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Homework Help: Head on collision

  1. Oct 31, 2004 #1
    A 4.5 kg object with a speed of 4.0 m/s collides head-on with a 11 kg object moving toward it with a speed of 3.0 m/s. The 11 kg object stops dead after the collision.

    (a) What is the final speed of the 4.5 kg object?
    (b) Is the collision elastic?

    so i tried initial momentum = final momentum but I get the wrong answer.
    (4.5 kg)(4.0 m/s) + (11 kg)(3.0 m/s)=(4.5 kg)(Vf)
    and solving for Vf = 11.333 m/s, but that is the wrong answer.
  2. jcsd
  3. Oct 31, 2004 #2
    Take into account that they have different directions. One momentum will be positive while the other negative.
  4. Oct 31, 2004 #3
    Conservation of Momentum is the correct property to use, but you forgot to factor in that the two objects are moving at different directions before the collision, so initial momentum = 4.5kg * 4 m/s - 11kg * 3 m/s

    For the second part, you should just see if Conservation of Energy is also upheld in the collision.

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