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# Momentum question watch

1. The question asks to give an expression for the vertical component of the momentum. What I got was

mVasin60 - mVbsin30

because of b going in an opposite direction to a.
But the mark scheme says it should be + not -. Can anyone please explain why?
2. Could it be plus because Vb is downwards so would be a negative? Using minus would cancel it out and imply that both particles are in the same direction
3. So a re-arrangement of mVaSin60 = -mVbSin30
4. (Original post by iow16)
Could it be plus because Vb is downwards so would be a negative? Using minus would cancel it out and imply that both particles are in the same direction
I think the reason is that because it is the total momentum you add them, but in this specific case 'vb' could be negative so it would be - but in general you would use +
5. (Original post by zainqwerty121)
I think the reason is that because it is the total momentum you add them, but in this specific case 'vb' could be negative so it would be - but in general you would use +
I guess you determined that the magnitude of the vertical components of the momentum was equal - so subtracting them to get to zero does sort of look sensible...

but really you get a total by addition and you should let the -ve velocity of one component turn it into an addition of +ve and -ve numbers... to get to zero.

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