PHYA4 - grav q

Q: NASA wants to recover a satellite that is on the moon's surface and to place it in orbit around the moon. NASA now decides to bring the satellite back to earth. explain why the amount of fuel required to return it back to earth will be much less than the amount required to send it to the moon originally?

so this is how i think the the situation would look, i could be wrong.
(earth)--------------R=0------(moon), im guessing the people are at somewhere between R=0 and the moon, if thats the case then how would the fuel be less? they are already feeling the gravitational pull of the moon.

Reply 1

hello calum

Original post by cooldudeman

I think the answer is just that the mass of the moon is much smaller than the earth, so the gravitational pull of the earth is stronger than that of the moons, so getting from the moon to earth is easier than the earth to moon. The gravitational force of earth is pulling it back which is easier than having it pulling against you.

F = \frac{GM_1M_2}{x^2}

because the earth has a larger mass, it has a larger force.

Reply 2

cooldudeman

Original post by hello calum

F = \frac{GM_1M_2}{x^2}

because the earth has a larger mass, it has a larger force.

so even if the probe is near the moon, the earths grav F is stronger than the moons?

Reply 3

hello calum

Original post by cooldudeman

so even if the probe is near the moon, the earths grav F is stronger than the moons?

Yeah, because the earth has a larger mass. the probe would have had to start from the earths surface. To go to the moon from the earths surface takes a lot more energy then to get from the moons surface to earth because of the masses.

Reply 4

cooldudeman

Original post by hello calum

ok, but in this question, where are they with the probe, when they made the decision to bring it to earth?

Reply 5

hello calum

Original post by cooldudeman

ok, but in this question, where are they with the probe, when they made the decision to bring it to earth?

I don't really understand what the question is saying to be honest. Is the question: why does it take less fuel to bring it back to earth than it does to take it to the moon?

What is the actual question word for word?

Reply 6

cooldudeman

Original post by hello calum

something like that but word for word, it is,
'NASA wants to recover a satellite that is on the moon's surface and to place it in orbit around the moon.(there is a graph q about gfs which is unrelated). NASA now decides to bring the satellite back to earth. explain why the amount of fuel required to return the satellite to earth will be much less than the amount required to send it to the moon originally.'

also, what would the line look like for this,

-----------(surface earth)--------------------(surface moon)-------> x-axis(distance)

and the y-axis is grav potential.

(edited 11 years ago)

Reply 7

hello calum

Original post by cooldudeman

im not sure about potential energies but for the force:
I think it would start very high on the y-axis. Then fall down like the graph

\frac{1}{x^2}

then rise back up but not nearly as high as at the start.... But the answer would surely be that the earth has a higher mass. Then you expand on that, I'm not sure why you're trying to graph it?

Reply 8

cooldudeman

Original post by hello calum

im not sure about potential energies but for the force:
I think it would start very high on the y-axis. Then fall down like the graph

\frac{1}{x^2}

then rise back up but not nearly as high as at the start.... But the answer would surely be that the earth has a higher mass. Then you expand on that, I'm not sure why you're trying to graph it?