# Two bodies with different masses, which one would move first due to gravity only?

1. Oct 7, 2011

### barryD

Hi, I'm reading physics in my spare time, I am reading about Newton and how he discovered the law of gravity F= G m1m2 / r2

I was wondering if the heaviest body moves as well when being attracted by the lighter body.

i.e. if two objects with different masses are positioned a distance r apart, and neglecting everything except gravity, why doesn't the heavier object move as well? Is only the lighter object moving?

2. Oct 7, 2011

### Janus

Staff Emeritus
Yes, you are right, both bodies will move toward each other.

3. Oct 7, 2011

### gsal

I think they both move...what does not move is the center of mass. So, the big mass moves a lot less, but moves; the little mass moves a lot more.

4. Oct 7, 2011

### DaveC426913

What may be throwing you is that we often look at systems where one mass is much larger than the other. Since the acceleration of the body is inverse to its mass (a=F/m) - a much larger body moves proportionally less - we can treat it as if only the smaller body is moving.

This is much simpler. It would really be a pain in the butt if, every time we calculated the trajectory of a tennis ball, we had to take into account the movement of the Earth toward the tennis ball.

So, for most intents and purposes, we treat the much larger body as unmoving.

5. Oct 7, 2011

thanks.