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!

Elevator Problem(dynamics)

  1. Oct 13, 2009 #1
    My answers for this problem aren't correct. Can anyone please help me spot my mistake. Thanks.


    Inside the elevator, masses m1 = 2.00kg and m2 = 4.00 kg are conncted by a cable A. Mass m1 is connected to the roof of the elevator by a cable B. The elevator has a downward acceleration of g/2. Calculate the tensions in cables A and B.

    m1 = 2.00 kg
    m2 = 4.00 kg
    a = 9.8/2

    I set up two free body diagrams.. One for m1 and one for m2

    So for m1 FBD

    SUMFy= 0 = Fb - 2(9.8/2) - Fa
    Fb = 9.8 N

    for m2 FBD

    SUMFy = 0 = Fa - mg
    Fa = 4(9.8/2) = 19.6 N NOW PLUGGING BACK IN
     
  2. jcsd
  3. Oct 13, 2009 #2

    kuruman

    User Avatar
    Homework Helper
    Gold Member

    There are no answers to check. When you post them, we will check them. Nevertheless, I can see mistakes already. You say

    SUMFy = 0 = Fa - mg

    Why is the sum of all the forces in the y-direction zero? Are the masses not accelerating?

    Also, when you say "mg" which mass are you talking about, 1 or 2?
     
  4. Oct 13, 2009 #3
    m1 = 2.00kg m2 = 4.00 kg

    ok let me try this over..

    FBD of m1....so here the mg will be 2kg (9.8) or would it be 2(9.8/2)?

    so SUMFy = Fb - Fa - mg = ma
    = Fb - Fa -(2)(9.8)=2(-9.8/2)
    Fb = -9.8 N which couldnt be possible since its in tension

    FBD of m2...so here mg will be 4kg (-9.8/2)???

    so SUMFy= Fa - 4(9.8)= 4(-9.8/2)
    Fa = 19.6 N

    again I am getting the same answers as before..
     
  5. Oct 13, 2009 #4

    kuruman

    User Avatar
    Homework Helper
    Gold Member

    How did you get -9.8 N? I don't get that when I put in the numbers. Check your algebra.

    This looks correct. Do you think it is not?
     
  6. Oct 13, 2009 #5

    The instructor said 19.6 N was not the correct answer. I am not really sure of the actual correct answer.

    I checked my algebra again for Fb and ended up getting 49N. I really am not sure what to think now.
     
  7. Oct 13, 2009 #6

    kuruman

    User Avatar
    Homework Helper
    Gold Member

    The Fa = 19.6 N is correct. I agree with your instructor. Now show me exactly what you do after you write down

    Fb - Fa -(2)(9.8)=2(-9.8/2)
     
  8. Oct 13, 2009 #7
    Fb - 19.6 -(2)(9.8)=2(-9.8/2)

    Fb = 19.6 + 98/5 - 49/5
    = 29.4 N
     
  9. Oct 13, 2009 #8

    kuruman

    User Avatar
    Homework Helper
    Gold Member

    Looks good. I think you are done.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Elevator Problem(dynamics)
  1. Elevator problem (Replies: 3)

  2. Elevator problem (Replies: 7)

  3. Elevator Problem (Replies: 5)

Loading...