When is momentum not conserved?

[theyellowking]

I tried to look through of the question I am about to ask, but have found no defining answer. While I know that momentum is mostly conserved, I am confused as to the situations in which it is not. It is stated that when no outside forces interact with the system, then the momentum is conserved. However, how would you know what to classify as an outside or closed system?

For instance, I stumbled across a hydraulics question asking whether energy and/or momentum is conserved when pressure is applied to one side of the hydraulics system. The problem I had with this question was, what do we consider the closed system? Should the force applied be a part of the outside or closed system?

For the question I mentioned, I am also assuming that the energy is conserved? But I'm not quite sure why.

I apologize if this question seems all over the place, but my head has been hurting over the closed vs. open system.

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[justadream]

I second this question.

From my experience in TBR, momentum is NOT conserved in pendulums (gravity is the external force) and springs (velocity varies with time).

But momentum is conserved in the Newton's cradle situation.

 

[yane4000]

I believe in the hydraulics example momentum would not be conserved because there is the outside force of pushing down. From my experience in doing momentum problems, it seems to be that momentum is only conserved during collisions of objects such as two balls striking each other, there is no other force involved here or in recoil problems with a bullet and gun. It's kind of like assuming that these objects just happen to collide into one another without any outside intervention pushing them towards each other. For example when a baseball bat strikes a ball, there exists the force of the bat hitting the ball, supplied by someone. Here, momentum would not be conserved because initial momentum won't equal final momentum.

Hope this helps!

 

[IslandStyle808]

I second this question.

From my experience in TBR, momentum is NOT conserved in pendulums (gravity is the external force) and springs (velocity varies with time).

But momentum is conserved in the Newton's cradle situation.

I could never for the life of me accept this statement from TBR, since gravity is affects the two end balls of the cradle.

 

[theyellowking]

Thanks @yane for the answer.

Anyone else?

 

[DrknoSDN]

I would agree with Yane that you should use the momentum equation to determine if momentum is conserved. If final momentum equals initial momentum then you have conservation.

For the newtons cradle I believe it is only talking about the time immediately before and after the impact. Reason being is because pendulums approximate SHM for small theta, and if delta T is infinitely small immediately before and after the balls collide then you are looking at a pendulum motion over a theta range of near zero (m1v1 + m2v2 = m1v1' + m2v2'). At the point of impact gravity is no longer accelerating the ball and PE is completely converted to KE. Momentum is conserved during the collision.

For the hydraulics it seems that the small side would displace a small volume at a high velocity. On the larger end the larger mass(volume) would be displaced at a slower velocity so conservation could apply. Also significant conservation of momentum would be required for the long distance propagation of underseas waves that result in a tsunami.

I am strictly talking about an "ideal MCAT world" scenario because in reality momentum would be lost. In an ideal MCAT world I don't see why a fluid compression wave could not propagate forever in an infinitely long tube. That is conservation of momentum, if you ignore real world factors like heat generation and molecular friction etc..

Edit:

For example when a baseball bat strikes a ball, there exists the force of the bat hitting the ball, supplied by someone. Here, momentum would not be conserved because initial momentum won't equal final momentum.

Im curious if your saying momentum is not conserved because a force is being applied during and/or after the collision because if a baseball bat was swung at a thrown ball then momentum would be conserved if you stopped applying any force or acceleration to the bat before the impact occurs. The bat would still exert a force on the ball even if the person wasn't.
At that point they are just 2 free flying objects that collide. How the objects are accelerated to their initial velocities is not a factor of conservation of mometum as long as no force is applied immediately before and after the collision.

 

[Cawolf]

Hydraulic lifts function as @DrknoSDN says - it is by Pascal's Principle that the pressure on the input is equal to the output pressure (with an ideal fluid).

 

[theyellowking]

I would agree with Yane that you should use the momentum equation to determine if momentum is conserved. If final momentum equals initial momentum then you have conservation.

For the newtons cradle I believe it is only talking about the time immediately before and after the impact. Reason being is because pendulums approximate SHM for small theta, and if delta T is infinitely small immediately before and after the balls collide then you are looking at a pendulum motion over a theta range of near zero (m1v1 + m2v2 = m1v1' + m2v2'). At the point of impact gravity is no longer accelerating the ball and PE is completely converted to KE. Momentum is conserved during the collision.

For the hydraulics it seems that the small side would displace a small volume at a high velocity. On the larger end the larger mass(volume) would be displaced at a slower velocity so conservation could apply. Also significant conservation of momentum would be required for the long distance propagation of underseas waves that result in a tsunami.

I am strictly talking about an "ideal MCAT world" scenario because in reality momentum would be lost. In an ideal MCAT world I don't see why a fluid compression wave could not propagate forever in an infinitely long tube. That is conservation of momentum, if you ignore real world factors like heat generation and molecular friction etc..

Edit:

Im curious if your saying momentum is not conserved because a force is being applied during and/or after the collision because if a baseball bat was swung at a thrown ball then momentum would be conserved if you stopped applying any force or acceleration to the bat before the impact occurs. The bat would still exert a force on the ball even if the person wasn't.
At that point they are just 2 free flying objects that collide. How the objects are accelerated to their initial velocities is not a factor of conservation of mometum as long as no force is applied immediately before and after the collision.

So if we're talking about a cradle, momentum would be conserved if we're looking at right before and right after? If we're talking about the momentum equation, the mass would not change, and the velocities would be the same magnitude?

 

[Cawolf]

Yes - in a non-inertial reference frame (i.e Earth) the conservation of momentum will apply in some infinitely small period of time before and after the collision.

 

[theyellowking]

Ah, thank you very much!