Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction on that system. Mathematically can be written as,

Here,
v = speed of the exhaust gases measured relative to the rocket.
= Rate of change of mass with respect to time
Our values are given as,


Replacing we have that


If the elevator's speed is constant (in ANY direction) and you can't see out of it, then there's NO measurement you can make inside that will tell you that it's moving. Any weight you measure is normal.
X rays because to see your bones