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ATTWOOD'S MACHINE (rope + pully)

  1. Mar 2, 2009 #1
    1. The problem statement, all variables and given/known data
    Determine the force of gravity acting on the masses (we want to end up with 9.8m/s2

    ---> My situation : A rope with 2 different masses hanging on each side, is held by a pulley. The heavier mass falls towards one end. (teacher said something about the mass having to cancel out afterwards) or so.

    2. Relevant equations
    d=v1t +1/2at2
    I think we can use Newton's second law, F=ma


    3. The attempt at a solution
    I used d=v1t +1/2at2 and got acceleration which came out to be .889m/s2

    I was wondering what steps I can take now
  2. jcsd
  3. Mar 2, 2009 #2

    Doc Al

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    Staff: Mentor

    How does the measured acceleration relate to the acceleration due to gravity (g)?

    You'll need to apply Newton's laws to analyze the system. Apply F = ma to each mass.
  4. Mar 2, 2009 #3
    awesome ^^ i just tried that juuust right before you replied haha thx Now I know im sorta on the right track^^
  5. Mar 2, 2009 #4
    ok im stuck again xD
  6. Mar 2, 2009 #5

    Doc Al

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    Show exactly what you did and where you got stuck.
  7. Mar 2, 2009 #6
    ok So,
    step 1:
    d = v1t +1/2at2
    got acceleration = .889m/s2

    Mass A:


    Mass B:

    I got stuck right there, like what can I do with it? can I find net force because If I subtract FA and Fb, it'd be 0N O_O, but since im 100% sure that the Net force isn't 0, what do I do? because Newtons second law states that, if the object is at rest, or at a constant velocity, the net force is 0. but the mass is acceleration towards the floor =P so it cannot be 0
  8. Mar 2, 2009 #7
    ehhh... anything? im still stuck there lol...been working on it and still working on it ^^
    Last edited: Mar 2, 2009
  9. Mar 2, 2009 #8

    Doc Al

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    You must first identify the forces acting on each mass, find the net force on each (symbolically), and then apply Newton's 2nd law (∑F = ma) to each. You'll get two equations (one for each mass) which you will combine.
  10. Mar 2, 2009 #9


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    I don't think you can finish it without knowing something about the masses.
    The wording is inconsistent, too - it asks for a force, then says it should have units of acceleration. Could you check the question again and make sure it has been copied word for word?
  11. Mar 2, 2009 #10

    Doc Al

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    This makes sense, assuming you solved for the acceleration correctly.

    But I don't know what you're doing here. Each mass is different, so call them m1 and m2. Your equations will look something like:

    ∑F(on m1) = m1*a
    ∑F(on m2) = m2*a

    Note that if m1 has an acceleration of a downward, then m2 must have an acceleration of a upward.
  12. Mar 2, 2009 #11
    MMMM, i just found out that we can use a value for mass...

    m1 = .1kg
    m2 = .2kg
    and now I need to figure out how to get g = 9.8
  13. Mar 2, 2009 #12


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    Oh, now you can do it knowing the masses!

    Sum of forces = ma, where m is the total mass.
    Put in an mg for each force.
  14. Mar 2, 2009 #13
    ok I'll try that ^^
  15. Mar 2, 2009 #14
    ok I got... for the sum of forces is the following...

    FA= mg
    FA= .1g N

    FB= mg
    FB= .2g N

    FNET = FB - FA
    FNET = .1g N

    Fsum of forces = .3g
    .1g = .3g????

    and now I don't know how this will get me to 9.8 ^^


    I also tried doing...

    Ftotal force = ma, where I subbed in the a that I obtained from before...

    0.1g = 0.3*0.889
    g = 2.6m/s2...

    But If I heard the teacher correctly, I believe I heard I'm supposed to get 9.8 or something lol
    Last edited: Mar 2, 2009
  16. Mar 2, 2009 #15


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    sum of forces = ma
    .2g - .1g = .3*0.889
    you can easily find the ACCELERATION of gravity.
    No, it isn't working out to 9.8 as you would expect . . . on Earth.
    Your question says find the FORCE of gravity - not sure what that means in this case since there are 2 different masses so two different forces of gravity. Check the wording again?
  17. Mar 2, 2009 #16
    sorry, the question just says

    Attwood's Machine - Measure Acceleration, calculate g.

    I also did the experiment physically too =P

    I made a pulley with a rope and a mass on either end. and I just let it drop, timed it, and measured the distance =P
  18. Mar 2, 2009 #17
    Are you sure the values of time and distance are accurate? Given those two masses g should be equal to 3a.
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