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Three Blocks on Frictionless Incline(Please check work)

  1. Oct 7, 2010 #1
    1. The problem statement, all variables and given/known data

    Three blocks are pushed up a frictionless plane that is inclined at an angle of 30 degrees with respect to the horizontal as shown below. The blocks move together with constant acceleration a = 3.0 m/s2. The masses of the blocks are: MA = MC = 1 kg, and MB = 2 kg. The next three questions pertain to this situation.

    [PLAIN]https://online-s.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?courses/phys100/fall10/hwb/06/01/fig4.gif [Broken]

    The magnitude of the force that block A exerts on Block B is greater than the magnitude of the force that block B exerts on Block A.


    Compare FAonB, the force Block A exerts on Block B, to FConB, the force Block C exerts on Block B,

    FAonB < FConB
    FAonB = FConB
    FAonB > FConB

    What is the value of F, the external force that is exerted on Block A?

    F = 3.0 N
    F = 4.9 N
    F = 12.0 N
    F = 31.6 N
    F = 39.2 N

    2. Relevant equations
    (Newton's laws)

    3. The attempt at a solution
    I have found answers for the questions, I'd just like my work to be checked

    Part 1:
    False, because Newton's second says for each force there is an equal and opposite reaction, therefore meaning the forces must be equal
    Part 2:
    Looking at the diagram, Faonb is greater because it has the forces of two blocks acting on it as opposed to just one in Fconb, follow Newton's second again.
    Part 3:
    I used F=ma
    to get 31.6 N

    If anyone could check my work it would be much appreciated!
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 7, 2010 #2
    It's almost correct, you miscalculated part 3. F = ma + mgsin30 = 4*3 + 4*10*0.5 = 12+20 = 32N. I don't know how you got 31.6....

    EDIT: sorry, I didn't realize part 3 also had options:) Yeah 31.6 is correct, but in your equation it shouldn't be 40sin(30), but 4*9.81*sin(30), you get 31.6 that way.
  4. Oct 7, 2010 #3
    Yeah, I had calculated it with 10 m/s^2 and then changed it to 9.8 but forgot to change it on my note sheet that I'm working the problems out on.

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