# Newtons law of universal gravatational

im stuck with this problem:
the question asked for the gravatational force of attraction between moon and earth while the moon's radius is not given.
do i need the moon's radius? since r is the distance from the center of moon to the center of earth (right?) and the second question of the same question is that i have to find the earth's gravitational field at the moon. so if i use GM/r=g, same, moon's radius is not given, so then i cannot calculate it??
my physics sucks a lot, so plz help~

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cristo
Staff Emeritus
Well, what are you given? Yes, you will need to know the radius of the moon (you need to know the distance between the center of masses of the moon and the earth). List the values you are given.

In future, please use the homework posting template with which you were provided.

the given is that the mass of moon and earth and the radius of earth and the distance between earth and moon

cristo
Staff Emeritus
Well, I did a quick search and found that the radius of the moon is 1.738 x 106 m. You can use this in your calculation.

ok tthx~~ its not in my book tho, im not sure if the teacher will take that... thx anyways

Maybe it's a poorly written problem and whoever wrote the problem neglected to think about adding in the radius of the moon. The moon is pretty small, and actually this distance won't affect your answer much anyway since the radius of the moon is less than 0.5% of the distance from earth to the moon! So the difference between the answers with and without the radius of the moon is less than 1%.

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cristo
Staff Emeritus