A helium balloon is tied to a brick and dropped into the lake. As the balloo sinks the buoyant force acting on decrease.
To understand why the buoyant force acting on the helium balloon decreases as it sinks, we need to consider the principle of buoyancy.
Buoyancy is the upward force exerted on an object submerged in a fluid (in this case, water) and is dependent on the density of the fluid and the volume of the submerged object. The buoyant force is equal to the weight of the fluid displaced by the object.
When the helium balloon is first dropped into the lake, it has a greater density than the surrounding air, causing it to rise. As the balloon ascends, it displaces air with a greater weight than its own weight, resulting in an upward buoyant force that exceeds the downward force of gravity. This causes the balloon to float upward.
However, as the balloon sinks deeper into the lake, it enters a region of water, which is denser than air. The density of the water is much closer to the density of the balloon, so the buoyant force decreases. The balloon is now displacing water with a lower weight than its own weight. As a result, the buoyant force acting on the balloon decreases, and gravity becomes the dominant force, causing the balloon to sink further.
In summary, the buoyant force acting on the helium balloon decreases as it sinks because it starts displacing water, which has a density closer to that of the balloon, resulting in a lower buoyant force compared to when it was displacing air.