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Homework Help: Help with newton's second and third laws

  1. Nov 4, 2014 #1
    1. The problem statement, all variables and given/known data

    Two blocks are pushed to the right so that they move together with increasing speed. Block B remains at the height shown. Ignore friction between the ground and block A but not between block A and block B. The mass of block A is 10kg and the mass of block B is 2kg. Let system S represent the system consisting of both blocks A and B. (use g=10m/s^2)

    A-for block a, b, and system s: draw free-body diagrams, identify any Newton's third law force pairs, and write out the algebraic form of Newton's second law.

    B-using only the forces in your free-body diagram for system S, calculate the magnitude of the force exerted on system C by the ground (N_sg).

    C-using only the forces in your free-body diagrams for block A and block B, calculate the magnitude of the force exerted on block A by the ground (N_ag).

    D- How should the value of N_sg compare to N_ag?

    2. Relevant equations
    Force = mass X acceleration

    3. The attempt at a solution

    This is my attempt so far. I'm stuck on C because I feel like I would use the same equation F=m X a but mass would be 10kg instead of 12 because it's asking about the force exerted on A. But the blocks move together so would I account for the weight of B as well? I've attached an image so you can see my attempt at parts A and B as well.

    http://oi58.tinypic.com/2z4gw11.jpg 2z4gw11.jpg
  2. jcsd
  3. Nov 4, 2014 #2

    Doc Al

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    Staff: Mentor


    The weight of B only acts directly on B, not A. Of course, the weight of B affects the force that B exerts on A. Set up two equations, one for A and one for B.
  4. Nov 4, 2014 #3
    Sorry if this is a stupid question, but do you mean write different force equations for A and B?
    So for A it would be F=10kg X 10m/s2=100N
    and for B it would be F=2kg X 10m/s2 = 20N
  5. Nov 4, 2014 #4

    Doc Al

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    Staff: Mentor

    I mean apply Newton's 2nd law to each block separately. (##\Sigma F = ma##.) You already have the forces identified, and even the equations written. (To answer part C, all you need are the vertical forces.)
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