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Homework Help: Momentum Conceptual Questions

  1. Jan 29, 2009 #1
    1. The problem statement, all variables and given/known data

    QUESTION 1 :A stationary bomb explodes in space breaking into a number of small fragments. At the location of the explosion, the net force do to gravity is 0 newtons. Which one of the following statements concerning the event is true?
    a) Kinetic energy is conserved in the process
    b) The fragments must have equal kinetic energies
    c) The sum of the KE's of the fragments must be 0
    d) The vector sum of the linear momenta of the fragments must be zero
    e) The velocity of any one fragment must be equal to the velocity of any other fragment

    QUESTION 2 : An object of mass 3m, initially at rest, explodes breaking into two fragments of mass m and 2m respectively. Which of the statements is true (after the explosion)?
    a) They may fly off at right angles
    b) They may fly off in the same direction
    c) The smaller fragment will have twice the speed of the larger fragment
    d) The larger fragment will have twice the speed of the smaller fragment
    e) the smaller fragment will have four times the speed of the larger fragment

    QUESTION 3: Car one is traveling due north and Car Two is traveling due east. After the collision shown, Car 1 rebounds due south. Which of the numbered arrows is the only one that can represent the final direction of Car 2.
    Arrow 1 - 180 degrees
    Arrow 2 - 150 degrees
    Arrow 3 - 90 degrees
    Arrow 4 - 30 degrees
    Arrow 5 - 0 degrees

    2. Relevant equations

    p = mv
    Impulse = force * time = change in momentum

    3. The attempt at a solution

    QUESTION 1 : Its obviously not an elastic collision because kinetic energy is lost in the explosion--which rules out a. Kinetic energy is always positive, so the sum can't be zero--not c. I don't understand b or e. I think its d, because momentum has to be conserved.
    QUESTION 2 : I think its c, because momentum must be conserved and that is the only way that both fragments have a momentum of 2ms after the collision
    QUESTION 3: I have no idea how to solve this problem. How can you find the direction of final velocity without pluggin in numbers?
  2. jcsd
  3. Jan 29, 2009 #2


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    Homework Helper

    The first 2 look OK.

    The third one, wants you to conserve momentum in both x and y.

    Before the collision you had +Y and -X momentum, after you have some -Y and ...
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