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Homework Help: Some questions(pulleys,rotation)

  1. May 15, 2009 #1
    1. The problem statement, all variables and given/known data
    Hi all!
    I'm attaching a picture with three diagrams,each for a question(they're numbered in green)
    so question number 1:
    i had to find all kinds of ratios between the accelerations and stuff, so this was good,
    the last part of the question was "Find the acceleration using Energy" , I don't have an idea ow to relate energy to acceleration , It's pretty easy to find using Newton's laws, but not energy, so any hint here could help.
    Info:no friction between any surface nor pulleys, neglect mass of pulleys,I defined mass 1 and mass 2 as numbered in sketch.
    question number 2:
    It's a multiply choice question :
    A sphere that rotates around his axis of rotation was put on an inclined plane(long),Center of mass velocity in the moment it was put on the inclined plane was 0 , direction of rotation is as indicated in the sketch.
    which of the following is true :
    a)there is a period of time that the acceleration is greater than g*sin(beta).
    b)there is a period of time that the acceleration is smaller than g*sin(beta).
    c)there is a period of time that the acceleration is equal to g*sin(beta).
    d)the angular velocity firs decreases then increases.

    question number 3:
    in the sketch, everything is ideal, the system is released from rest,the distance between M and 2M is:
    a)not changing in system number 1, and increasing in system number 2.
    b)decreases in system 1, increases in 2.
    c)constant in both systems.
    d)increases in both.

    2. Relevant equations
    Lots of!
    W(conservative)=change in kinetic energy.
    W(non conservative) = change in total energy.
    rolling no slipping c.O.M velocity V=WR.
    Acceleration of a sphere on an inclined plane when rolling no slipping =(2/3)gsin(beta) if i remember correctly.
    3. The attempt at a solution
    ok question number 1 left only the last part which is what i wrote, here I have almost no idea, I just found the velocity ratio, and i think I need to attach work of tension and change in PE and Ek some how ... can't find a way, so a hint will help :}
    wow this one is annoying , so what I have so far is , the velocity is 0 initially , so it's slipping, till it will have velocity of W*R , so there is acceleration downwards the plane, but i think there should be friction as well, so the answer should be b)period when acceleration is smaller than gsinB??? or it's d? :S or else XDD
    Truly, that many pulleys make me sick, I would really like to know what steps are involved in this kind of questions, are You doing it in the intuitive way?
    well anyway , I'll start from system number 2(left).
    0.5M is definitely going up, so either M and 2M going down or only one of them.
    in the first system , i think 3M goes down.2M and M going up.
    well I'm lost in this question.
    any hints/stories/jokes are welcome!

    Attached Files:

    Last edited: May 16, 2009
  2. jcsd
  3. May 16, 2009 #2
    bump,any one?
  4. May 16, 2009 #3
    Ok I've got the answer for question 3(constant distance), any one can help me with the others? :S
  5. May 17, 2009 #4
    Really nobody knows? or It's just cause the thread is to long? XD
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