Physics help please

Hi,
Please could I have some help on question 6c of this paper. I’m not really sure about how to answer it.
Paper: https://pmt.physicsandmathstutor.com/download/Physics/A-level/Past-Papers/CAIE/Paper-4/QP/June%202015%20(v2)%20QP.pdf
Thanks!

Reply 1

BlakeTerror

A nice question. It comes down to momentum being a vector quantity, and you may find this easier to answer if you include a vector triangle as an illustration. Before the collision there is a single momentum vector pointing to the right (for the photon, which is the only thing moving). After the collision there are two arrows, one pointing down and to the right (for the electron) and another pointing up and to the right (for the deflected photon). Conservation of momentum means that these 3 arrows form a vector triangle. The question wants us to explain why the length of the electron arrow is not equal to the difference in the lengths of the other 2 arrows (which is pretty much self-evident from the geometry of the triangle). So, as an answer to the question, I would first make a statement that momentum is a vector quantity, then draw the vector triangle and say that |b| is not equal to |a|-|c|.
In case you're on the look out for more practice questions, I've got an ever-expanding collection of them here: Exam Practice Questions - YouTube

Reply 2

lordaxil

Remember momentum is a vector quantity.

Reply 3

anonymous56754

Original post by lordaxil

Remember momentum is a vector quantity.

Yes, the markscheme said that but I’m not sure what to make of it, please could you elaborate? thanks!

Reply 4

lordaxil

If you imagine two vectors placed head-to-tail, their resultant length is not necessarily the same as the sum of their inidvidual lengths (only true if the vectors are parallel).