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Force and motion

  1. May 8, 2008 #1
    1. The problem statement, all variables and given/known data
    A 2.0-kg mass and a 3.0-kg mass are on a horizontal frictionless surface connected by a massless spring with spring constant k = 140 N/m. The large mass is on the right of the spring: the small mass on the left. A 15-N force is applied to the large mass. How much does the spring stretch? The book answer is 4.3 cm.

    2. Relevant equations
    F = -kx
    F = ma
    W = mg

    3. The attempt at a solution

    Total force on the system is 15 N
    2 kg + kx + 3 kg = 15 N
    2 (9.8) + 140 x + 3 (9.8) = 15 N
    Somehow I am getting the signs wrong because I cannot come up with 4.3 cm.
  2. jcsd
  3. May 8, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Not sure what you're doing here:
    (1) The weights of the masses act vertically, not horizontally; they aren't relevant to this problem.
    (2) The spring force is internal to the system, so it's net effect on the system as a whole cancels out.

    Hint: Answer these questions:
    What's the acceleration of the system?
    What forces act on the larger mass?
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