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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.

    TRUE
    FALSE

    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
    Fnet=ma
    (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:
    Faonb>Fconb
    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
    F-40sin(30)=4(3)
    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.

    Thanks!
     
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