vth is the RMS instantaneous velocity. Taking pictures at intervals gives an averaged velocity, which is slower because the particle wastes some time going in different directions that cancel out.v1 is going to be near the instantaneous velocity, but still a little slower, since the velocity is still going to decay slightly, even over 1/10th of the decay time.v2 is going to be significantly slower. If we make the time step even slower than 10 ms, we expect the RMS velocity to go roughly as the inverse square root of the timestep. Anyway, the answer should be 3:
EDIT: added spoiler formatting
vth is the RMS instantaneous velocity. Taking pictures at intervals gives an averaged velocity, which is slower because the particle wastes some time going in different directions that cancel out.v1 is going to be near the instantaneous velocity, but still a little slower, since the velocity is still going to decay slightly, even over 1/10th of the decay time.v2 is going to be significantly slower. If we make the time step even slower than 10 ms, we expect the RMS velocity to go roughly as the inverse square root of the timestep. Anyway, the answer should be 3:
v2<v1<vth