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Pulley Problem

  1. Mar 26, 2014 #1
    1. The problem statement, all variables and given/known data
    Two masses are attached by a light string and looped over two frictionless pulleys. Object 1 (200g) accelerates upwards at a rate of 4.90 m/s squared. Determine mass of object 2.


    2. Relevant equations

    3. The attempt at a solution
    I tries doing this but don't think its right.

    Fnet= F1 + F2
    ma= m1 + m2
    -m2g = m1g – ma
    m2g= m(a-g)
    m2=(.2kg)(4.90m/s 2)
    m2=.98 kg or 980 grams
  2. jcsd
  3. Mar 26, 2014 #2
    Yes, that solution is not correct. The problem has only two masses but your equation "-m2g = m1g – ma" has three different masses in it. you must fix that. Besides, there is also a sign mistake on your net force calculation. Make sure you draw a diagram before attempting to solve any problem like that. Include all the relevant forces in your diagram.

    Also (a-g)/g is not equal to a. That's just basic algebra
  4. Mar 26, 2014 #3
    You did not do this correctly.

    Start by setting up two different equations by ∑F=ma for each mass.
  5. Mar 26, 2014 #4
    Was anything right?
  6. Mar 26, 2014 #5
    The units look right.
  7. Mar 26, 2014 #6
    Lol ok thanks. I shall endeavour to solve this enigmatic question.
  8. Mar 27, 2014 #7
    Ok so would finding out the net force of object 1 help?
  9. Mar 27, 2014 #8
    It's a start.
  10. Mar 27, 2014 #9
    Ok so I have set up two equations for each mass.



    Fnet1=(.2kg)(4.90m/s squared)

    Fnet1=.98 N

    However I am stumped what to do with Fnet2. I go Fnet2=m(-4.90 m/s squared) then ma=m(-4.90m/s squared) but don't know what to do next.
  11. Mar 27, 2014 #10
    Did you do a free body diagram of each mass? Include all the forces acting on each mass. These will add up to Fnet for each case.
  12. Mar 27, 2014 #11
    Yes but I can't seem to get much from that....I have free body diagram of object 1 and 2 which is a block with Fg acting down and Ft acting up.
  13. Mar 27, 2014 #12
    I can't get it. I'v tried all sorts of ways but its that darn net force that I can't get! If I could just get that I'd have it.
  14. Mar 27, 2014 #13


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    Are the gravity forces acting on object 1 and 2 the same? If not, then don't call them both Fg. What about Ft? Is that the same for both objects?
  15. Mar 27, 2014 #14
    Great, now sum them up to get Fnet.
  16. Mar 27, 2014 #15
    mass of object 2 = 600 grams
  17. Mar 27, 2014 #16
    Yeah, that's good to know. But sum up all the forces you outlined above in an equation and set it equal to Fnet.
  18. Mar 27, 2014 #17
    I don't need to. All I need is mass of object 2.
  19. Mar 27, 2014 #18
    that's not correct. You have to sum up all the forces and set it to ma.
  20. Mar 27, 2014 #19
  21. Mar 27, 2014 #20
    There is a minus sign missing in lines 1 through 3. Everything else seems correct. You should wait until the very last step before plugging in any values such as the m1 and g. Algebra is easier than arithmetic.
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