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Uniform circular motion

  1. Aug 7, 2015 #1

    faiziqb12

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    1. The problem statement, all variables and given/known data
    i was going through uniform circular but a confusion popped out of my head .
    as i have read before the Rutherford atomic model was only rejected due to
    the fact that electrons moving in circular orbits need to accelerate constantly
    to maintain a uniform speed but the acceleration causes electrons to radiate
    the energy and they would eventually lose all their energy and fall into the
    nucleus ..... so my question is in uniform circular motion there is a constant
    acceleration which keeps a uniform speed..is it centripretal acceleration?
    how?............ and how to calculate the magnitude and direction of this
    acceleration? ...... and is it a constant?
     
  2. jcsd
  3. Aug 7, 2015 #2

    tms

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    What are your answers?
     
  4. Aug 7, 2015 #3

    faiziqb12

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    i am empty
    i was trying to add up the the gravitation,friction and centripretal acceleration vector
    but it proved of no use
     
  5. Aug 7, 2015 #4

    faiziqb12

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    i have ran out of my mind
    and here are no websites that can help me
    please help!
     
  6. Aug 8, 2015 #5
    What is centripetal acceleration ? What causes it ?
     
  7. Aug 8, 2015 #6

    faiziqb12

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    NO MY QUESTION ACTUALL IS THE SAME AS I HAVE POSTED
    the centripetal force just allows a body to be fixed in a circular orbit but what causes the object to keep a fixed velocity in that circular orbit
     
  8. Aug 8, 2015 #7
    Is the velocity constant ? Does uniform circular motion maintain constant speed , or constant velocity ?
     
  9. Aug 8, 2015 #8

    faiziqb12

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    oh sorry constant speed
     
  10. Aug 8, 2015 #9
    Okay , so it is now clear that velocity of the object is changing .

    Now , for constant mass systems , if velocity of object is changing , then what does that say about object's acceleration - is it zero ?
     
  11. Aug 8, 2015 #10

    faiziqb12

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    yes
     
  12. Aug 8, 2015 #11
    No , it isn't !

    What causes change in velocity ?
     
  13. Aug 8, 2015 #12

    faiziqb12

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    What causes change in velocity ?
    i was talking about magnitude...
    now you come on the question
    i can see two types of acceleration
    one that keeps the object the object in orbit{centripedal acceleration}

    my question is about the second one ..that one which keeps a uniform speed or say a uniform magnitude of velocity
     
  14. Aug 8, 2015 #13

    faiziqb12

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    you are right
     
  15. Aug 8, 2015 #14
    Consider a pebble tied to a string , rotated about a fixed point . As it starts to spin , tension acts as a centripetal force , consequently causing an acceleration - which is the centripetal acceleration .

    This acceleration is responsible for circular motion . For uniform circular motion , this is the pebble's only acceleration .
    It causes change in velocity of the pebble . As this is perpendicular to initial velocity of pebble , it only causes a change in the pebble's direction of motion of the pebble , working like this throughout the circular motion .

    Thus , my question to you is this - how is the acceleration changing the velocity ( we have already seen that velocity is changing ) ?
    Also , how is velocity ( a vector ) related to speed ( a scalar ) ?
     
  16. Aug 8, 2015 #15

    faiziqb12

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    THE centripetal force is only responsible for keeping the body in orbit not for increasing,decreasing or maintaining its speed.
    in uniform circular motion there is constant speed due to the force applied by us repeatedly....and this force is the second type of acceleration
    now my question is how can i calulate the amount of such acceleration needed to maintain uniformity of speed
    i hope you would have understood my question now
     
  17. Aug 8, 2015 #16
    Please understand , I have got your question . What my problem is that you seem to be making mistakes in your basic concepts .

    So please answer these -
     
  18. Aug 8, 2015 #17

    faiziqb12

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    velocity has direction along with magnitude so it is a vector and speed has only a magnitude so it is a scalar
    also acceleration is changing the velocity by changing its direction
     
  19. Aug 8, 2015 #18
    So , in the entire motion , only direction of the velocity is changing , and not it's magnitude . So if it's magnitude isn't changing , speed isn't changing .

    So , to finally answer your question , no acceleration is causing uniform speed . Speed was never changing , and hence no acceleration was ever required to do so .
     
  20. Aug 8, 2015 #19

    faiziqb12

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    what you wanna say is if we stop rotating the string in the example , but not let the string go.then the ball will again be in uniform motion.

    no, it wont

    we revolve our hands to give the desired force always to cancel the effect of gravitational force and the frictional force in the example given

    and this force is actually the another accleration i was talking about
     
  21. Aug 8, 2015 #20
    ???

    How exactly would you do this ?
     
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