And the last physics question for today?

Question

I always need help working out physics problems... would appreciate it...

The earth and moon are separated by a center-to-center distance of 3.85 x 10^8 m. The mass of the earth is 5.98 x 10^24 kg and that of the moon is 7.35 x 10^22 kg. How far does the center of mass lie from the center of the earth? (Chapter 7)

Answer 1

Center of mass is a weighted average; that is, the center of mass of a system of masses is

(m1 d1 + m2 d2 + m3 d3 ... ) / (m1 + m2 + m3 ...)

So to find the distance to the center of mass relative to the center of the Earth, solve

d = M(moon) * D(moon) / (M(moon) + M(Earth))

Note that since we are measuring this distance using the center of the Earth as the origin, D(Earth) is zero.

OK Pinky I've done four or five for you tonight so give me some "best answers!"