If a body is falling freely and during the time of its fall a change takes place in the relative ⋅ positions of its component parts within the body, the body's centre of gravity continues to move with the acceleration of free fall, since displacements which take place as a result of the effect of internal forces cannot alter the position of the centre of gravity. Therefore the centre of gravity of the system test-tube-plus-fly will also move as before with the acceleration of free fall. Consequently, when the fly flies up from the lower part of the test-tube to the upper part, the bottom of the test-tube will be lowered somewhat relative to the centre of gravity of the whole system (test-tube-plus-fly). Thus the bottom of the test-tube will strike the ground sooner than if the fly were to remain motionless. In other words, the length of time of the fall is reduced.