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Homework Help: Conservation of momentum

  1. Aug 5, 2010 #1
    Ok so I want to know if conservation of momentum can be applies in a case of shooting a bullet to the wall I am talking about in the x direction of course since gravity is an external force in the y direction so a let say I shoot a bullet in a straight angle to the wall so it has huge x component of velocity and it hit the wall for but it get stuck in the wall and doesn't fall down what can the laws of impulse and conservation of momentum be applied in this case?
    I mean I= F delta t but the bullet is got stuck so how can I know the time of the collision also the bullet had a velocity of maybe 360m/s or something and when it hit the wall it had 0 velocity so and there was no external force so how was momentum conserved? ( I am thinking that since the bullet dug through the wall the little pieces of the wall gained the velocity or something right?)
  2. jcsd
  3. Aug 5, 2010 #2
    Usually when looking at systems such as the one you are describing, professors will say that you can't use conservation of momentum; I never liked this because while it's not useful to solve problems of this nature, it implies that moment is not conserved (which is wrong; momentum is always conserved).

    Anyways, your thinking (if I understand what you wrote) is correct. Say you start driving your car East. By doing so, you cause the Earth to spin going West. Keep in mind though, the mass of the Earth is >>>>>>>>>>> then the mass of a car.
  4. Aug 5, 2010 #3
    you mean momentum is always conserved unless we have an external force acting on the system right?
  5. Aug 5, 2010 #4
    Depends on how you look at it. Say you threw a ball straight up into the air. Is momentum conserved in that case? Yes.

    When you throw the ball up, the ball gains momentum p and the Earth gains momentum -p. Now, from the moment the ball leaves your hand, it starts to slow down because the external force of gravity acts on it. Is momentum conserved? Yes! As the Earth pulls the ball to it, the ball pulls the Earth to it. When they collide at the original point of acceleration, they have equal and opposite momentum vectors.

    Again, momentum is always conserved IF you view the system properly.
  6. Aug 5, 2010 #5
    umm what about friction I mean when slide something on a a table which has a very high friction coefficient wouldn't the the energy get turned into heat and not kinetic energy so momentum wont be conserved right? I mean the energy that got transferred to the table was heat not kinetic.
  7. Aug 5, 2010 #6
    What is heat? ;)
  8. Aug 5, 2010 #7
    So you are saying that since gaining heat means that molecules are getting faster which means that the momentum was used for that?(or something like that?)
    Edit: also I forgot I was asking in the original question what can be considered time of impact in the impulse equation considering the bullet?
  9. Aug 5, 2010 #8

    For the time for impact, it would be the time it takes for the bullet to reach zero velocity starting from the moment of impact.
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