Gravitation - Why don't planets smash into each other?

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Discussion Overview

The discussion revolves around the question of why planets do not collide with each other despite the universal law of gravitation, which states that all masses attract each other. Participants explore the dynamics of planetary motion, the role of gravitational forces, and the concept of energy in gravitational systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why the Sun does not move towards the planets, suggesting that if everything pulls on everything else, the Sun should be affected by the gravitational forces from the planets.
  • Another participant introduces Newton's cannonball as a way to conceptualize gravitational motion and refers to Feynman's Lectures for a numerical approximation of planetary motion.
  • A different participant emphasizes that if two planets are stationary relative to each other, they would collide, prompting a discussion about the energy components in a gravitational system.
  • This participant also notes that when considering two bodies of approximately equal mass, they will orbit around a common center of mass, indicating mutual gravitational attraction.
  • One participant shares a link to an experiment demonstrating that two masses placed at rest will collide, suggesting a practical illustration of gravitational attraction.

Areas of Agreement / Disagreement

Participants express various viewpoints on the dynamics of gravitational attraction and planetary motion, with no consensus reached on the underlying reasons for the stability of planetary orbits versus potential collisions.

Contextual Notes

Participants discuss the role of energy in gravitational systems, but the conversation does not resolve the complexities of gravitational interactions or the assumptions underlying their arguments.

physio
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Gravitation -- Why don't planets smash into each other?

Why don't planets smash into each other? The universal law of gravitation states that everything pulls everything else in the universe. Using this logic shouldn't the Sun move because it should be experiencing the pulling force from so many planets?

Using the charge analogy if a positive charge is kept in the vicinity of a negative charge, there is a force on the charges and both of them accelerate towards each other, why doesn't this apply to the planets?
 
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An interesting way to see this is Newton's cannonball.
http://en.wikipedia.org/wiki/Newton%27s_cannonball
http://waowen.screaming.net/revision/force&motion/ncananim.htm

In the Feynman Lectures, Feynman talks you through a simple numerical approximation of the motion of a planet which is also very instructive.

For a more technical derivation see any book on classical mechanics (Goldstein for example, or Landau has a very nice exposition as well).

The case of electrically charged particles is completely equivalent, they to do not collide provided their velocities are not along the line joining the particles (and ignoring radiation).


A_B
 


physio said:
Why don't planets smash into each other? The universal law of gravitation states that everything pulls everything else in the universe. Using this logic shouldn't the Sun move because it should be experiencing the pulling force from so many planets?

If I take two planets and place them at a distance apart, and they are stationary to each other, then yes, they will smash into each other.

Now, go back to the situation of planets, etc. What is different here than what I just described? Is the energy of the system ONLY in the attractive potential energy, or is there ANOTHER energy component here that might balance that out?

Secondly, if two bodies are off approximately the same mass, the system will consist not of one body orbiting the other, but both of them moving around some point in between them (the center of mass). So in this case, there is a clear evidence that, yes, they are pulling on each other, and each one is being pulled by the other.

Zz.
 

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