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Momentum help

  1. Oct 18, 2005 #1
    Hi, I need help witht these 2 questions. My answers have *********** next to them. Any help is appreciated. Thanks.

    1. A rocket is propelled forward by ejecting gas at high speed. The forward motion is a consequence of
    a) conservation of energy.
    b) conservation of momentum. ***********
    c) both of the above.
    d) neither of the above.

    2. In an inelastic collision
    a) both energy and momentum are conserved.
    b) energy is conserved.
    c) momentum is conserved. ***********
    d) neither is conserved.

    Thank you.
  2. jcsd
  3. Oct 18, 2005 #2


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    Gold Member

    Momentum is conserved in all such reactions.

    Energy is also conserved (ignoring mass energy conversions). However, not all of it may go into motion of the objects of interest. Some of it ends up as heat or energy of deformation.
  4. Oct 18, 2005 #3
    For question 2, this might help:

    When objects collide momentum is always conserved so long as no external forces act on the objects.

    An elastic collision means that the total kinetic energy before and after the collision is the same. A perfectly elastic collision can only occur between molecules or smaller particles because a collision between larger objects (such as vehicles) results in heating of the objects and their surroundings, thus an overall loss of kinetic energy. A significant loss of kinetic energy means that the collision is inelastic, however if the loss of kinetic energy is very small you can sometimes treat the collision as elastic to make calculations easier.

    For question 1:

    From the previous explanation because a rocket is so large there will be an overall loss of energy to the surroundings as heat, sound and light. Therefore energy can't be conserved, so it can't be both of the above. The reason momentum is conserved is because the mass*velocity of the gass being ejected is equal to the mass*velocity of the rocket moving forwards.

    I hope this helps, and if any of this is wrong please correct me.
  5. Oct 18, 2005 #4
    i agree with you. on question 1 and believe that only momentum is conserved. As for question 2, since it asks for inelastic collisions, i guess we have to assume that kinetic energy is not conserved at all.
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