Subatomic particles called muons can be created in the upper atmosphere by colli
ID: 2304889 • Letter: S
Question
Subatomic particles called muons can be created in the upper atmosphere by collisions of cosmic rays (energetic particles coming from astrophysical sources). As we shall see in a few lectures, muons quickly decay (in 2.2 microseconds on average) into other particles. Consider a muon that was created at a height the same as the top of a nearby mountain, traveled straight down towards the Earth through the atmosphere at 0.9 times the speed of light, and then decayed at a point the same height as that of a tree it reaches, after exactly 2.2 microseconds in the muon frame.
a) How fast in fraction of the speed of light was the muon moving in its own rest frame?
b) In the muon’s rest frame, how fast in fraction of the speed of light is the tree moving?
Explanation / Answer
a] In its own frame of reference, the muon was traveling at a velocity of v = 0 m/s since that is its rest frame.
b] In the frame of reference of the muons, the trees are the ones that are moving and they are moving at a velocity of
v = 0 + 0.9c = 0.9c.