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Homework Help: How to calculate weight of object swinging a radius?

  1. Sep 8, 2011 #1
    1. The problem statement, all variables and given/known data
    20 lbs. on the end of a 30 inch arm. Dropping from 2oclock position to 5oclock position.

    How much wieght will be developed if this was dropped on a scale measuring lbs.

    2. Relevant equations
    I am unsure how to calculate this.

    3. The attempt at a solution
    If there is a formula I can try to calculate myself.
    I just don't know where to start.
  2. jcsd
  3. Sep 8, 2011 #2
    Hi there...
    Let me see if I get this straight:, by
    did you mean that at that juncture, on this sort of pendulum, the thread/rope(whatever) is suddenly cut, and the object is dropped on a weight measure?
    If that's the case, then you must consider what sort of forces act upon an object moving in such a structure; Essentially(and this is quite a hint), you need to think about circular motion...
    Give it a try, it's bound to work...
    Good luck,
  4. Sep 8, 2011 #3
    Ok. i researched that and found this formula.


    w in my angular velocity. that is what i need to know.

    t is the period for 1 rotation.

    Is the rotation in rpm.

    I know my rpm is 19.

    If so then w would be .33

    If my angular velocity is .33

    How does that convert to lbs?
  5. Sep 9, 2011 #4
    Well, let me simply rearrange what you wrote, if you don't mind...
    You're correct in positing that: [tex] \omega = \frac{2\pi}{t} [/tex], Where t is the time of a single rotation(however, in SI units, this is ALWAYS measured in seconds); Therefore, RPMs won't do.
    My advice is(based on my personal experience) is to try and convert any problem to metric units(SI based), and if the solution explicitly requires Imperial/non-standard representation, only then(after having the proper resolution), should you turn it back to whatever's necessary(but that's my opinion, and you're of course free to do what you're comfortable with).
    As for those pesky Lbs, think about what a force suggests, its equation, and what do Lbs represent in this case(Newton's 2nd law would be helpful).
    You're on the right track though,
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