conservation of energy vs conservation of momentum questions

[thebillsfan]

these questions, i think, are really tricky. the question goes like, the phenomena described in the passage is an example of which of the following principles?
a. conservation of momentum
b. conservation of energy
c. and d. -- not related

since momentum and energy are both always conserved, how do you know when one is more applicable than the other? they both apply. does momentum only apply to collisions, or can it apply to something like gravitational orbit or spring motion?

thanks--that may have been the last question i ever post on this forum, but we'll see. 🙂

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[Vihsadas]

these questions, i think, are really tricky. the question goes like, the phenomena described in the passage is an example of which of the following principles?
a. conservation of momentum
b. conservation of energy
c. and d. -- not related

since momentum and energy are both always conserved, how do you know when one is more applicable than the other? they both apply. does momentum only apply to collisions, or can it apply to something like gravitational orbit or spring motion?

thanks--that may have been the last question i ever post on this forum, but we'll see. 🙂

Momentum can be applied to any situation with mass and movement, and even that's not completely accurate since it's applicable to EM waves as well!
Are you sure this question didn't say something like, "Conservation of kinetic energy"?
In some types of collisons (inelastic), Kinetic energy can be converted to heat, sound, light, and deformation.

 

[thebillsfan]

No, it definitely said conservation of energy.
but the question had to do with the spring system slowing down over time...so the energy would be conserved but is momentum still conserved in this situation?

 

[ratatat]

No, it definitely said conservation of energy.
but the question had to do with the spring system slowing down over time...so the energy would be conserved but is momentum still conserved in this situation?

I think the way you do those types of problems is you determine whether it's an elastic or inelastic collision. If it's elastic, you can simply use conservation of energy. If it's inelastic, you have to use conservation of momentum. I always use conservation of momentum though.

EDIT

For that particular question, I would say conservation of energy, because although it's slowing down, some of the kinetic energy from its movement will be converted into internal energy (friction and heat) as far as I know. What was the answer?

 

[thebillsfan]

I think the way you do those types of problems is you determine whether it's an elastic or inelastic collision. If it's elastic, you can simply use conservation of energy. If it's inelastic, you have to use conservation of momentum. I always use conservation of momentum though.

EDIT

For that particular question, I would say conservation of energy, because although it's slowing down, some of the kinetic energy from its movement will be converted into internal energy (friction and heat) as far as I know. What was the answer?

it was energy. i understand the reasoning, but what i dont understand is since momentum is always conserved, how does that not play a role

 

[549]

Momentum is only conserved when there are no external forces present. In problems dealing with SHM such as the spring problem, there is an external force being applied by the spring, so momentum isn't necessarily conserved.

I'm pretty sure that's right, but this topic does confuse me a bit, so can someone confirm this??

 

[thebillsfan]

Momentum is only conserved when there are no external forces present. In problems dealing with SHM such as the spring problem, there is an external force being applied by the spring, so momentum isn't necessarily conserved.

I'm pretty sure that's right, but this topic does confuse me a bit, so can someone confirm this??

id generally agree with you. however, i did have one question one time about how a pendulum when its going through its starting point (x=0, maximum speed and minimum acceleration), why does it continue to keep going tot he other side? the answer was conservation of momentum and NOT energy in this case.

 

[bacotell]

where did the question come from?

 

[bacotell]

well i guess it makes sense after thinking about it since the net force(gravity) applied to swing it back to one side is then cancled out by the force (gravity) pushing it in the opposing direction, thus the over all force net = 0, therefore momentum is conserved.

Becuase energy is conserved once it reaches the bottom of its path (accel = max, where all PE is converted to KE), but momentum explains why it keeps swinging back to max PE.

Hope that helps. Better yet, hope it makes sense!

 

[minutemen11]

well i guess it makes sense after thinking about it since the net force(gravity) applied to swing it back to one side is then cancled out by the force (gravity) pushing it in the opposing direction, thus the over all force net = 0, therefore momentum is conserved.

Becuase energy is conserved once it reaches the bottom of its path (accel = max, where all PE is converted to KE), but momentum explains why it keeps swinging back to max PE.

Hope that helps. Better yet, hope it makes sense!

i thought you could never restore the max. inital PE of the pendulum.. this is true when air resistance is involved right? but if you ignore air resistance, if you released a pendulum at h=2m, it will be 2m again on the other side of the pendulum?

 

[ezsanche]

No, it definitely said conservation of energy.
but the question had to do with the spring system slowing down over time...so the energy would be conserved but is momentum still conserved in this situation?

Is there a external force acting on the object. because in that case momemtum is not conserved

 

[ezsanche]

id generally agree with you. however, i did have one question one time about how a pendulum when its going through its starting point (x=0, maximum speed and minimum acceleration), why does it continue to keep going tot he other side? the answer was conservation of momentum and NOT energy in this case.

I remember a question like that but the answer wasn't conservation of momemtum it was because it GAINED momemtum when it reached the bottom of it swings.

 

[thebillsfan]

I remember a question like that but the answer wasn't conservation of momemtum it was because it GAINED momemtum when it reached the bottom of it swings.

huh? you mean as the swing goes from the top of its flight back to bottom, it is gaining momentum. but at the pt at its bottom, no more momentum is to be gained, and it continues going to the other side because of the momentum it already had. BUT once could argue it continues going to the other side b/c it had KE which needed to be converted to PE. honestly, how the hell can you answer such a question?

 

[549]

You can look at it as momentum increasing as a result of the conservation of energy. But either way, and as mentioned a few times here, with external forces present (gravity, restoring force...) momentum will not be conserved.

 

[ezsanche]

huh? you mean as the swing goes from the top of its flight back to bottom, it is gaining momentum. but at the pt at its bottom, no more momentum is to be gained, and it continues going to the other side because of the momentum it already had. BUT once could argue it continues going to the other side b/c it had KE which needed to be converted to PE. honestly, how the hell can you answer such a question?

yeah that was my point. Since there is a varying external force acting on the object momentum can not be conserved. Just think it about this way. The velocity of the top of the swing zero and the velocity at the bottom of the swing is some max number. Since the mass is the same through out the swing the momentum is different at each point of the swing. HENCE the momentum can not be conserved.