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Circular motion

  1. Nov 6, 2009 #1
    Hi guys i have a rather weak concept on circular motion so i wish you guys can help me. As you know when something goes round in circle, there is a net force towards the center right? If that is the case, why this force doesn't push the particle to the center causing it to spiral towards the center? Secondly, why does planetary motion around the sun elliptical? Why can't they go round the Sun in circle?

    Lastly this has got no connection with circular motion but rather a though i had when youtubing some video. Some guy in some video on youtube said that if we were to take a marble to represent the nucleus of the atom and a strand of hair to be the electron, their distance would be 20 miles apart. If this is so, why are we able to hold things? Wouldn't the space between the nucleus and electron be too big??
  2. jcsd
  3. Nov 6, 2009 #2
    In case of objects on a string: The force is only there, because the object would go in a straight line if unaccelerated, and it is prevented from doing so by the rope. The object can't go further from the centre because of the rope, and if it ever got even a tiny bit closer to the centre, the rope would go slack, and the centripetal force would disappear. Because there now is no centripetal force, the object would go in a straight line again, it would threaten to fly out of the circle, but it is once again stopped by the rope, etc.

    A circle is only a special case of an ellipse. All planets go in ellipses that are nearly circular.
    I believe this comes from how the solar system formed from a rotating disk of gas.

    If you give a motion to an object 10000 km from the earth, perpendicular to the line to the center of the earth, the following can happen

    1. the speed is too slow for circular motion, and you get an ellipse with the point where you released the object as the most distant point from the earth.
    2. the speed is just right, so v^2/r balances the acceleration of gravity and you get a circle.
    3. the speed is higher than that, so you get an ellipse with the point where you released it as the closest point to the earth.
    4. the speed is higher than the escape velocity, and the object flies off.

    For now, it's problem best to accept that if you try to overlap the electron orbits of 2 atoms, this is very hard and requires a lot of energy. (unless you get a chemical bond).
    If you want to know more you could look at quantum mechanics, but I wouldn't recommend this now.
  4. Nov 6, 2009 #3


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    When we "hold" a ball, the atoms in our hand are interacting with the atoms in the ball.
    But it is not the nuclei of the atoms that are interacting, it is the electron shells that are interacting. They interact through mutual repulsion.

    At the macroscopic hand/ball scale, the electrons effectively form a shell around the atom, making the whole thing like unto a small ball whose internal structure is irrelevant.
  5. Nov 6, 2009 #4
    That is like trying to compare a giant fishing net to a coffee filter...

    The coffee filter has little holes that allow the water to pass through, but prevents the coffee grounds.

    Now if you drastically increased the size of the coffee filter it would be the size of a large fishing net. Which of course would be absolutely worthless for filtering coffee, as everything would fall through.

    Size does matter!!! You cant increase the size of something and expect it to maintain its original function.

    PS: DaveC426913 answered the question perfectly if you understand atom configuration and a little bit of chemistry.
    Last edited: Nov 6, 2009
  6. Nov 7, 2009 #5
    Hmm I don't understand this, if the speed is too low why does the object goes into a elliptical orbital instead of just crashing into Earth? So what do we need to do to make some crash into Earth?

    Oh so the reason is due to electronic repulsion?

    Hmm coffee filter.... I believe i just magnify the atom to make my point why something so small can't pass through such a big gap. I don't see what's wrong with the analogy if you are magnifying the coffee filter to a fishing net then you have to magnify the coffee bean into basketballs and water molecules maybe into coffee bean so it will just seep through.
    Last edited: Nov 7, 2009
  7. Nov 7, 2009 #6
    The diameter of a hydrogen atom is about 5*{10}^{-8)mm.. Which means you increased the size by a few "gazilion" times. You did not mention increasing the size of the hand, because if you did it would actually be the same situation. The hand would be about the size of the planet and the atom roughly 20 miles across or whatever.

    Either way the electron clouds would be proportionally the same size, and the electrons of the ball would repel the electrons of your hand.
    Last edited: Nov 7, 2009
  8. Nov 7, 2009 #7


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    There's some underlying preconception that you have that is not quite coming through, but it's confusing you. I'm not quite sure what is in your mind in the way of a picture.

    As "seen" by other atoms, the electron shell of a given is effectively solid. In a classical and extremely oversimplified manner of speaking, the electron moves about the proton so fast that it is basically "everywhere at once". So other atoms do not "see" into the core of an atom - all they see is the shell.

    As far as the atoms in the ball are concerned, your hand is made of a giant rugged surface of these hard little balls. Not only are they all hard little balls, but most of them are glued togther (in the form of molecules - the skin of your hand is made of very complex molecules).

    You could no more pass the ball through your hand than you could cram one house made of bricks through the side of another house made of bricks.
  9. Nov 9, 2009 #8
    Hmm i always thought we are made up of atoms so shouldn't the atoms of the hand be magnify as well? Oh well never mind that i understood the concept thanks.

    So regarding the other question, why doesn't the planets crash into the Sun but instead speeds up and the orbital becomes an ellipse.
  10. Nov 9, 2009 #9


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    We are made of atoms. And they would be magnified as well. Why would you think otherwise. And more importantly, why do you think that would change anything?

    The planets are trying to crash into the sun. They fall toward it. But their their velocity causes them to miss it. Same with the Moon and other satellites around Earth.

    I think you have some misconceptions about orbits and gravity. Why don't you describe in more detail what you expect to see happening and we'll try to sort out where the confusion lies?
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