Physics Momentum AS QS! Help!

[Saturation]

Please explain this qs to me!: (Number 3(bi))
http://www.xtremepapers.me/CIE/Internat ... _qp_21.pdf

I thought that the change in momentum is = mv - mu
If I do that, I get -1.33, while the ms gives +1.33
Help?

MS:
http://www.xtremepapers.me/CIE/Internat ... _ms_21.pdf

This is not the only paper, even a question in M/J 2009 gives the change of momentum as a positive, while i get the same answer in negative!

This change in momentum idea is killing me!!! :bad: :bad: :bad: HELP!

 

[libra94]

change in momentum
(140/1000 * 5.5 ) - (140/1000 * -4.0)
=1.33Nm
as momentum is a vector quantity, we take the directions into consideration
so we take -4 as the ball is moving in opp. direction (u can see that frm the graph)

 

[Talha]

dude always remember dat momentum values r always quoted as positive values.. the -ve sign is basically telling u da direction dat da particle vil be following!

 

[Saturation]

Thanks but the thing is shouldn't we include the direction aswell?

 

[Saturation]

change in momentum
(140/1000 * 5.5 ) - (140/1000 * -4.0)
=1.33Nm
as momentum is a vector quantity, we take the directions into consideration
so we take -4 as the ball is moving in opp. direction (u can see that frm the graph)

Your doing initial momentum - final momentum right? isn't
momentum change = final momentum - initial momentum?

and btw, is it wrong if we write -1.33 ? (cause the ms doesnt mention anyting about it )

 

[AayushR]

You have to write down the magnitude.

 

[sumbouudy]

momentum = m(v-(-u))
= m(v+u)

 

[sniper7137]

Its simple. If something is initially travelling in the forward direction then its velocity is positive. But after collision, if it starts to travel backwards then its velocity is negative. The common mistake here is that when we look at the velocity time graph we tend to ignore the sign with the velocity. Change in momentum is indeed given by m1v1 - m2v2. All you need to do is to put in the correct sign along with the magnitude of the velocity.

Check out this link:
http://www.s-cool.co.uk/a-level/physics ... momentum-0

And change in momentum is always quoted in terms of its magnitude. The sign is not quoted.

Hope this helps.

 

[Saturation]

But it doesnt mention magnitude!? Is -1.33 right!??