1. Limited time only! Sign up for a free 30min personal 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!

Homework Help: Tension Problem: Two boxes with two ropes

  1. Sep 22, 2010 #1
    here's a problem I did earlier today. I think I got it right, but I'd appreciate it if somebody could check. Also, is there a different way to solve this problem?

    1. The problem statement, all variables and given/known data
    Two masses Ma and Mb are connected through a heavy rope with mass Mr. The top mass Ma is hanging from a from a massless rope that is that is attached to a helicopter that moves upwards with acceleration a. Find the tension in the top rope as well as the tensions in the bottom (heavy) rope.

    I labeled the tension of the top rope T1, the tension of the heavy rope pulling downward on Ma T2.1, and the tension pulling upward on Mb T2.2.

    2. Relevant equations
    For mass A: Fnet = Ma * a
    --> T1 - T2.1 - Ma * g = Ma * a
    --> T1 - T2.1 = Ma * a + Ma * g

    For mass B: Fnet = Mb * a
    --> T2.2 - Mb * g = Mb * a
    --> T2.2 = Mb * a + Mb * g
    This already gives us the tension from the heavy rope pulling upwards on the second mass.

    For the entire system, without the top rope:
    Fnet = (Ma + Mb + Mr) * a
    --> T1 - (Ma + Mb + Mr) * g = (Ma + Mb + Mr) * a
    --> T1 = (Ma + Mb + Mr) * (a + g)

    I plugged this in to the earlier equation with T1 and T2.1:

    (Ma + Mb + Mr) * (a + g) - T2.1 = Ma * (a + g)
    --> T2.1 = (Mb + Mr) * (a + g)

    So we have:
    T1 = (Ma + Mb + mr) * (a + g)
    T2.1 = (Mb + Mr) * (a + g)
    T2.2 = Mb * (a + g)

    3. The attempt at a solution

    See above.

    Is this correct?
  2. jcsd
  3. Sep 22, 2010 #2
    That looks good.
    You can check your answer in limiting case. When mass of the rope goes to zero, your answer gives the same result as massless rope.
  4. Sep 22, 2010 #3
    Cool, thanks.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook