1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Circular Motion -- Swinging keys on a string in a vertical circle

  1. Nov 13, 2014 #1
    1. The problem statement, all variables and given/known data
    Assuming the keys are moving in uniform circular motion!!!!!

    Keys with a combined mass of 0.100 kg are attached to a 0.25 m long string and swung in a circle in the vertical plane.

    a)What is the slowest speed that the keys can swing and still maintain a circular path?

    b)What is the tension in the string at the bottom of the circle?


    2. Relevant equations
    Given: V =10m/s, R=25m, Fc = mv^2/r, Ff = UFn



    a) Centripetal Force is Fg, so Fg = MV^2/r
    So, mg = mV^2/r
    0.1(9.8) = (0.1)(V^2/0.25)
    Mass cancels out..
    9.8 = V^2/0.25
    So, (0.25)(9.8) = V^2
    Therefore, V^2 = 2.45, so V = 1.56m/s
    The slowest speed the keys can travel and still maintain circular motion is 1.56m/s.

    b) b) Fnet = Ft - Fg or Ft = Fnet + Fg (assuming this is uniform circular motion!!)
    Ft = mV^2/r + mg
    Ft = [ (0.1)(1.56)^2 / 0.25 ] + (0.1)(9.8)
    Ft = 0.97N
    Therefore, assuming that the keys are moving in uniform circular motion, the Ft at the bottom of the circle is 0.97N.

    Ok.. Really having a tough time with this one.
    Been looking over it for too long and eyes are gone blurry.. any help is appreciated!!!
     
  2. jcsd
  3. Nov 13, 2014 #2

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Some mistake there, surely.
     
  4. Nov 13, 2014 #3
    At the top it has a certain velocity, but you need to think about what happens as it goes downward (following the circular path).
     
  5. Nov 13, 2014 #4
    you can't assume uniform circular motion in this case . it gains kinetic energy at the bottom
     
  6. Nov 13, 2014 #5

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Judging from the calculations, dylanjames was not really assuming uniform circular motion, merely moving in a circle.
     
  7. Nov 14, 2014 #6
    if he wasn't why would he use the result from a) in b) for the keys' velocity at the bottom? I'm a bit confused here .
     
  8. Nov 14, 2014 #7

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Sorry, you're right.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Circular Motion -- Swinging keys on a string in a vertical circle
Loading...