Gravity: Is it Always the Lightest Body Moving?

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Discussion Overview

The discussion centers around the behavior of two bodies with different masses under the influence of gravity, specifically whether both bodies move toward each other when attracted by their mutual gravitational force. The scope includes conceptual understanding of gravitational interactions and the implications of mass on motion.

Discussion Character

  • Conceptual clarification, Technical explanation

Main Points Raised

  • One participant questions why the heavier body does not appear to move when attracted by a lighter body, suggesting a misunderstanding of gravitational interactions.
  • Another participant confirms that both bodies move toward each other, indicating that the heavier body does indeed move, albeit less significantly.
  • A different viewpoint emphasizes the concept of the center of mass, explaining that while both bodies move, the heavier mass moves less due to its larger mass, leading to the perception that only the lighter body is moving.
  • Another participant notes that in many practical scenarios, the larger mass is treated as stationary for simplicity, as its movement is minimal compared to the smaller mass.

Areas of Agreement / Disagreement

Participants generally agree that both bodies move toward each other, but there is a nuanced discussion regarding the extent of their movements and the implications of mass on their motion. The treatment of the larger body as stationary in certain calculations is also acknowledged.

Contextual Notes

The discussion touches on the limitations of simplifying assumptions in gravitational calculations, such as neglecting the movement of larger bodies in certain contexts, which may not hold in all scenarios.

barryD
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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?
 
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Yes, you are right, both bodies will move toward each other.
 
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.
 
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.
 
thanks.
 

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