Physics Paper 1

[InvisionFree]

In question 11 the correct answer is B
http://www.xtremepapers.com/CIE/Interna ... 4_qp_1.pdf
I dont get it. The correct answer should be D.
Please help.
Thanks

 

[hancock]

Take one direction to be positive eg : The direction in which the particle is moving. So the initial momentum is mv.
As the collision is perfectly elastic, the final velocity and initial velocity will be the same i.e. v. But, in the opposite direction.

So, Final momentum = -mv

Change in momentum = Final momentum - Initial momentum
=-mv-(mv)
-2mv

As we took the direction to the right to be positive, the answer will be 2mv to the left. Thus, B