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Homework Help: Newton's Law question, Not sure if right

  1. Oct 7, 2011 #1
    1. The problem statement, all variables and given/known data

    [PLAIN]http://img208.imageshack.us/img208/9987/forcediagram.png [Broken]

    In the figure above, two small objects (magnets for instance) of the same mass exert forces on each other of a magnitude F. When object A is subjected to an external force of 3.6x10^-2 N, A accelerates at 1.6 m/s/s and B accelerates at 2.1 m/s/s as shown. Find F. Neglect other forces, assume accelerations are parallel.

    2. Relevant equations
    ƩF = ƩmƩa

    3. The attempt at a solution

    I'm not sure if the solution i got is entirely correct but this is it

    ƩF(A) = 3.6x10^-12
    ƩF(B) = (3.6x10^-2) + F

    I got this because B is accelerating faster which means the force on B must be greater if the masses are equal.

    Using this, I found the mass of A, by F = ma

    3.6x10^-2 = m(1.6)
    m = .0225 kg, so A and B are .0225 kg

    To find F, I substitute the mass into F = ma for F(b)


    (3.6x10^-2)+ F = .0225*2.1
    F = 1.13x10^-2 N

    Is that correct?
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Oct 7, 2011 #2


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    Homework Helper

    You have not allowed for F acting on each body. being as B accelerates faster than A - moves away from it - the force F must be repulsive.

    For A, 3.6 x 10-2 N acts to the right, anf F acts to the left.
    For B, just F acts to the right.

    From that you can get what you are after.
    Last edited by a moderator: May 5, 2017
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