An inverted test tube partially filled with air in a plastic water-filled soft drink bottle as shown in the figure. The amount of air in the tube has been adjusted so that it just floats. The bottle cap is securely fastened. A slight squeeze of the plastic bottle will cause the test tube to sink to the bottom of the bottle. Explain this phenomenon.
Initially, the test tube is just floating because the tube is in equilibrium, ie the upward force and downward forces are equal
downward force= weight of tube + pressure of air inside the bottle
buoyancy force = weight of tube + pressure of air inside the bottle
when the bottle is squeezed the volume of bottle reduces, which causes an increase in pressure inside the bottle. Thus the downward force due to pressure increases and the tube sinks.