# Physics question momentum

Hi, please could i have help on this question? I don’t understand where i went wrong? The question is asking to calculate the speed after the collision. Change in momentum= mass times change in velocity. The change in velocity will be final-initial velocity so surely the markscheme for 2 ci should read v-2.8 instead of v+2.8 (2.8 was the initial velocity?
Here is the question: https://app.gemoo.com/share/image-annotation/632443787613372416?codeId=DWlKz2oB9XWZr&origin=imageurlgenerator
https://app.gemoo.com/share/image-annotation/632444133144338432?codeId=vJ3dJ4xog5mrO&origin=imageurlgenerator
Here is the markscheme: https://app.gemoo.com/share/image-annotation/632444400736735232?codeId=DWlKzlKG9R2wr&origin=imageurlgenerator

Thanks!!
Question seems to be asking for speed?
Question seems to be asking for speed?
Original post by Joinedup
Question seems to be asking for speed?

Would that make a difference, would it still not be final speed-initial speed?
It's bouncing back in the opposite direction... subtracting a negative number is the same as adding a positive number.
Original post by Joinedup
It's bouncing back in the opposite direction... subtracting a negative number is the same as adding a positive number.

But if it is bouncing back in the opposite direction, wouldn’t the final velocity be negative so it should be (-v2-+2.8) so (-v2-2.8) which still does not give the same answer?
Change in velocity would be the change required to reduce the 2.8 m/s to zero and then the change required to send it back in the opposite direction added together wouldn't it?

Sorry if I'm missing something, I'm trying to look at the screenshots on a mobile and tbh they look like garbage with bits missing.
Original post by anonymous294
Hi, please could i have help on this question? I don’t understand where i went wrong? The question is asking to calculate the speed after the collision. Change in momentum= mass times change in velocity. The change in velocity will be final-initial velocity so surely the markscheme for 2 ci should read v-2.8 instead of v+2.8 (2.8 was the initial velocity?
Here is the question: https://app.gemoo.com/share/image-annotation/632443787613372416?codeId=DWlKz2oB9XWZr&origin=imageurlgenerator
https://app.gemoo.com/share/image-annotation/632444133144338432?codeId=vJ3dJ4xog5mrO&origin=imageurlgenerator
Here is the markscheme: https://app.gemoo.com/share/image-annotation/632444400736735232?codeId=DWlKzlKG9R2wr&origin=imageurlgenerator

Thanks!!

Original post by anonymous294
But if it is bouncing back in the opposite direction, wouldn’t the final velocity be negative so it should be (-v2-+2.8) so (-v2-2.8) which still does not give the same answer?

If we use the definition
change in momentum = final momentum - initial momentum
we can notice a few things:
the direction of final momentum is pointing to the left after collision
the direction of initial momentum is pointing to the right before the collision
let use "arrows" to do the subtraction of the momenta, change in momentum will be
$\leftarrow - \rightarrow = \leftarrow + (-\rightarrow) =\leftarrow + (\leftarrow) = \leftarrow$
the change in momentum is pointing to the left and the change in momentum is given as positive in the question, so we can assume that the positive direction is pointing to left

As a result, we can write
change in momentum = mass × (final vel initial vel) = m(v (−2.8)) = m(v + 2.8)

The last part (v + 2.8) “can” be viewed as the addition of final speed and initial speed to compute the change in momentum.
Original post by Eimmanuel
If we use the definition
change in momentum = final momentum - initial momentum
we can notice a few things:
the direction of final momentum is pointing to the left after collision
the direction of initial momentum is pointing to the right before the collision
let use "arrows" to do the subtraction of the momenta, change in momentum will be
$\leftarrow - \rightarrow = \leftarrow + (-\rightarrow) =\leftarrow + (\leftarrow) = \leftarrow$
the change in momentum is pointing to the left and the change in momentum is given as positive in the question, so we can assume that the positive direction is pointing to left
As a result, we can write
change in momentum = mass × (final vel initial vel) = m(v (−2.8)) = m(v + 2.8)
The last part (v + 2.8) “can” be viewed as the addition of final speed and initial speed to compute the change in momentum.

Ahhh that makes a lot more sense, thank you! But is there an easier way to work out that the positive direction was left because we have to know this to get the right answer otherwise if it was to the right, the answer would be completely different?