Two points of view *
In Scenario 1, a mass m moving with speed v collides with a mass 2m that is initially at
rest. They combine to form a particle with mass M . In Scenario 2, a mass 2m moving with
1
speed v collides with a mass m that is initially at rest. They combine to form a particle with
mass M . These two scenarios describe the same setup, just viewed in different reference
2
frames. So it must be the case that M = M . Verify this explicitly by doing the appropriate
1 2
calculation in the two different frames.