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The center of mass of a set of points

Given several compact objects of known mass and position, you can find the center of mass of the group in the following way:

or, in a more compact notation,

This method will work well as long as the size of each object is much smaller than the distance between the objects.


Example: given the 3 small objects in the picture below, with masses A = 5 kg, B = 2 kg, C = 4 kg, what is the location of their center of mass?


The Earth sits on the origin, at position x = 0. Where is the center of mass of the Earth/Moon system? Is it inside or outside the Earth? (Hint: see Appendix C).


Joe makes a circle of radius R out of sheet metal. He then cuts out a smaller circle of half the radius, R/2, so that it just touches the edge of the big circle.

Where is the center of mass of the remaining metal?

Creative Commons License Copyright © Michael Richmond. This work is licensed under a Creative Commons License.