer 3

oh shoot i read the question wrong....
anyways..
ohhhh i think i understand it now...