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# Negative momentum? Watch

1. In some calculations in conservation of momentum. If two objects are travelling in opposite directions.

One will have a -ve velocity. Okay

But why does this have to equate to a positive momentum. As the answer required that:

-p = m x -v

Therefore a negative velocity results in a positive momentum .
But why should it have to equate to -ve p? Is momentum not a vector? Surely a negative momentum is the result of a negative velocity.
2. (Original post by Feynboy)
In some calculations in conservation of momentum. If two objects are travelling in opposite directions.

One will have a -ve velocity. Okay

But why does this have to equate to a positive momentum. As the answer required that:

-p = m x -v
Therefore a negative velocity results in a positive momentum .

But why should it have to equate to -ve p? Is momentum not a vector? Surely a negative momentum is the result of a negative velocity.
I'm not sure I understand what your saying..

Can you provide an example?
3. Therefore a negative velocity results in a positive momentum .
It does not. Momentum, like velocity is a vector quantity and can therefore take positive or negative sign in any coordinate direction. According to the equation p = mv, the momentum is in the same direction as velocity.

If your velocity is -5 m/s North and your mass is 2 kg then your momentum is -10 kg m/s North. All this tells you is that the direction of velocity/momentum points opposite to the reference direction (North). A momentum of -10 kg m/s North is +10 kg m/s South.
4. (Original post by Feynboy)
In some calculations in conservation of momentum. If two objects are travelling in opposite directions.

One will have a -ve velocity. Okay

But why does this have to equate to a positive momentum. As the answer required that:

-p = m x -v

Therefore a negative velocity results in a positive momentum .
But why should it have to equate to -ve p? Is momentum not a vector? Surely a negative momentum is the result of a negative velocity.
When dealing with momentum questions you would leave it as p if it's positive or negative. You wouldn't write -p= -6kgms^-1 you just leave it as p=. And yes a negative velocity will always results in a negative momentum.

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5. (Original post by mik1a)
It does not. Momentum, like velocity is a vector quantity and can therefore take positive or negative sign in any coordinate direction. According to the equation p = mv, the momentum is in the same direction as velocity.

If your velocity is -5 m/s North and your mass is 2 kg then your momentum is -10 kg m/s North. All this tells you is that the direction of velocity/momentum points opposite to the reference direction (North). A momentum of -10 kg m/s North is +10 kg m/s South.
That was my thought too, my question book seems to disagree.
6. (Original post by Feynboy)
That was my thought too, my question book seems to disagree.
What is the book actually saying? I find it hard to believe that they'd be wrong about something so basic (was it maybe asking for the magnitude of the momentum or something?).
7. (Original post by Plagioclase)
What is the book actually saying? I find it hard to believe that they'd be wrong about something so basic (was it maybe asking for the magnitude of the momentum or something?).
It was just the answer to a question, I got the correct magnitude but I thought it was negative.The teacher justified this by saying that

-p = m x -v so the answer would actually be positive.I disagreed but it was the answer in the book
8. (Original post by Feynboy)
It was just the answer to a question, I got the correct magnitude but I thought it was negative.The teacher justified this by saying that

-p = m x -v so the answer would actually be positive.I disagreed but it was the answer in the book
What was the exact question and the exact answer in the book? That's the only way we'll be able to tell you if the book is wrong

is the same exact statement as
9. -p = m*-v is also true because it's p=mv with both sides multiplied by -1. I'm sure it's a misunderstanding, but we can't get to the bottom of it without a specific question or statement.

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Updated: September 21, 2016
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