Abeam of π+ particles (mπ = 140 mev/c2) is accelerated to a momentum of pπ+ = 3.1 gev/c in the laboratory frame. a π+ particle decays into a positive muon (mμ = 105 mev/c2) and a muon neutrino (mνμ = 0 mev/c2).

a. what is the energy of the muon in the rest frame of the π+?
b. what is the energy of the muon in the laboratory frame if it is found traveling in the beam direction?
c. what is the energy of the muon in the laboratory frame if it is found traveling at and angle of 250 mrad with respect to the beam direction?

russian is not my first language, so i'm afraid i can't write out an answer in russian, but hopefully you can find a way to do so. here's my best translation of the problem: you're given a plot of a particle's position in one-dimensional space over time, and you're asked to find either the total distance traveled by the particle after 6 seconds had elapsed, or the total displacement of the particle after 6 seconds.

if you want the total distance traveled, you would first need to determine the particle's velocity as a function of time, then integrate that over the 6-second interval. we know the position function is quadratic with roots at 0 and 6, so we have

where is unknown. because the position is parabolic, there is symmetry about the midpoint at , and we know that

so that the position function is

which means the velocity is

then the total distance traveled is

so the total distance traveled is 20 m.

on the other hand, if you want to find displacement: the particle starts at and ends at , so its displacement is 0.