The diagram shows a muon (μ−\mu^-μ−), a negatively charged lepton with charge −e-e−e, moving with velocity v v\,v along the positive yyy-axis. It enters a region of uniform magnetic field B B\,B directed along the negative xxx-axis.
In which direction does the magnetic force act on the muon at the instant it enters the magnetic field?
Along the +z+z+z direction (out of the page)
Along the −z-z−z direction (into the page)
Along the +x+x+x direction
Along the −y-y−y direction