- #1

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m1v = (m1+m2)Vf -----> 0 = (m1+m2)Vf - m1v1

1/2(m1+m2)Vf^2 - 1/2m1v^2 = (m1+m2)Vf - m1v1

Or totally wrong? Thanks!

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- Thread starter xxphysics
- Start date

- #1

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m1v = (m1+m2)Vf -----> 0 = (m1+m2)Vf - m1v1

1/2(m1+m2)Vf^2 - 1/2m1v^2 = (m1+m2)Vf - m1v1

Or totally wrong? Thanks!

- #2

Chestermiller

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Totally wrong. The units don't even match.

- #3

Nugatory

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So when you ask whether the change in kinetic energy is equal to the difference between the initial and final momentum, you're asking whether the change in kinetic energy is equal to zero.

This would be a good time to stop and think about the definition of "inelastic collision".

- #4

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In an inelastic collision is the change in kinetic energy equal to the difference of final and initial momentum if one of the objects is initially at rest?

OK, my take on this is that, this is a rather odd question. You're asking if

ΔK = K

This is odd because that is the DEFINITION of ΔK!

Zz.

- #5

nasu

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This is not what the quoted sentence says. :)

- #6

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This is not what the quoted sentence says. :)

I am aware that the OP is mixing momentum with kinetic energy. I was hoping that this was an oversight, and not out of ignorance.

Zz.

- #7

sophiecentaur

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It's more than possible that he didn't actually know??I was hoping that this was an oversight, and not out of ignorance.

- #8

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ZzOK, my take on this is that, this is a rather odd question. You're asking if

ΔK = K_{f}- K_{i}

This is odd because that is the DEFINITION of ΔK!

Zz.

- #9

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ZzI am aware that the OP is mixing momentum with kinetic energy. I was hoping that this was an oversight, and not out of ignorance.

Zz.

- #10

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Totally wrong. The units don't even match.

Thank you :) I just thought they were both looking at the change in velocity and in both equations there is a way to account for the differences in mass (before and after collision) so I didn't think was absurd to wonder if there is a connection between the formulas.It's more than possible that he didn't actually know??

- #11

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No my question was if you could relate the momentum equation of an inelastic collision to the change in kinetic energy of that collisionOK, my take on this is that, this is a rather odd question. You're asking if

ΔK = K_{f}- K_{i}

This is odd because that is the DEFINITION of ΔK!

Zz.

- #12

sophiecentaur

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I see what you are after. There isn't a 1:1 relationship between the two quantities. Particular circumstances will give particular relationships.No my question was if you could relate the momentum equation of an inelastic collision to the change in kinetic energy of that collision

Two situations with the same total momentum and different KE transferred to the collision. For convenience I have chosen to bring the motion to a halt. :

Two equal masses m&m, travelling towards each other at v and -v (Earth frame of reference) will have a total momentum of zero and a total KE of mv

This two trivial cases are enough to show that your idea can't be relied on. It's the squaring of the velocity that upsets things.

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