1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Rotational Motion question has me stumped

  1. Nov 3, 2007 #1
    Hey, I've puzzled my puzzler sore on this question for my physics assignment:

    A 0.50-kg ball that is tied to the end of a 1.5-m light cord is revolved in a horizontal plane with the cord making a 30° angle with the vertical. (a) Determine the ball’s speed. (b) If the cord can withstand a maximum tension of 9.8 N, what is the highest speed at which the ball can move?

    If anyone could help me out that would be awesome.
  2. jcsd
  3. Nov 3, 2007 #2
    Let Ft = force of tension Fg = weight Fnet = net centripetal force

    The key is to break the tension force into vertical and horizontal components. The vertical component will be equal to the weight. The horizontal component will be equal to the net centripetal force. If you write an equation for each of these, and then divide the second one by the first one you get:
    tan 30 = v^2 / (rg) where r = 1.5 m sin 30

    Does that help?
  4. Nov 3, 2007 #3
    Thanks a lot, I understand how to find the velocity, but I'm still doing something wrong to get the max possible V if max T can be 9.8N. Ty = mg= 4.9N. What I'm doing to find max V is finding max Tx...so sqrrt(9.8^2 - 4.9^2) = 8.49N = (m X v^2)/ 1.5msin30. This is apparently wrong...the correct answer is 4.7m/s.
  5. Nov 3, 2007 #4


    User Avatar
    Homework Helper

    When the tension is 9.8N and the weight is 4.9N, the angle between srting and the vertical is not 30 degree but 60 degree. Try it again.
    Last edited: Nov 3, 2007
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook