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Homework Help: Help with elastic collisions/kinetic energy problems

  1. Mar 1, 2013 #1
    (1)Block A of mass mA = 1kg is moving at velocity +v (from left to right)
    towards block B of mass mB = 2kg which is at rest. To the right of block B there is block
    C. It is at rest and its mass is mC = 1kg. Find the ultimate velocities of all three masses
    assuming all collisions are elastic (i.e. total kinetic energy does not change).

    (2) A 0.05 kg marble is fired vertically upward using a spring gun. The spring
    must be compressed 8 cm if the marble is to just reach a target 20 m above the marble’s
    position on the compressed spring. (a) What is the change ∆Ugrav in the gravitational
    potential energy of the marble-Earth system during 20 m ascent? (b) What is the change
    ∆Uelas in the elastic potential energy of the spring during its lunch of the marble? (c)
    What the elastic (spring) constant of the spring?

    (3) A person of mass M = 32.5 kg stands on ice and disdainfully throws my
    favorite Leo Tolstoy’s book ”Anna Karenina” weighing m = 2.25kg with the initial velocity
    v0 = 12m/s. The book is thrown from the ice level, so the initial height is zero. When the
    book lands on the ice the total distance between the book and the offender is l = 15.2 m.
    At what angle was this excellent book thrown? How fast is the offender moving?
  2. jcsd
  3. Mar 1, 2013 #2
    Show us your attempt for each problem and where you're having trouble.
  4. Mar 1, 2013 #3


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    Hello PhilliesFan. Welcome to PF !

    You need to make an honest attempt at a solution before we can help you -- it's a rule of this Forum.

    It's also probably best to post each problem in its own thread.

    The folks who set up this Forum have included a template to be used for each new thread. Please use it:
    1. The problem statement, all variables and given/known data
    Your statement of the problem goes here.

    2. Relevant equations
    Relevant equations, principles, etc. go here.

    3. The attempt at a solution
    Finally, show your attempt and/or what you know/don't know.

    This is a very good site for getting help.
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