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Homework Help: Linear Collisions; Conservation Law

  1. Sep 22, 2012 #1
    Hello Physics Forums!

    1. The problem statement, all variables and given/known data
    The student sets up a collision experiment with two masses on a horizontal air track, with two springs on each end. The mass of A is 100 g and B is 200 g. Each spring has a constant of 20 N/m. Spring 1 (left side) was initially compressed 5 cm
    to start mass A moving.

    a. Determine the speed of mass A as it leaves spring 1 [0.71 m/s]
    b. If A has a velocity of 20 cm/s to the right after it collides with B, determine the velocity of mass B. [0.25 m/s right]
    c. Determine the maximum compression of spring 2 when mass B collides with it. [2.5cm]

    2. Relevant equations
    Ep = 1/2kx2
    p = mv

    3. The attempt at a solution
    I solved part a readily using the conservation of energy law (Ep = Ek). When I tried to solve part b using conservation of momentum, however, I obtained an incorrect answer:

    Total momentum before = Total momentum after
    [(0.1kg)(0.71m/s)+(0.2kg)(0)-(0.1kg)(0.02m/s)]/(0.2kg)= v4
    v4 = 0.35 m/s right

    Am I reading the question wrong? Are my formulae incorrect?​
    Last edited: Sep 22, 2012
  2. jcsd
  3. Sep 22, 2012 #2
    20 cm/s would be 0.20 m/s, not 0.02 m/s
  4. Sep 22, 2012 #3
    Hahahaha thanks for your help; I'll have to be more diligent with unit conversions next time!
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