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Homework Help: Masses and combined pulleys

  1. Sep 23, 2016 #1
    1. The problem statement, all variables and given/known data
    Snap1.jpg
    1) Which mass accelerates faster than g
    2) What's m1's acceleration
    3) In which condition m3 will move upwards, if they start from rest

    2. Relevant equations
    Mass-acceleration: F=ma

    3. The attempt at a solution
    $$\left\{\begin{array}{l} (1)~a_3=2a_2+2a_1 \\ (2)~2T_3+m_2g-T_3=m_2a_2 \\ (3)~m_1g-2T_3=m_1a_1 \\ (4)~ T_3-m_3g=m_3a_3 \end{array}\right.$$
    $$\rightarrow~a_2=\frac{m_1m_2+4m_2m_3+3m_1m_3}{m_1m_2+4m_2m_3-2m_1m_3}g$$
    The fraction is always greater than 1, so a2 accelerates faster than g.
    $$a_1=\frac{m_1m_2-6m_2m_3-2m_1m_3}{m_1m_2+4m_2m_3-2m_1m_3}g$$
    3) eq' (1):
    $$a_3=2a_2+2a_1~~\rightarrow~~a_3=\left( \frac{2m_1m_2-2m_2m_3+m_1m_3}{m_1m_2+4m_2m_3-2m_1m_3} \right)2g$$
    I made eq' (4) as m3 ascending, so if a3 is positive-it ascends. from (5) two conditions are necessary, either both, the nominator and denominator are positive or are they negative. the first scenario:
    $$\left\{ \begin{array}{l} (6)~2m_1m_2-2m_2m_3+m_1m_3>0 \\ (7)~m_1m_2+4m_2m_3-2m_1m_3>0 \end{array}\right.$$
    From (6) i get (i multiply by 2): ##4m_1m_2+2m_1m_3>4m_2m_3##
    From (7): ##m_1m_2+4m_2m_3>2m_1m_3##
    I combine those 2 to get: ##5m_2>0##, but this is always true.
     
  2. jcsd
  3. Sep 23, 2016 #2
    I don't match your kinematic equation (1).
     
  4. Sep 26, 2016 #3
    Snap1.jpg The initial length of the rope equals to it's length after the change:
    $$x_1+x_2+(x_1-x_2)+x_3=(x_2+\Delta_2)+(x_1+\Delta_1)+(x_1-x_2-\Delta_2+\Delta_1)+(x_3-\Delta_3)$$
    $$\rightarrow~2\Delta_1=\Delta_3\rightarrow~2a_1=a_3$$
     
  5. Sep 26, 2016 #4
    Call ##y_1## the distance between the center of the upper pulley and the center of the middle pulley, ##y_2## the distance between the center of the middle pulley and the center of the bottom pulley, and ##y_3## the distance between the center of the middle pulley and mass 3. Then the total length of rope (excluding the parts wrapped on the pulleys, which are constant) is $$L=2y_1+2y_2 +y_3$$ Since the length L is constant, $$2\frac{d^2y_1}{dt^2}+2\frac{d^2y_2}{dt^2}+\frac{d^2y_3}{dt^2}=0$$In terms of these parameters, $$a_1=\frac{d^2y_1}{dt^2}+\frac{d^2y_2}{dt^2}$$
    $$a_2=\frac{d^2y_1}{dt^2}$$
    $$a_3=-\left(\frac{d^2y_1}{dt^2}+\frac{d^2y_3}{dt^2}\right)$$
    From these equations, if follows that $$a_3=2a_1-a_2$$
     
    Last edited: Sep 28, 2016
  6. Sep 26, 2016 #5
    I worked the problem and I agree with Karol, concerning the constraint equation. I have a hard time believing the the delta l2 cancels out but I used graph paper and tried to draw the system and the system at a later time and I got Karol's constraint equation. Mass 2 still has greater than acceleration g.
     
  7. Sep 26, 2016 #6

    TSny

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    You've defined ##x_3## as the displacement of ##m_3## from one of the moving pulleys. So, the acceleration determined by ##x_3## will be the acceleration of ##m_3## relative to that pulley rather than relative to the earth frame of reference.
     
  8. Sep 27, 2016 #7
    Show us what you did. TSny already demonstrated an error in Karol's result, so this must be a common feature of yours as well.

    If there is something wrong with the simple and straightforward analysis I presented in post #4, please point it out.
     
  9. Sep 28, 2016 #8
    Why don't you take into account ##\ddot y_2## in:
    $$a_3=-\left(\frac{d^2y_1}{dt^2}+\frac{d^2y_3}{dt^2}\right)$$
     
  10. Sep 28, 2016 #9
    In post.#4, I meant to say that y3 is the distance between the middle pulley and m3. I've gone back and changed it.
     
    Last edited: Sep 28, 2016
  11. Sep 28, 2016 #10
    You didn't change anything, and it's like you've said in post #4:
    Snap1.jpg
     
  12. Sep 28, 2016 #11
    Yes, I did. I changed the word "lower" to "middle" in referring to y3. y3 is the distance between the center of the middle pulley and m3.
     
  13. Sep 28, 2016 #12
    Yes, that fixes it, thank you mpresic, TSny and Chestermiller.
    But i forgot i didn't finish. m1's acceleration is ##a_1=\frac{a_3-a_2}{2}##
    For m to accelerate upwards, from the relation: ##a_3=2a_1-a_2~~\rightarrow~~2a_1>a_2##
    Now i will translate it to masses
     
    Last edited: Sep 28, 2016
  14. Sep 28, 2016 #13
    $$\left\{\begin{array}{l} (1)~a_3=2a_1-a_2 \\ (2)~2T_3+m_2g-T_3=m_2a_2 \\ (3)~m_1g-2T_3=m_1a_1 \\ (4)~ T_3-m_3g=m_3a_3 \end{array}\right.$$
    $$\rightarrow~a_1=-\frac{m_3}{5m_2},~~a_3=\frac{m_3}{10m_2}-g$$
    For m to move upwards, according to the direction in the drawing, a3 must be positive:
    $$a_3>0~\rightarrow~~\frac{m_3}{10m_2}>g$$
     
  15. Sep 28, 2016 #14
    That's not what I get.
     
  16. Sep 28, 2016 #15

    TSny

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    I agree with the 4 equations in post #14, but not with the results for a1 and a3.
     
  17. Sep 28, 2016 #16
    I now agree with the four equations of Chestermiller, but I did have the wrong constraint equation earlier, I had a3 = 2 a1 (without a2). I looked at the length relative to the moving (pully), and not relative to the fixed (top) pulley. I now agree with the newest 4 equations
     
  18. Sep 29, 2016 #17
    Thank you TSny,:
    $$\left\{\begin{array}{l} (1)~a_3=2a_1-a_2 \\ (2)~2T_3+m_2g-T_3=m_2a_2 \\ (3)~m_1g-2T_3=m_1a_1 \\ (4)~ T_3-m_3g=m_3a_3 \end{array}\right.$$
    $$(2)\& (4):~(5)~a_2=\frac{m_3}{m_2}a_3+\frac{m_3}{m_2}g+g$$
    $$(3)\& (4):~(6)~a_3=\left( \frac{m_1-2m_3}{2m_3} \right)g-\frac{m_1}{2m_3}a_1$$
    $$(1)\& (5)\& (6):~(7)~\left( \frac{m_1m_2+2m_3^2}{2m_2m_3} \right)g+\frac{m_3}{m_2}a_3=\left( 2+\frac{m_1}{2m_3} \right)a_1$$
    $$(6)\& (7):~(8)~a_1=\frac{(m_2+m_3)g}{m_1m_2+m_1m_3+4m_2m_3}$$
    $$(6)\& (8):~a_3=\frac{1}{2m_3}\left( m_1-2m_3-\frac{m_1(m2+m_3)}{m_1m_2+m_1m_3+4m_2m_3} \right)g$$
    For m3 to move up it must be positive, in accordance with my drawing:
    $$a_3>0~~\rightarrow~~m_1>2m_3+\frac{m_1(m_2+m_3)}{m_1m_2+m_1m_3+4m_2m_3}$$
     
  19. Sep 30, 2016 #18
    Try again. The units don't match.
     
  20. Sep 30, 2016 #19
    $$\left\{\begin{array}{l} (1)~a_3=2a_1-a_2 \\ (2)~2T_3+m_2g-T_3=m_2a_2 \\ (3)~m_1g-2T_3=m_1a_1 \\ (4)~ T_3-m_3g=m_3a_3 \end{array}\right.$$
    $$(2)\& (4):~(5)~a_2=\frac{m_3}{m_2}a_3+\left( 1+\frac{m_3}{m_2} \right)g$$
    $$(3)\& (4):~(6)~a_3=\left( \frac{m_1}{2m_3}-1 \right)g-\frac{m_1}{2m_3}a_1$$
    $$(1)\& (5)\& (6):~(7)~a_1=\left( \frac{m_1m_2+2m_3^2}{m_2(m_1+4m_3)} \right)g+\left( \frac{2m_3}{m_1+4m_3} \right)a_3$$
    $$(6)\& (7):~(8)~a_3=\frac{m_1^2-2m_1m_3-10m_3^2-m_1m_2}{4m_3(m_1+2m_3)}$$
    For m3 to move up it must be positive, in accordance with my drawing:
    $$a_3>0~\rightarrow~m_1^2>2m_1m_3+m_1m_2+10m_3^2$$
     
  21. Sep 30, 2016 #20
    I get $$m_3<\frac{m_1m_2}{(m_1+4m_2)}$$
     
  22. Oct 1, 2016 #21
    $$\left\{\begin{array}{l} (1)~a_3=2a_1-a_2 \\ (2)~2T+m_2g-T=m_2a_2 \\ (3)~m_1g-2T=m_1a_1 \\ (4)~ T-m_3g=m_3a_3 \end{array}\right.$$
    $$(1)\& (4):~(5)~~T-m_3g=2m_3a_1-m_3a_2$$
    $$(2):~~T=m_2a_2-m_2g,~~(2)\& (5):~(6):~~2a_1=\left( \frac{m_2+m_3}{m_3} \right)a_2-\left( \frac{m_2+m_3}{m_3} \right)g$$
    $$(2)\& (4)~:(7)~~a_2=\frac{m_3}{m_2}a_3+\left( 1+\frac{m_3}{m_2} \right)g$$
    $$(3)\& (4):~(8)~~a_1=\left( 1-\frac{2m_3}{m_1} \right)g-\frac{2m_3}{m_1}a_3$$
    $$(6)\& (7)\& (8):~(9)~~a_3=\frac{m_1m_2-m_1m_3-4m_2m_3}{m_1m_2+m_1m_3+4m_2m_3}g$$
    $$a_3>0~\rightarrow~~m_3<\frac{m_1m_2}{m_1+4m_2}$$
    Thank you Chestermiller
     
  23. Oct 21, 2016 #22
    Hello ,

    Let y1 be the distance of m1 from fixed pulley , y2 be distance of m2 and y3 be distance of m3 .Let h be fixed distance between m2 and middle pulley .

    Since length of string is constant ,

    y1 + y1 - y2 - h + y3 - y2 - h + y2 = constant

    Differentiating twice I get ,

    a3 = a2 - 2a1 . This has sign difference from the result you have obtained . I surely must be making a mistake somewhere .

    Could you please tell my error .

    Thanks
     
  24. Oct 21, 2016 #23

    TSny

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    In the original figure, ##a_3## is defined as positive in the upward direction while ##a_1## and ##a_2## are defined as positive in the downward direction.
     
  25. Oct 21, 2016 #24
    Oh !

    In that case what I have done is correct assuming downward is positive for all the masses ?
     
  26. Oct 21, 2016 #25

    TSny

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    Yes. Looks good.
     
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