While running these tests, crall notices a similarity in the velocity measured at the ground independent of whether the tennis ball is thrown upward or downward. she decides to see if her hypothesis that the final speed is independent of the sign of the initial velocity is correct. crall and whipple decide to run the experiment several times using different initial speeds of the elevator each time comparing the final velocity of the tennis ball when the elevator is traveling upward vs. downward at the same speed?

At the ground the ball will always have velocity along the direction of gravity. If upward motion is taken positive it will always have negative velocity at the ground because, if the ball was given an initial upward velocity then gravity will decelerate it and bring it down with a negative final velocity. If the ball is given an initial downward velocity then the ball will be further accelerated by gravity in the downward direction only, again maintaining negative direction. The magnitude however in both cases will be different. the final velocity at the ground will have higher magnitude in case of elevator moving downwards.