# How far does the center of mass lie from the center of the earth?

The earth and moon are seperated by a center-to-center distance of 3.85x10^8m. The mass of the earth is 5.98x10^24kg and that of the moon is 7.35x10^22kg. How far does the center of mass lie from the center of the earth?

I know you have to use the equation
Xcm=m1x1+m2x2/m1+m2

I don't know what x1 or x2 and i am not exactly sure what you measure x1 and x2 from and that is probably why i can't figure out how to use the equation.

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OlderDan
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rmarkatos said:
The earth and moon are seperated by a center-to-center distance of 3.85x10^8m. The mass of the earth is 5.98x10^24kg and that of the moon is 7.35x10^22kg. How far does the center of mass lie from the center of the earth?

I know you have to use the equation
Xcm=m1x1+m2x2/m1+m2

I don't know what x1 or x2 and i am not exactly sure what you measure x1 and x2 from and that is probably why i can't figure out how to use the equation.
Draw a line through the earth and the moon. Pick any point you want on that line to call x = 0. Then do your calculation. When you find the CM, find the distance from there to the center of the earth. Some choices for x = 0 are more convenient than others, but you are free to make any choice you want.

the line you drawn is in between the earth and moon? and you pick a point that is on line in between the moon and earth?

I really do not understand what you mean by that, my textbook does not do a good job of explaining it

OlderDan