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B Do objects in circular motion also *have* to undergo rotation?

  1. Nov 23, 2016 #1
    1. 20141128-142420766-6583-circular.gif
    2. circulrv (1).gif
    Well, these are two pics on circular motion, i found on net...
    If the objects had a zero rotational motion before circular motion, and the next moment suddenly a force acts ( perpendicular to their linear momentum ) such that they move in circular motion,... which of these pictures would it resemble?... 1 or 2?
     
  2. jcsd
  3. Nov 23, 2016 #2

    DrClaude

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    Circular motion requires constant acceleration. What force is causing that acceleration?
     
  4. Nov 23, 2016 #3
    Yeah... constant force say,.. gravity towards a point in space,.... so should the motion resemble 1 or 2
     
  5. Nov 23, 2016 #4

    jbriggs444

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    Pictures 1 and 2 are interchangeable. They depict the exact same scenario. There is no difference other than the fact that one object is rectangular and one is round and that one object has an acceleration vector drawn in and the other has a velocity vector drawn in.

    What difference do you see between the two pictures that you wish us to comment on?
     
  6. Nov 23, 2016 #5

    DrClaude

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    It will depend very much on the way the force is impacted, but the general case will be closer to picture 1. Only in the case where the rotational period is the same as the orbital period will the result be picture 2.
     
  7. Nov 23, 2016 #6

    DrClaude

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    I take picture 1 to show no rotation of the object.
     
  8. Nov 23, 2016 #7
    Well,... the squared object undergoes 1 rotation about its axis for every i rotation around a fixed point in space
    the round object does not rotate about its axis....
    so they are different....
     
  9. Nov 23, 2016 #8
    So, it doesn't rotate about its own axis... thanks for the clarification
     
  10. Nov 23, 2016 #9

    jbriggs444

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    Thanks, I missed seeing the three holes on the bowling ball.
     
  11. Nov 23, 2016 #10
    Yeah... OK !
    Say, i have an object which is in linear motion and is also not in any kind of rotational motion...
    Say that suddenly, i have gravity pulling it to its center elsewhere in space, will the object be like image 1 or image 2 ?
     
  12. Nov 23, 2016 #11
    The way we usually accomplish these things, (e.g. swinging a rock on a rope) it almost always winds up as you show in the second picture. The conservation of angular momentum as planets accrete from the dust around a star have them spinning much faster. Tidal locking eventually slows the moon and the planets down to the second situation. You are almost never shown a case where the object isn't spinning as it orbits either in lock step like case 2 or faster in the same direction.

    So it is with no small bit of evidence that one might expect this is necessary. It is not. If we could plop a planet down and just shove in the orbital direction without spinning it, it would orbit without spinning as in picture 1. The force it takes to put the object in orbit need not impart any torque.

    The easiest place to see this is with a playground tether ball.
     
  13. Nov 23, 2016 #12

    Andy Resnick

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    Interesting question... never thought of this before, it's probably useful to incorporate into my Intro class. Note that there are 2 reference frames involved- the stationary lab frame, and the rotating frame of the velocity (and/or acceleration) vector. In the upper case, the object is not rotating in the lab frame but is rotating with respect to the acceleration vector; the opposite occurs for the lower case.

    As an analogy, the Hubble Space Telescope orbits the earth like the top diagram, while AFAIK the International Space Station (and Space Shuttle, back in the day) orbit like the lower diagram [LHLV (local horizontal-local vertical) attitude].
     
  14. Nov 23, 2016 #13

    A.T.

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    It will change from 1 to 2 over time, and increase the distance:
    https://en.wikipedia.org/wiki/Tidal_locking

    Depends on how the force is applied. If that force creates no torque around the objects center of mass, then it will stay in state 1 forever.
     
    Last edited: Nov 23, 2016
  15. Feb 18, 2017 #14
    I recently found out that for an object to undergo rotation ;
    1. All it's particles should be moving in a 2D circular path or a part of a 2D circular path
    3. All the center of circles of the moving particles should be on a line
    2. This line should be perpendicular to all the planes of motion of the particles ( axis of rotation )
    using these conditions, the first picture is not undergoing rotation as the 3rd condition is not satisfied
    the 2nd picture is undergoing rotation since it satisfies all the 3 conditions
    hope that helps !
     
  16. Feb 18, 2017 #15

    sophiecentaur

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    There are two separate issues here. If the 'object' is spherical and the force is applied at the CM, then there is nothing to make the object rotate about its own axis. IF the force is applied elsewhere then there is a couple to make the object rotate about the point of application. Its rotation on the end of a string would cause an angular oscillation about the mean radial direction of the string.
     
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