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Forces: mass on a string circular motion

  1. Apr 19, 2014 #1
    Hi, if I have a mass on a string and I swing it around my head at a constant speed, what is the resultant force on the mass (I'm looking for a qualitative rather than quantitive answer)?

    My inital thoughts were towards the centre of rotation, due to the tension on the string (and since the mass is accelerating towards centre). So there must be a resulting force towards the centre?

    But also if I was to sit on the mass I would feel an outward force, so I'm not quite sure??

    Thanks for any help :)
  2. jcsd
  3. Apr 19, 2014 #2


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    Science Advisor

    Yes, you feel an outward pull (the "centrifugal" force) due to the "equal and opposite" rule. You are applying an inward pull to the string and that feels to you like the same force outward. The force on the mass is inward, the force on you is outward.
  4. Apr 19, 2014 #3


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    Gold Member

    It depends on the reference frame you choose. The most common are (1) a fixed inertial frame observing the circular motion and (2) a non-inertial accelerating frame such as your mass at the end of the string. You've already recognized both.

    Exactly. This is an example of observing the circular motion from an inertial frame (you). The mass accelerates inward towards a central point due to centripetal force. The balancing reactionary force outwards is centrifugal.

    And that's the second example, when viewed from the non-inertial accelerating frame (the mass). Here the fictitious force you feel is centrifugal. The use of the term centrifugal is distinct from the first example, IOW different when used for the inertial frame #1 and the non-inertial frame #2.

    I hope I got this mostly right :biggrin: and it helps!
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