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Homework Help: Newton's Third law of motion problem.

  1. Apr 25, 2012 #1
    1. The problem statement, all variables and given/known data
    An object is free to move on a table, ex-
    cept that there is a constant friction force f
    that opposes the motion of the object when
    it moves. If a force of 10 N pulls the object
    across the table, the acceleration is 2 m/s2. If
    a force of 20 N pulls the object across the ta-
    ble, the acceleration is 6 m/s2. What is the
    force of friction f?

    a) 3.33 N
    b) 1 N
    c) 5 N
    d) 10 N
    e) none of these

    2. Relevant equations

    F= ma
    F= -F'

    F= μS where μ= coefficient of static friction

    3. The attempt at a solution

    i first find the mass in both case

    F= 10 N and a= 2m/s/s
    m= F/a = 10/2
    m= 5kg

    and again

    F= 20N and a= 6m/s/s

    m= F/a = 20/6
    m= 3.33kg

    what i do next..
    either i must go to

    F= μ S

    please help me to solve it.. how i go further?
  2. jcsd
  3. Apr 25, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    10 N is the applied force, not the net force. To use Newton's 2nd law you need the net force.
  4. Apr 25, 2012 #3
    then Doc Al

    how will i find the net force..
    will it be

    Fnet = 20+10 = 30N ...
    what i do next?
  5. Apr 25, 2012 #4

    Doc Al

    User Avatar

    Staff: Mentor

    No. In the first case, what two forces act? One is the applied force of 10 N. What's the other force?

    You'll end up with an equation with 2 unknowns.

    Then you'll get a second equation, using the second case (with the applied force of 20 N).

    You'll solve those two equations for the two unknowns.
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