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Conservation of Momentum Conceptual Questions

  1. Dec 5, 2011 #1
    1. The problem statement, all variables and given/known data
    1. A moving object collides with a stationary object.
    (a) Is it possible for both objects to be at rest after the collision? If "yes," give an example. If "no," explain why not.
    (b) Is it possible for only one object to be at rest after the collision? If "yes," give an example. If "no," explain why not.


    2. Relevant equations

    p tot = ptot'
    p = mv

    3. The attempt at a solution

    (a) ptot = m1v1 + m2v2
    m2v2 is 0,
    ptot' = m1v1' + m2v2'
    ptot = ptot'
    m1v1 = m1v1' + m2v2'
    Now I'm stuck, I think the answer is no, i think both of them will move, but how do I show this?

    (b) same problem, I'm stuck!

    1. The problem statement, all variables and given/known data

    2. A wet snowball of mass m, travelling at speed v, strikes a tree. It sticks to the tree and stops. Does this example violate the law of conservation of momentum? Explain.


    2. Relevant equations
    p= mv



    3. The attempt at a solution
    this is what I thought m1 is snowball m2 is tree
    m1v1 + m2v2 = (m1+m2)'v12'
    LS = m1v1
    RS = (m1+m2)' v12'
    LS does not equal right side, therefore LOC of M does not hold, but I feel that I have done a fallacious step somewhere.

    1. The problem statement, all variables and given/known data

    5. An object of mass m has an elastic collision with another object initially at rest, and continues to move in the original direction but with one-third its original speed. What is the mass of the other object in terms of m?

    2. Relevant equations

    p = mv
    Ek = 1/2mv^2

    3. The attempt at a solution
    Given
    m1 =?
    m2= ?
    v1 = ?
    v2 = 0
    v1' = 1/3(v1)
    v2' = ?

    m1v1 + m2v2 = m1v1' + m2v2'
    m1v1 = m1v1" + m2v2'
    m1v1 = m1(1/3v1) + m2v2'
    Now I am stuck, How do I solve for m2? Do I use Ek = 1/2mv^2 eqns and stuff.
     
  2. jcsd
  3. Dec 5, 2011 #2

    Delphi51

    User Avatar
    Homework Helper

    Welcome to PF!
    Maybe it is just too late at night, but all these symbols don't seem to be very clarifying.
    Why not just say initial p = m1*v1 ≠ 0. So, if momentum is conserved in the collision, then momentum is not zero afterwards either.

    (b) Time to go play pool or curling. If you must play with equations, don't bother writing terms that are known to be zero.
     
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