1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why doesn't the Earth collide with the Sun?

  1. Nov 4, 2009 #1
    I am in AP Physics and we just got done with Centripetal force and Universal Gravitation. I was wondering since for the planets to stay in motion they need to have the same force as the object they are orbiting around. Since a bigger mass means a bigger force, how does the Earth stay in motion around the Sun.
     
  2. jcsd
  3. Nov 4, 2009 #2

    Doc Al

    User Avatar

    Staff: Mentor

    I'm not sure I understand your point. Consider a planet in circular orbit about the Sun. The force needed to maintain the orbit equals mv2/r (the "centripetal" force), so the more massive the planet the greater the force needed. But gravity is proportional to mass, so it works out just right.
     
  4. Nov 4, 2009 #3
    also, technically the sun and earth are both revolving around one another, which may be a helpful way of seeing the situation.
    or not.
     
  5. Nov 4, 2009 #4
    I wouldn't look at it like that, that just leads to confusion in my opinion.

    Newtons first law states an object will remain still or continue at a constant velocity in a straight line unless acted on by an unbalanced force.

    Think of the earth as originally moving in a straight line, the gravitational force of the sun acted on it, but not enough to pull it straight into it, it pulled it around in a curve, and the velocity stays great enough that it will keep going round and round.

    Think of a hammer throw, the object is getting pulled but not enough to go straight to the center, only enough to move in a circle.
     
  6. Nov 4, 2009 #5

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Because it is not in the way.
     
  7. Nov 4, 2009 #6

    Doc Al

    User Avatar

    Staff: Mentor

    That's the best answer so far! :smile:
     
  8. Oct 8, 2010 #7
    No force needed to stay in motion.

    The Earth orbits the Sun in high vacuum. So there's very little resistance if any to slow Earth's orbit significantly in its billion years existence.
     
  9. Oct 8, 2010 #8

    Doc Al

    User Avatar

    Staff: Mentor

    You might want to rethink that. It's the gravitational attraction of the Sun on the Earth which allows the Earth to remain in orbit. If there were no force, the Earth would take off in a straight line.
     
  10. Oct 8, 2010 #9
    I have definitely misunderstood the first post!

    I was thinking since Porky had just gone through centripetal, etc. what's the point of asking the obvious. I thought he's looking for something else:!!)
     
  11. Oct 8, 2010 #10

    Doc Al

    User Avatar

    Staff: Mentor

    Also realize that this thread is almost a year old. That ship has sailed and porky is long gone.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook