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What is the reaction force of centripetal force

  1. Jul 23, 2015 #1
    According to newton's 3rd law, force exists in pair. When I attach a ball to a rope and rotate it, I am giving the ball an centripetal force to keep it moving in circular motion, so is the tension in the string the reaction force I am asking?

    I also want to ask about the reason why centrifugal force is considered as a pseudo-force. My explanation is that when you stand on the ball (imagine), you experience both the centripetal force (because you are rotating with the ball) and the centrifugal force (because you feel a force pushing you outward), but noting is actually providing the centrifugal force, and this also makes the rotating frame of reference an non-inertial frame. Is it correct?

    Last edited: Jul 23, 2015
  2. jcsd
  3. Jul 23, 2015 #2
    Centripetal force is not a new kind of force . It only refers to that force which is reponsible for circular motion of an object - In this case , the tension .

    *In inertial frame -
    Tension is not equal to some other force mw2r . Acceleration is caused due to tension and this tension is simply equal to m*a , and a = w2r .

    *In non inertial frame of the ball , you now add a pseudo force , simply following Newton's law for the non-inertial frame .
    You call this force the centrifugal force .

    It doesn't make the circular motion impossible . Would you consider your accelerating car ( from inertial frame ) to be moving while sitting in it ( a non inertial frame ) ?
    Now , coming to your question , if you sat on the ball , would you see the ball accelerating ? As you said , Fnet = 0 and I just said , a =0 .

    So is Newton's law being invalidated ?

    *Just so you know , there's a like button near the bottom which you can press if somebody's post helps .:cool:
  4. Jul 23, 2015 #3
    Just to check my concept, in inertial frame where F = ma is obeyed, there is no reaction force for centripetal force because the force is just tension, while in non-inertial frame, centrifugal force is added to "balance" the centripetal force as stated by the newton 3rd law; otherwise, the law will be invalidated.

    Hope its correct this time

    Thanks anyway
  5. Jul 23, 2015 #4
    You may not experience. You just feel like something pull you out, which you stated it after this, a pseudo force. This experience is one of the key for Einstein to establish General Relativity.
  6. Jul 23, 2015 #5
    Of course not, for tension is made by the molecules on the rope! It's a real force so it must have a reaction, which is the force from the rope.
  7. Jul 23, 2015 #6
    so does that mean the reaction force for centripetal force is the force from the rope, and tension is kind of like a "medium" to provide centripetal force?
  8. Jul 23, 2015 #7
    The tension is the centripetal force .
  9. Jul 23, 2015 #8
    Yes, it is.
    Yes. The rope makes the force, a centripetal force that the rope pulls you, so the reaction must be a force you pull the rope.
  10. Jul 23, 2015 #9


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    Note that these two are different things (see table in the first link). The modern use of "centrifugal force" usually means the later (inertial force existing in rotating frames only).
  11. Jul 23, 2015 #10
    Yeah - that's correct .
  12. Jul 23, 2015 #11
    Thank you all guys I think I have had a better grasp of the concept!
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