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Collision question involving velocity, Kinetic energy and conservation

  1. May 11, 2014 #1
    1. The problem statement, all variables and given/known data
    A 50 gram steel ball moving on a frictionless horizontal surface at 2.0m.s^-1 hits a stationary 20 gram ceramic ball. After the collision the ceramic ball moves off at a velocity of 2.5m.s^-1.
    (i) Calculate the velocity of the steel ball after the collision.
    (ii) Calculate the total kinetic energy of the balls before the collision and again after the collision.
    (iii) From your results in part (ii) has the kinetic energy been conserved? If not, where has this energy gone?

    2. Relevant equations

    initial momentum = final momentum

    3. The attempt at a solution
    For the first sub-question I got initial momentum=50*2=100 and final momentum i got 50v+2.5(20)
    equating these I got 100=50v+50 => v=1ms^-1

    Would the 20 be correct in that calculation of final momentum?

    initial KE = ½ * 50 * 2^2 = 100
    Final KE = ½ * 50 * v^2 + ½ * m * 2.5^2
    Last edited: May 11, 2014
  2. jcsd
  3. May 11, 2014 #2
    Where does "20" come from? It was not specified in the statement of the problem.
  4. May 11, 2014 #3
    Oh It's the mass of the stationary ceramic ball, sorry about that, forgot to write it in I guess. Would it be 50 or 20 in this case?
  5. May 11, 2014 #4
    Then your solution for the steel's ball velocity after the collision is correct.
  6. May 11, 2014 #5


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    Homework Helper

    Yes but it should be remembered that the units are not in standard units (joules) since you didn't make the conversion from grams to kilograms. The answers would have to be divided by 1000 to be in joules (it's a good habit to pay attention to units).
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