Is momentum always conserved?

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Mcat35

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I know this is kinda of a basic question, but I've always had problems with momentum. I know that in inelastic collision and elastic collision momentum is conserved while kinetic energy is conserved for elastic collision.

Can there be an instance where momentum is not conserved?
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Momentum is always conserved in a closed system, yes.

This is recklessly pulled from wikipedia, but a good example is the Newton's cradle toy. It would go on forever, due to conservation of momentum, except for loss of energy to friction, sound, etc.
 
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If there is no external force on a system, then momentum of the system cannot change. This is a basic statement of Newton's Second Law.
 
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Catburr said:
Momentum is always conserved in a closed system, yes.

This is recklessly pulled from wikipedia, but a good example is the Newton's cradle toy. It would go on forever, due to conservation of momentum, except for loss of energy to friction, sound, etc.
Click to expand...

Eh, it would go on forever if the collisions were 100% elastic, but they're not. Slightly different from momentum conservation. Momentum is conserved in that situation, you're just artificially shrinking the system - the system is the universe.
 
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For the purposes of the MCAT, unless the question tells you otherwise, you should assume that there is conservation of momentum.
 
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Don't always assume yes though when you are asked if momentum is conserved.

I did a problem recently in TBR where a block of a certain mass collided with another block that was attached to a spring, and then they stuck together and oscillated after the collision. They said momentum was not conserved because the velocity keeps changing as they oscillate.

To be honest I'm still somewhat confused by this problem.
 
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Once they began oscillating, there was an external force, so momentum was not conserved for the two blocks.
 
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attixx said:
Don't always assume yes though when you are asked if momentum is conserved.

I did a problem recently in TBR where a block of a certain mass collided with another block that was attached to a spring, and then they stuck together and oscillated after the collision. They said momentum was not conserved because the velocity keeps changing as they oscillate.

To be honest I'm still somewhat confused by this problem.
Click to expand...

That's a good point -- there could be a tricky question where momentum is not conserved. However, playing the odds, and given that there are no calculators allowed on the MCATs, a question not involving conservation of momentum would be tough to solve. If so, then you have to play the odds and just guess on such a question instead of struggling with it. Focus on getting the easy ones right because you don't get more points for getting tougher questions right.
 
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yankswin2011 said:
That's a good point -- there could be a tricky question where momentum is not conserved. However, playing the odds, and given that there are no calculators allowed on the MCATs, a question not involving conservation of momentum would be tough to solve. If so, then you have to play the odds and just guess on such a question instead of struggling with it. Focus on getting the easy ones right because you don't get more points for getting tougher questions right.
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It's not really a tricky question at all. It is just understanding that once external forces are added, momentum has to change. When collisions happen, there is no external force, or external force can be neglected, for the time period we are interested in. Obviously nothing goes on forever, so momentum isn't conserved for all of eternity. But when we talk about momentum being conserved in problems, we mean for the time period right after the collision.
 
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Rabolisk said:
It's not really a tricky question at all. It is just understanding that once external forces are added, momentum has to change. When collisions happen, there is no external force, or external force can be neglected, for the time period we are interested in. Obviously nothing goes on forever, so momentum isn't conserved for all of eternity. But when we talk about momentum being conserved in problems, we mean for the time period right after the collision.
Click to expand...

Good point, I guess there could be a qualitative question regarding when momentum in not conserved. However, have you come across any quantitative questions regarding when conservation of momentum is not conserved?
 
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attixx said:
Don't always assume yes though when you are asked if momentum is conserved.

I did a problem recently in TBR where a block of a certain mass collided with another block that was attached to a spring, and then they stuck together and oscillated after the collision. They said momentum was not conserved because the velocity keeps changing as they oscillate.

To be honest I'm still somewhat confused by this problem.
Click to expand...
I am still baffled by the same problem too.
 
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BestDoctorEver said:
I am still baffled by the same problem too.
Click to expand...

As Rabolisk has pointed out, momentum is not conserved when there is a net force acting on the system. As he also stated, this concept is built into Newton's second law. Basically, if a force is acting on an object (or system), then the object (or system) is experiencing an acceleration, which means it's velocity is changing. If the linear velocity changes for an object of constant mass, then the linear momentum changes. (The linear part is not the key part here, but for the system in question it will help).

Once the moving block collides with the stationary block resting against the spring, the two blocks will move together against the resistive force of the spring. That means there is a force acting on the two-block system (namely the restoring force of the spring). That force is against the motion of the blocks, so the blocks will slow down, and thereby lose velocity (and momentum). Momentum was not conserved after collision, because of the spring force acting on the system (F = -kx).
 
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according to kaplan on one of their FL, they posit that when gravitation force is involved in a collision, it can be an external force such that momentum will not be conserved. also friction would do the same.
 
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should have thanked you all sooner.

there are some very nice responses in this post..

Cleared some stuff up for me

Thanks!
 
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Mcat35 said:
I know this is kinda of a basic question, but I've always had problems with momentum. I know that in inelastic collision and elastic collision momentum is conserved while kinetic energy is conserved for elastic collision.

Can there be an instance where momentum is not conserved?
Click to expand...

To make it simple MOMENTUM IS ALWAYS CONSERVED as long as an external force does not act on the system.
 
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