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Homework Help: Classic Hit and Stick momentum?

  1. Oct 10, 2013 #1
    1. The problem statement, all variables and given/known data

    Three balls of clay have masses m1 = 106 g, m2 = 148 g, m3 = 120 g, and speeds v1 = 1.48 m/s, v2 = 1.19 m/s, v3 = 1.85 m/s. They move in the directions: (1) at 45° from the horizontal, (2) horizontally left, and (3) vertically up. They collide simultaneously and stick together. Calculate the speed and the direction (angle) of the resulting blob of clay.

    2. Relevant equations


    3. The attempt at a solution

    Vx = 106 * 1.48 * cos(45) - 148 * 1.19 = -65.189882 m/s

    Vy = 106*1.48sin(45) + 120*1.85 = 332.0309 m/s

    V = sqrt(332.0309^2 + [-65.189882]^2) (answer?)

    angle = arctan (Vy/Vx)

    This isn't right is it,
  2. jcsd
  3. Oct 10, 2013 #2

    Andrew Mason

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    First of all we have to know the direction of the 45° ball. Is it 45° to the left or to the right?

    It is not clear what you are doing in calculating vx and vy.

    I suggest you write out the equations for momentum before and after the collision and then relate them. How are they related?

  4. Oct 11, 2013 #3
    No that isn't right, remember that the conservation of momentum applies, use it. Calculate the total momentum in the x direction before the collision, then it them equal to the total momentum in the x direction after the collision, then do the same for the y direction. Then you will have two equations with two unknowns, use algebra to solve for both. What you did was incorrect, you cannot find the total momentum in the x direction and set it equal to the velocity in the x direction. If in doubt, check the units.
  5. Oct 11, 2013 #4

    mv = m1 * v1 + m1*v2 + m2*v3? in the x and y directions then use trig to find speed and direction?
  6. Oct 11, 2013 #5


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    Yes. What you calculated in the OP (assuming m1 was moving up and to the right, which is not clear) were the x and y momenta, not the x and y speeds. You just needed to divide by the combined mass.
  7. Oct 17, 2013 #6
    Cool, I got it!
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