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Homework Help: Conservation of momentum and SHM of a spring

  1. Nov 18, 2008 #1
    1. The problem statement, all variables and given/known data

    A spring of negligible mass
    and force constant k = 400 N/m is hung vertically, and a 0.200-kg
    pan is suspended from its lower end. A butcher drops a 2.2-kg
    steak onto the pan from a height of 0.40 m. The steak makes a
    totally inelastic collision with the pan and sets the system into ver-
    tical SlIM. What are (a) the speed of the pan and steak immedi-
    ately after the collision;

    2. Relevant equations

    P = Mv

    3. The attempt at a solution

    I cant really see how linear momentum can be conserved in this case. Doesnt the weight of the steak constitue a net external force on the system? How then can momentum be conserved? I can do the rest of the problem if i clarify this doubt. Thanks
  2. jcsd
  3. Nov 18, 2008 #2


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    Staff Emeritus
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    Gold Member

    At the *instant* the steak collides with the pan, there is no net external force on the pan-steak system. Therefore, one may apply conservation of momentum at the *instant* of the collision to determine the initial velocity of the system. However, as you correctly point out, both immediately before and after the collision there is a net external force acting on the pan-steak system and therefore momentum is not conserved.

    Does that make sense?
  4. Nov 18, 2008 #3
    But doesnt the weight of the steak constitue a net external force even at the instant of collision?
  5. Nov 18, 2008 #4

    D H

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    Staff Emeritus
    Science Advisor

    Yes, but it is finite. The collision occurs over a very short period of time [itex]\Delta t[/itex]. If this is sufficiently short, the collision forces [itex]\Delta \boldsymbol{p}/\Delta t[/itex] will be much, much greater in magnitude finite external forces. In the limit of an instantaneous collision, you can ignore those external forces during the collision process because the internal collision forces become infinitely large (they are impulses). The meat+pan system is effectively a closed system during the infinitesimally short collision process. While conservation of kinetic energy does not apply here (the collision is inelastic), conservation of momentum does.
  6. Nov 18, 2008 #5
    Thanks for the clarification. I get it now.
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