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

Net Force on a Spinning Ball Attached to a String Question

  1. Apr 7, 2012 #1
    Hi all,

    I have a conceptual question about the net force of a rotating or spinning ball attached to a string. If we have a situation such as a tether-ball game where the ball is being swung around a string at an angle to the vertical, is the net force the tension in the string, or the component of the tension that acts as the centripetal force? To me personally it seems that the net force should be the centripetal force as this is what is causing the balls acceleration, but then again the only force that can act on the ball (aside from gravity) is through the string.

    Any help in clearing this up is greatly appreciated, thanks in advance for any answers.
  2. jcsd
  3. Apr 7, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi D.Strauss! Welcome to PF! :smile:
    This is why I personally think that nobody should ever mention centripetal force … it's confusing. :redface:

    There is no separate centripetal force … there's only two forces: tension and gravity.​

    By definition, centripetal force must be towards the centre, and as you say, that's horizontal.

    The net force is the tension plus gravity, and in equilibrium (but not if the ball is rising or falling), that is horizontal.

    So in equilibrium, the net force is centripetal, and otherwise it isn't.​
  4. Apr 7, 2012 #3
    Thank you for the welcome and quick reply. :)

    I agree with your opinion on the topic, not only is it confusing, it's also pretty rare to find good explanations.
    Thank you for clearing this up though, like you said we can only take into account tension and gravity, so the net force must be the sum of the two, which in this case is the centripetal force, right?
    I think a lot of the confusion comes from the fact that the name centripetal force suggests a separate force which, as you said, it isn't.
  5. Apr 7, 2012 #4


    User Avatar
    Homework Helper

    Only if there is no vertical component of velocity (or acceleration). The ball would have to move in a circle of constant radius along a horizontal plane.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook