Question

You take an empty glass jar and push it into a tank of water with the open mouth of the jar downward, so that the air inside the jar is trapped and cannot get out. If you push the jar deeper into the water, does the buoyant force on the jar stay the same? If not, does it increase or decrease? Explain.

Answer 1

Answer: it does not remain the same

Step-by-step explanation:

Immediately the jar enters the water, the buoyant force on the jar is equal to the weight of the water displaced by the jar + trapped air.

While the jar is pushed down into the water, the air trapped into the jar is compressed and its pressure increases. The compressed air now pushes down the surface of the water inside the jar, reducing the volume of the water displaced and its corresponding weight. Since the weight of the water displaced is reduced, so do the buoyant force while the jar is being pushed down into the water, but not fully submerged.

When the jar is fully submerged, the weight of the water displaced is constant and so is the buoyant force.