Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why do natural Moons revolve around planets?

  1. Jan 30, 2015 #1
    Why do Moons revolve around planets? If because of gravitational pull, why won't they clash with planets. Please help me get answer to this question!?
     
  2. jcsd
  3. Jan 30, 2015 #2

    RUber

    User Avatar
    Homework Helper

  4. Jan 30, 2015 #3

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    Hi officalmanjosh,

    Here's a picture of 'Newton's cannonball':
    Newton_Cannon.ABC_.png
    There are three trajectories depicted. In all three cases the gravity is pulling the cannonballs in towards the centre of the Earth.
    And yet, trajectory C is not crashing onto Earth.

    Can you tell us what you think is the main difference between the three? What makes B fall farther than A? What does the cannon do with the cannon ball?
     
  5. Jan 30, 2015 #4
    Hello to you too Bannersnatch, the answer is Centripetal force. Even acceleration of ball C is correct. Am I right about why canon ball C today's like that.?
     
  6. Jan 30, 2015 #5

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    The only force acting on all three balls is the gravitational force. In the case of ball C this force will act as the centripetal force. But let's not get into that now.

    Try and think of what makes ball B fall farther than ball A. What does the cannon do with the balls?
     
  7. Jan 30, 2015 #6
    Angle depression of canon and force of which it shoots canon balls varies from ball a to b to c. Angle of depression of firing ball C is @horizontal line
     
  8. Jan 30, 2015 #7

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    Alright. Notice that the picture depicts (or at least should depict) the cannon as shooting at the same angle in all three cases. That leaves the 'force of a shot', or acceleration it imparts to the ball in the barrel.

    Therefore, as the three balls leave the barrel and are no longer accelerated, they all have different velocities.

    That's the only difference.

    Can you now formulate the reason for the ball C not crashing into Earth, and similarly, the reason why satellites are not crashing either?
     
  9. Jan 30, 2015 #8

    Nugatory

    User Avatar

    Staff: Mentor

    No. Although the scale of the picture makes it hard to see, in all three cases the cannonball leaves the cannon at the same angle (horizontal) and only the muzzle velocity is different. The slowest-moving ball (A) seems to making a steeper angle, but what's really going on is that because it's moving more slowly it doesn't travel as far left to right in the time that it takes it drop by a given amount as do the other two.

    [edit: Bandersnatch beat me to it :) ]
     
  10. Jan 30, 2015 #9
    It would be better if you finally say complete answer bro! ;)
     
  11. Jan 30, 2015 #10

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    Sorry, that's not how this site works (most of the time). We're here to help people understand physical concepts, and not being spoon-fed the answers is how people learn best. Getting there yourself is how you develop intuition.
    If you want a straight answer, just type your question in google.
     
  12. Jan 30, 2015 #11
    I hope I've got the answer. Hey Bannersnatch, is answer mass?
     
  13. Jan 30, 2015 #12
    Hey am I right bandersnatch bro????
     
  14. Jan 30, 2015 #13

    Bandersnatch

    User Avatar
    Science Advisor
    Gold Member

    It's the same in all three cases.
     
  15. Jan 30, 2015 #14

    RUber

    User Avatar
    Homework Helper

    So imagine that the moon is the cannonball. As it is introduced at some initial time into our system.
    In cases A and B from Bandersnatch's diagram above, the moon crashes into the earth and we all die. In case C, it falls into a nice orbit.
    I would argue that there would even be a case d where the moon goes off into space and does not come back.
    In all cases, the mass, angle, etc. are all the same, only one difference as stated in posts 7 & 8 is velocity.
    Luckily air resistance in space is negligible, so the moon will likely continue to orbit for a while.
    What force does velocity impart on an object?
    That is the force that has to balance with gravity to keep it orbiting.
     
  16. Jan 30, 2015 #15

    Nugatory

    User Avatar

    Staff: Mentor

    There is - with enough initial velocity the cannonball would head off into space on a hyperbolic trajectory.
     
  17. Feb 1, 2015 #16

    Drakkith

    User Avatar

    Staff: Mentor

    This is incorrect. Velocity does not 'impart a force'. There is no force working against gravity to keep something in orbit. The object simply has to go fast enough so that as it falls it continually misses the ground.
     
  18. Feb 6, 2015 #17
    Something in orbit has lots of energy and angular momentum

    Gravity preserves total energy and angular momentum

    So something in orbit will tend to stay in orbit until acted upon by some other huge force
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why do natural Moons revolve around planets?
  1. Why do planets rotate? (Replies: 18)

Loading...