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!

Pulley question -- 2 pulleys and 2 weights

  1. Jul 26, 2015 #1
    THIS THREAD WAS INCORRECTLY PLACED IN THE CLASSICAL PHYSICS FORUM, SO THERE IS NO TEMPLATE

    Hi, everyone.
    my teacher stuck me with these questions and i am need of a little help answering question 2.

    here's my answers to question 1 first.
    1a) a=0, as system would be in equilibrium when F=0
    b) F=5mgN, as for a=5m/s the acceleration of B has to equal 2.5m/s and as its in equilibrium F has just got to equal m*a which is 2.5*2mg=5mgN
    c) F=mga

    i think my answers are right but if im making a simple mistake let me know.

    now my attempts at 2.

    2a) first, B has to x/2 above the ground for the full system to work so original potential energy is 2mgx/2=mgx assuming A starts on the ground. now its the following kenetic energy bits that are confusing me because there will be two different V's one for A and one for B. VA = 2VB so assuming no energy gets lost to heat via friction whatever all of the gravitational energy plus energy input from F has to turn into kinetic energy so im thinking.

    mgx + Fx = 1/2mVA 2 + mVB 2

    which kind of works if your given enough information. but part B doesn't i don't think.

    2b) substituting values into my answer to 2a

    98 + 196 = 1/2[ VA 2 + 2VB 2 ]
    so VA 2 + 2VB 2 = 588N
    and VA = 2VB
    so with further manipulation i get VA = 19.8m/s

    but using logic from question 1. if F=19.6 and B has a mass of 2kg, the acceleration of B = 9.8m/s 2 therefore acceleration of A = 19.6m/s 2 and then using SUVAT.

    s=10m u=0 v=? a=19.6 m/s 2
    v 2 = u 2 +2as
    v = 23.2m/s

    which is different. iv tried multiple times and i can't figure out what im doing wrong. im pretty sure its the WEP part not the SUVAT part as im better at that but i do not know.

    please help.
     

    Attached Files:

    Last edited by a moderator: Jul 26, 2015
  2. jcsd
  3. Jul 26, 2015 #2

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    It would be nice if you had posted the text of the questions as well.

    As your post reads now, we have no idea if you are correct or not.
     
  4. Jul 26, 2015 #3
    iv updated it now so the picture of the question is on. sorry about that. it quite late haha
     
  5. Jul 28, 2015 #4

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Your answer to 1b) is wrong.
    Write out the full force/acceleration equations for the two masses, including T as a common unknown.
    Taking shortcuts often leads to errors.
     
  6. Jul 28, 2015 #5
    okay, here i go.

    For mass A: T-mg = 5m =X2=> 2T-2mg = 10m
    For mass B: F+2mg-2T= -2.5(2m) = -5m

    adding the two equations:

    F=5m

    is that correct?
     
  7. Jul 28, 2015 #6

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Check your signs in the mass B equation. Think which way the acceleration is.
     
  8. Jul 28, 2015 #7
    i have taken up to be my positive direction so a is positive for A and negative for B.

    is that not right?
     
  9. Jul 28, 2015 #8

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    The left hand side of your mass B equation takes down to be positive.
     
  10. Jul 28, 2015 #9
    okay then. would that them make F=15m ?
     
  11. Jul 28, 2015 #10

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Yes. But you should include units since the 15 is not dimensionless.
     
  12. Jul 28, 2015 #11
    Does that then make the answer to 1c) F=3ma N ?
     
  13. Jul 28, 2015 #12

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Yes, but this time you should leave out the units. That's because m is a mass and a an aceleration in unspecified units, so it's just F=3ma.
     
  14. Jul 28, 2015 #13
    so was my WEP question right?
     
  15. Jul 29, 2015 #14

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Do you mean it has to start x/2 above the ground? It certainly needs to be at least that.
    You can't say what the original PE is unless you define a zero level. The loss in PE by B will be mgx.
    How far does F move?
     
  16. Jul 31, 2015 #15
    assume the zero lvl is at the base of mass A and B is exactly X/2 above the ground.
    the PE of the entire system is then: 2mgx/2 = mgx
    the energy by F is: Fx/2
    KE of A afterwards is 1/2*m*va2
    KE of B afterwards is 1/2*2m*(vb/2)2 = m*(vb/2)2

    as vb = 1/2va

    KE of B can also be written as: m*(va/4)2 = mva2/16

    so the equation should be:
    mgx + Fx/2 = 1/2mva2 + mva2/16

    which can be simplified to:
    mgx + Fx/2 = 9/16mv2

    is that right or am i missing somthing?
     
  17. Jul 31, 2015 #16

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Why vb/2?
     
  18. Aug 2, 2015 #17
    i was thinking as it is half Va but then i did it again. does that then make the end, mgx + Fx/2 = 3/4mv2 is that right?
     
  19. Aug 2, 2015 #18

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Yes.
     
  20. Aug 3, 2015 #19
    for part 2b im still getting different answers depending on which method i use.

    if F=3ma, 19.6=3(1)a, a=6.53.
    using v2 = u2+2as
    v2 = 2(6.53)(10) = 130.67
    so, v= 11.43 m/s

    but if we use mgx + Fx/2 = 3/4mv2
    1(9.8)(10) + 19.6(10/2) = 3/4(1)v2
    196 = 3/4v2
    v2=261.33
    v = 16.17 m/s

    whats wrong?
     
  21. Aug 3, 2015 #20

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    I missed another error in the OP.
    Think again about the total change in PE.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Pulley question -- 2 pulleys and 2 weights
Loading...