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quick question about change in momentum

If an object of constant mass slows down and you are asked to find the change of momentum, if mv-mu is negative would you express your answer as a positive value or negative value? I know that you can have a negative momentum as its a vector and a negative force too.
Also if you do write the positive value down then when you find the force, by dividng by time, would you express this as a negative value or write in the ..... direction?
Reply 1
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bornab3
I don't fully understand what you're asking considering i haven't seen the question or the wording of the question. But if the momentum is negative then don't write it as a magnitude positive value as this is wrong. However most of the time it will ask you for the magnitude of the force so you can assume that they want a positive value. But again, I wouldn't advise you to just write all force values as positive. Always pay attention to the wording of the question as this is what gives it away most of the time.
Reply 2
Avatar for Lay-Z
Lay-Z
OP
20
Original post by bornab3
I don't fully understand what you're asking considering i haven't seen the question or the wording of the question. But if the momentum is negative then don't write it as a magnitude positive value as this is wrong. However most of the time it will ask you for the magnitude of the force so you can assume that they want a positive value. But again, I wouldn't advise you to just write all force values as positive. Always pay attention to the wording of the question as this is what gives it away most of the time.

I mean if the momentums are both positive but the final momentum is less than the original momentum.
Eg a object of 10kg initially moves at 5ms-1 then moves at 2ms-1
or a object of 10kg initially moves at 10ms-1 then rebounds at a speed of 20ms-1

in the first one mv-mu = 20-50 = -30
in the second one -mv-mu = -300

would you write both these answers as negative values or positive values?
Reply 3
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bornab3
Yes, for the CHANGE in momentum you have to always express whether it's negative or positive as if you don't it means the completely wrong thing. But if it asks you for the overall momentum, most of the time put the magnitude which is always positive


Posted from TSR Mobile
If the question asks what the change in momentum was then
Yes, the change in momentum is negative in both those cases.
In practice it means that the force producing it was negative, opposite in direction to the two velocities, as they were both positive.
Velocity and momentum are both vectors, So is force, and as force is given by rate of change of momentum, the sign (direction) of the force is governed by the sign (direction) of the change in momentum.
Reply 5
Avatar for Lay-Z
Lay-Z
OP
20
Original post by bornab3
Yes, for the CHANGE in momentum you have to always express whether it's negative or positive as if you don't it means the completely wrong thing. But if it asks you for the overall momentum, most of the time put the magnitude which is always positive


Posted from TSR Mobile


Original post by Stonebridge
If the question asks what the change in momentum was then
Yes, the change in momentum is negative in both those cases.
In practice it means that the force producing it was negative, opposite in direction to the two velocities, as they were both positive.
Velocity and momentum are both vectors, So is force, and as force is given by rate of change of momentum, the sign (direction) of the force is governed by the sign (direction) of the change in momentum.


Thanks both

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