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

Question about Centripetal force

  1. Apr 7, 2008 #1
    If a string is attached to a weight and swung around overhead, is the force that creates tension on the string centripetal, centrifugal, or both?
     
  2. jcsd
  3. Apr 7, 2008 #2

    DaveC426913

    User Avatar
    Gold Member

    What do you think?
     
  4. Apr 7, 2008 #3
    Centripetal. Centrifugal force can only be measured in a rotating frame of reference.

    Pete
     
  5. Apr 8, 2008 #4
    I think centripetal would always exist in the string. But, centrifugal (reactive) force would depend on the rigidity of the string.
     
  6. Apr 8, 2008 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Still insist on using that archaic meaning of centrifugal force as the "reaction" force to centripetal force? If so, then it's simple: The string pulls the object (centripetal) and the object pulls the string ("centrifugal"). Can't have one without the other.

    One more time though: This is an old-fashioned usage of the term centrifugal. The modern usage is as a "fictitious" force that appears when viewing things from a rotating frame of reference.
     
  7. Apr 9, 2008 #6
    Just to add to what Doc Al said, the concept of "force" is actually a very slippery one. Technically, a force is the rate of change of momentum away from what the natural trajectory would have been. In other words, it doesn't really make sense to talk about a force without fixing trajectories first. Normally, in an inertia frame, things move in straight lines (ignoring gravity). So if something moves in a circle, there must be a force to make it do so --- we call that "centripetal". However, if we're in a rotating frame, things tend to move away from the centre, so if something doesn't then you need a force --- we call that one "centrifugal".
     
  8. Apr 9, 2008 #7
    archaic? Was it true? For the example which nuby explain,
    What are the action-reaction forces? Say the one is centripetal force, what is the other?
     
  9. Apr 9, 2008 #8

    Doc Al

    User Avatar

    Staff: Mentor

    The "action/reaction" pair is: String pulls on object & object pulls on string. When considering the motion of the object, we call the first force the centripetal force. The second force has no particular name. (Some folks call that second force the "centrifugal" force--but that's nonstandard usage. Old-fashioned and out of date!)
     
    Last edited: Apr 9, 2008
  10. Apr 9, 2008 #9

    DaveC426913

    User Avatar
    Gold Member

    I thought the second one was inertia - the object's tendency to continue moving in a straight line.
     
  11. Apr 9, 2008 #10

    Doc Al

    User Avatar

    Staff: Mentor

    Inertia is not a force.
     
  12. Apr 9, 2008 #11
    Ok. centripetal-centrifugal forces pair never action-reaction forces.

    Other hand, for this example, I suppose the pair of centripetal force is a force like so; say we pull a body by means of a string in space. There is no other force(friction, weight) then we apply. But anyway string will have has a tension. If this true, this is inertia force. But it is a pair-less fictitious force. Then how will we call it? :-/
     
  13. Apr 9, 2008 #12
    Net force?
     
  14. Apr 9, 2008 #13

    Hootenanny

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The string "pulls" on the body and the body "pulls" on the string, this forms a force-reaction pair. Equally, we "pull" on the string and the string "pulls" on us, again a force-reaction pair.

    What is this fictitious force your talking about?
     
  15. Apr 9, 2008 #14

    Doc Al

    User Avatar

    Staff: Mentor

    As Hootenanny already explained, the string pulls on the object and the object pulls on the string. Two very real forces--nothing "fictitious" here.
     
  16. Apr 9, 2008 #15
    Is it possible centrifugal force doesn't exist in the string at all? If you take the ball way and spin the string around by itself, the string would probably just get tangled around the axis. Would there be any centrifugal force in this example?

    Maybe the string only transfers force to the weight? i.e. Weight pulls on spinning axis through string, and the rigidity of the string controls the rate force is transfered.
     
  17. Apr 9, 2008 #16

    Doc Al

    User Avatar

    Staff: Mentor

    According to your nonstandard definition of "centrifugal" force: NO. The string can't pull without being pulled.
     
  18. Apr 9, 2008 #17
    I tried to understand second force on weight which asked in first post. Doc_Al said has no particular name and DaveC426913 said inertia. You and Doc_Al are right. I must be mad. Don't know how and why confused so much. Ok.
     
  19. Apr 9, 2008 #18
    centrifugal force is a misnomer. what people refer to as centrifugal force is actually centrepetal force to the best of my knowledge.
     
  20. Apr 9, 2008 #19

    DaveC426913

    User Avatar
    Gold Member

    No, centripetal force is the force pulling inward. Centrigual force is plain-old garden variety inertia as seen from the rotating reference frame. (i.e. if the string tied to the rotating bucket of water were to break, the bucket would continue in a straight line tangential to the bucket's former circular path).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Question about Centripetal force
Loading...