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: 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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook