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Homework Help: Mechanical Principles help please.

  1. Jun 21, 2008 #1
    1. The top pulleys of a Weston differential pulley block have diameters of 210mm and 190mm. Determine the effort required to raise a load of 150kg if the efficency of the system is 35%.

    What is the work done in overcoming friction when the load is raised through a height of 2.5m?

    I did some working out and got:

    effort = load/number of pulleys = 150/2 = 75 (what units?)

    and work done= (150x9.8) x 2.5 = 3677.6 N/M

    I did 150x9.8 to get newtons.

    Is this correct?

    Cheers Joe
  2. jcsd
  3. Jun 21, 2008 #2
    you must work out using these formulas

    i only no this as i did the same question in class

    you need to work out the velocity ratio of the machine first

    vr = 2 d1/(d1-d2)

    then the mechanical advantage

    effiency of the system = mechanical advantage/ velocity ratio x 100 %

    you need to transpose this

    effort of the machine = load / mechanical advantage

    this is for the first part

    second part is

    wd = force x distance moved

    then effiency of the system = work output/work input

    you also need to transpose this
    i no how you feel this is a hard subject and i am also finding it hard !!!!
  4. Jun 21, 2008 #3
    I take it d stands for diameter? Should it be this or should it be circumfrence?

  5. Jun 21, 2008 #4
    d is the diameter of each pulley
  6. Jun 21, 2008 #5
    Should I use diameter or circumfrence? As i seem to remember circumfrence being mentioned in class.
  7. Jun 21, 2008 #6
    when i calculated these i used the diameters and the answers were correct compared to the answers i had.
  8. Jun 21, 2008 #7
    Ok cheers.
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