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Homework Help: Having trouble finding the tension of this newton law problem

  1. Sep 29, 2011 #1
    Block A in the figure below has mass mA = 4.5 kg and is sliding down the ramp. Block B has mass mB = 2.2 kg. The coefficient of kinetic friction between block B and the horizontal plane is 0.50. The inclined plane is frictionless and at angle 30°.

    part a is to find the tension

    the equation I have is Ma(g)sin theta-f(a)=ma a
    i also found fa of to be 19.096, which I used [itex]\mu[/itex]k mag cos theta

    I found the b part which was to find the acceleration: 1.68 m/s^2

    using my first equation for tension i keep getting t=13.99
    webassign does not like that answer and I can't figure out what I am doing wrong

    thanks for any help you guys can give me
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 29, 2011 #2


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    Staff: Mentor

    From the problem description only one of the blocks is on the slope, the other is on a horizontal surface. So I'm not seeing the reason for you're use of the cosine function when you calculate the frictional force.

    You should be able to arrive at two expressions relating the tension and acceleration by analyzing the Free Body Diagrams of the two blocks.
  4. Sep 29, 2011 #3
    if use sin to find Fa it gives me 11.025 which makes the tension equal 3.56 which is also wrong.
    maybe I am not writing the equation right.

    I am getting Tension to be T=ma g sin theta- fa-ma a

    which I am getting 4.5(9.8)sin 30-19.096-(4.5)(9.8) which = 13.99

    am I setting the equation wrong??
  5. Sep 29, 2011 #4


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    Staff: Mentor

    You should be getting two equations for the tension, one for each block, from their FBD's. These equations will each have two unknowns: the tension and the acceleration. From them a single equation for the tension can be assembled (by substituting the acceleration from one into the other).

    Can you write the two equations from the FBD's?
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