1. Not finding help here? Sign up for a free 30min 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!

Three blocks and two pulleys

  1. Oct 10, 2009 #1
    1. The problem statement, all variables and given/known data
    three blocks are relased at t=0 from the position shown in the figure. Assume that there is no friction between the table and M2, and that the two pulleys are massless and frictionless. The masses are: M1 = 3.0 kg, M2 = 9.0 kg, M3 = 5.0 kg.

    M2 is the middle block connected to M1 (to the left) and M3 (to the right)

    Calculate the speed of M2 at a time 1.75 s after the system is released from rest.


    2. Relevant equations
    Since they will all have the same acceleration, i was trying to look for a and then multiply that by the t.



    3. The attempt at a solution

    I set up this equation:
    FM1 = Tension - M1*g = M1a
    FM3 = Tension - M3*g = (M1 + M2 + M3)a

    and i get a to be 1.4m/s^2 and Vf to be 2.45m/s but it is incorrect. Any help pls?
     
  2. jcsd
  3. Oct 10, 2009 #2

    rl.bhat

    User Avatar
    Homework Helper

    Tension in the two strings are not the same.
    Note down which mass goes up and which mass goes sown.
    Accordingly write down three equations for three masses.
    You have three unknowns, T1, T2 and a. Solve for them.
     
  4. Oct 10, 2009 #3
    wait, how are the two tensions different though?
    I know that FM1 = Tension - M1*g = M1a is correct, so is tension for Fm3 gonna be (m1+m2)a?
     
  5. Oct 10, 2009 #4

    rl.bhat

    User Avatar
    Homework Helper

    M3*g - T1 = M3*a ...(1)
    T1 - T2 = M2*a....(2)
    T1 - M1*g = M1*a ...(3)
    Solve for a.
     
  6. Nov 19, 2010 #5
    if i found a, how do i find the speed of m2?
     
  7. Nov 19, 2010 #6

    rl.bhat

    User Avatar
    Homework Helper

    Use v = vo + at. vo is zero.
     
  8. Nov 19, 2010 #7
    i tried it doesnt work, i have the same question but but different m
    m1=2 m2=8 m3=4
    i get zero for answer and its wrong... i did exactly the same way you did above.....
     
  9. Nov 19, 2010 #8

    rl.bhat

    User Avatar
    Homework Helper

    What is your value of a?
     
  10. Nov 19, 2010 #9
    i solved id and got 0 for a
    m3g-m1g-m1g=4-2-2=0
     
  11. Nov 19, 2010 #10

    rl.bhat

    User Avatar
    Homework Helper

    4g - T2 = 4a...(1)
    T2 - T1 = 8a...(2)
    T1 - 2g = 2a...(3)

    From these equations find a.
     
  12. Nov 20, 2010 #11
    i did it doesnt wark i just dont know what do i do wrong......
    i got a=1.962
    t= 1.45
    v=2.84 m/s
    why is it wrong??
     
  13. Nov 20, 2010 #12

    rl.bhat

    User Avatar
    Homework Helper

    If you add (1) and (3), you get

    T1 - T2 + 2g = 6a.

    2g = 6a + T2 - T1 = 6a + 8a = 14a.

    So a = 2g/14 = 2 * 9.8/14 = 1.4 m/s^2

    Now find the velocity of m2.
     
  14. Nov 20, 2010 #13
    yeahhhhhhhhhhh it fianely worked, thank you so mach!!
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Three blocks and two pulleys
Loading...