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: Find acceleration of this system with friction

  1. Oct 31, 2008 #1
    1. The problem statement, all variables and given/known data
    Body A weighs 102 N, Body B weighs 32 N. The coefficeints of friction between A and the incline are .56 for static, .25 for kinetic. Angle is 40 degrees. Find the acceleration of the system when block A is at rest... when it is moving up the incline, and when it is moving down the incline.

    (Block A is on the ramp attached to a string that goes up to the top of the ramp, down a pulley that attaches block B)


    2. Relevant equations
    Fnet=MA
    Ff=Fnormal(coefficient of friction)
    Fnet=T-Ff
    Fnormal=Mgy


    3. The attempt at a solution
    I have done problems just the same as this very recently, but for some reason, I cannot think of how to get the tension in the string/rope. I dont know why, but it just seems to be escaping me, and after getting T, the rest of the problem is already solved. I have the friction, equal to mgy multiped by the coefficient. So the acceleration would be the T-friction, all divided by the mass of the system.
     
  2. jcsd
  3. Oct 31, 2008 #2
    Re: Tension/pulley

    You have three cases - block A at rest, moving up, and moving down.

    Each will have a different free body diagram. You'll need to treat each case separately, rather than trying to lump them together with the same equations.
     
  4. Oct 31, 2008 #3
    Re: Tension/pulley

    hey heth,
    Yeah i just stared at this for some reason and didn't even think about it.
    I took a little break and revisited it and got something that looked like this...
    (just for A not in motion)

    Fnet=mgax+ff-T
    Fnet=T-mgb

    so mgax+ff-T=T-mgb

    solved for T

    T=(mgax+mgb+Ff)/2

    then take this tension find the unbalanced force, and divide it by the systems mass.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook