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

Pseudo forces on a person in a park-swing

  1. Sep 17, 2012 #1
    This is a very basic question,I also attempt to learn more from it.Some additional info would also be of great help.
    A girl whose mass is 40kg is using a swing set.The diameter of the wire used for constructing the links of the chain is 5mm.Determine the average normal stress in the links at the bottom of the swing,assuming that the INERTIAL FORCES CAN BE NEGLECTED.

    Q.The question is regarding the INERTIAL FORCES.I have drawn the resolution of forces diagram(attached and I am not sure it's correct).Here,where does one include the inertial/pseudo forces in this accelerating frame of reference?

    Q.Is this inertial force- 'pseudo' in the sense that the sagging of the swing appears from a imaginative force taking the swing as the reference frame?

    Hope the questions are clear.Please view the attachment diagram.

    Attached Files:

    Last edited: Sep 17, 2012
  2. jcsd
  3. Sep 17, 2012 #2


    User Avatar
    Science Advisor

    Ignoring inertial forces in this problem means that tension is due to supported weight only. You basically solve this problem for swing at lowest point and v=0.
  4. Sep 18, 2012 #3


    User Avatar
    Homework Helper

    Ignoring the mass of the chain and the swing, the only inertial force would be equal to the reaction force of the girl's 40kg body to centripetal acceleration, which is exerted onto the swing. From the swings rotating frame of reference the inertial force is the apparent centrifugal force exerted on the girls body. The swing exerts a real and non-inertial force onto the girl to oppose both the inertial force (swing frame of reference, or reaction force if using an inertial frame of reference) and weight.

    As K^2 mentioned, it appears that you're supposed to assume that the girl and the swing are at rest, and not moving, but that isn't a very interesting problem, since the only force opposed by the chain is due to the girl's weight.
    Last edited: Sep 18, 2012
  5. Sep 18, 2012 #4


    User Avatar
    Science Advisor
    Gold Member

    I find it very misleading to talk about inertial forces as reaction forces. Inertial forces are never part of a Newtons 3rd action-reaction pair.
    There is no centripetal acceleration in the rest frame of the swing, where inertial forces appear. The swing and body are static here.
    This is correct. The real centripetal froce on the body by the swing is opposing the inertial centrifugal force on the body. But they are not an action-reaction pair. They both act on the same object (the body), unlike action-reaction force pairs which act on a different object each. The reaction force to the real centripetal force on the body by the swing is the real reactive centrifugal force on the swing by the body.
  6. Sep 18, 2012 #5


    User Avatar
    Homework Helper

    My last edit to my post didn't take (the forum froze on me and I forgot to check my post later). I re-editted my previous post back to what I intended to post before, with some clarification. I only meant to state that the inertial force from the swing's frame of reference was equal to the reaction force from an inertial frame of reference.
    Last edited: Sep 18, 2012
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Pseudo forces on a person in a park-swing
  1. Mg= A Pseudo Force? (Replies: 18)