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: Dynamic Pulley System

  1. Oct 3, 2009 #1
    1. The problem statement, all variables and given/known data
    http://img53.imageshack.us/img53/6141/41622697.png [Broken]

    2. Relevant equations
    F = ma, all that basic stuff.

    3. The attempt at a solution
    g = 32.2
    Ma = 150/g = 4.66lb
    Mb = 75/g = 2.33lb

    so Box A
    |....}==== 2T

    box B

    -----|....|______ 60lb

    Sum of Fx = ma

    so for A
    2T = 4.66aA
    for B
    60 - 3T = 2.33aB

    then for the lengths of the wire I did

    so L = 2sa + 3sb
    0 = 2aA + 3aB
    but clearly aA and aB must have opposite signs so
    2aA = 3aB

    using simultaneous equations with the following you get
    60 = 2.33aB +3T
    0 = 4.66(3/2)aB - 2T
    which makes aB = 7.22ft s-2 (ANS)
    and T = 14.4 lb

    then i figured there were two ways to go about work done

    KEi(A+B) + Wd = KEf(A+B)
    or Wd = 2T*sA + (60-3T)*sB

    Which is great cause you get to doublecheck your result

    the problem is that for method 1 I get 334.1J
    and for method 2 I get 216.6J

    So, yeah, any help is appreciated.

    EDIT1: Formatting Errors
    EDIT2: I found my mistake. My simultaneous eqs were wrong, it was 6.99, not 4.99.
    Now the answers are aA = 7.02 ft s-2 and W = 140.4J
    I'd still appreciate some double-checking on my method since I'm not really sure if I'm doing this right.
    Last edited by a moderator: May 4, 2017
  2. jcsd
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Can you offer guidance or do you also need help?
Draft saved Draft deleted