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Momentum, 2 blocks and a spring

  1. Nov 27, 2006 #1
    Diagram here ---> http://capaserv.physics.mun.ca/msuphysicslib/Graphics/Gtype16/prob25_En_Mom_conserve.gif

    A spring of negligible mass is compressed between two masses on a frictionless table with sloping ramps at each end. The masses are released simultaneously. The masses have the same volume, but the density of M1 is greater than that of M2. (Select G-Greater than, L-Less than, E-Equal to. If the first is G and the rest L, enter GLLLLL).

    A) The momentum of M1 is ... the momentum of M2 once they both lose contact with the spring.
    B) The kinetic energy of M2 is ... the kinetic energy of M1 once they both lose contact with the spring.
    C) The force exerted by the spring on M1 is ... the force it exerts on M2.
    D) The final height up the ramp reached by M1 is ... the height reached by M2.
    E) The speed of M2 is ... the speed of M1 once they both lose contact with the spring.
    F) The duration of the force exerted by the spring on M1 is ... the time the force acts on M2.

    The force exerted by the spring is an internal force so that the total momenum immediately after the masses separate is equal to the total momentum when the spring is compressed (and the blocks are at rest). Once the masses have separated, there are no non-conservative forces and mechanical energy is conserved.

    From this I said EGGLGG for A-F respectively.

    Momenutm is conserved therefore I said both are equal for A.

    In B, M2 has a higher velocity since it has less mass and same momentum, therefore the KE derived from 0.5mv^2 would be greater than that of M1.

    For C, since M1 has a greater mass a greater force is exerted on it by the spring (? unsure on this one).

    And D, because M1 has a lesser speed than M2, it would travel a shorter distance up the slope than M2.

    For E, as stated before, since both have same momentum and M1 has greater mass, then M2 must have a greater velocity.

    Finally in F, the duration of force would be longer for M1 since it has a greater mass and more force is required to move it (? unsure again)

    I'm not exactly sure what ones are wrong and what ones are right, if anyone can shed some light on this problem it would be much appreciated.

    Last edited by a moderator: Apr 22, 2017
  2. jcsd
  3. Nov 27, 2006 #2


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    The ones you are unsure about are the ones you need to think about. Newton's third law says?????
  4. Nov 27, 2006 #3
    Thanks, I got it now, both of those should be Equal :)
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