PRIDE CHEKERAFig 1 showing four lifts in four different conditions in revealing
principle of equivalence.
(1) We have the above lift which is at rest on the surface of the
Earth in Scenario (1). In this situation, the initial length of the
spring will change as it extended due to weight attached to it.
Thus the length increase with an extension ‘x’. This is due to the
force of gravity acting on the spring load.
(2) Let's take the same lift to a height of about 600km above
Earth's surface and allow it to free fall to Earth's surface. In free
fall the effect of gravity seems to be cancelled such that no
gravity is felt and no extension of the spring, Scenario (2).
(3) Now let's take the lift into deep space where there is no
gravitational attraction at all. Here as in scenario (2), the spring
is not extended and thus extension x is zero, as shown in
Scenario (3) above.
(4) In this last scenario the same rocket is in deep space where
there is no gravitational attraction, Scenario (4). Then we put a
rocket inside the lift; as the rocket accelerates upwards (towards
the roof of the lift, since there is no upwards or downwards in
space) it does so with the lift. Here as in scenario (1) the spring
is extended by an extension x as the weight attached to it move
towards the floor of the lift.