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AP Mech problem about energy and momentum (Urgent ! )

  1. Feb 11, 2009 #1
    1. The problem statement, all variables and given/known data

    A 2kg block and an 8kg block are both attached to an ideal spring for which k=200 N/m and both are initially at rest on a horizontal frictionless surface. In an initial experiment a 0.1kg ball of clay is thrown at the 2kg block. The clay is moving horizontally with speed v when it hits and sticks to the block. The 8kg block is held still by a removable stop. As a result, the spring compresses a maximum distance of 0.4m.

    In a second experiment, an identical ball of clay is thrown at another identical 2kg block, but this time the stop is removed so that the 8kg block is free to move.

    e. State the principle(s) that can be used to calculate the velocity of the 8kg block at the instant that the spring regains its original length. Write the appropriate equation(s) and show the numerical substitutions, but do not solve for velocity.

    2. Relevant equations


    3. The attempt at a solution

    So I use conservation of momentum and get

    mclay*vclay + mblocks * 0 = ( mclay+mblocks)* vf

    However, the rubric said I need to include also conservation of energy so how will it fit in here ??
  2. jcsd
  3. Feb 11, 2009 #2


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    Homework Helper

    Until the first weight returns to normal relaxed spring detent, the spring contains PE. Only as it crosses the equilibrium point is it not to be accounted for.

    What happens to the speed of the larger block after the clay laden block crosses that point going backward?
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