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!

Incline with Pulley, find the mass of one block.

  1. Jun 19, 2013 #1
    1. The problem statement, all variables and given/known data

    A 20.0 kg block rests on a frictionless inclined plane of slope angle 30.0 degrees. A light cord attached to the block passes over a frictionlesss pulley at the top of the plane and is attached to a second block. What must be mass of the second block if the system is to be accelerating up and to the right at 2.00 m/s2?

    https://dist-ed.waketech.edu/course...63858134f05bd87414264516909/InclineAtwood.png

    Is a link to the image.

    I found in class that the answer is 17.7 kg.. however I am stumped on where the numbers should go.



    2. Relevant equations



    3. The attempt at a solution
    http://hyperphysics.phy-astr.gsu.edu/hbase/incpl.html#c1 is the link where I use my formulas

    Well I do know that if I isolate the mass on the incline

    ƩFnetx = -m1g * sin(30°) + Fτ = 2*20

    I got 2*20 because of ma, and I know for m1g I substitute (20*9.8) I also know that acceleration is going up the incline, and if I were to remove the pulley and put them on the x-axis, it would be going to the right which is positive, therefore, the acceleration is positive.

    Solving for that I get, Fτ = 138N which I know to be true

    Now.. isolating m2 I get
    ƩFnetx = -Fτ + m2g = 2m

    however I have two masses??

    In a previous problem when I had to find the mass of m2 if I wanted the objects to be at rest or constant velocity (so I assumed F=ma, but the sum would be 0). I ended up finding the weight of M2 being 98N.. would I plug that in for m2g? I guess not considering it doesn't give me the answer.. but I just wanted some feedback on this.
     
  2. jcsd
  3. Jun 19, 2013 #2

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Your last equation should read ƩFnetx = -Fτ + (m_2)g = 2(m_2). Now do the algebra correctly to solve for m_2!
     
  4. Jun 20, 2013 #3
    So I write the Equation..

    m_2 = -138 + m_2g
    --------------------
    2

    However, I still scratch my head at this, and I'm not sure if substituting would work either. Sorry if my algebra skills seem to be lacking... it is 1:37 a.m. where I am at currently.
     
  5. Jun 20, 2013 #4
    Bring all of the mass dependant quantities to one side and factor it out. Divide the -Ft by accel - grav. You should end up with m_2 = (-138.1)/(2-9.81)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted