As balloon goes up, the weight of air displaced becomes less and less. This means that the upthrust...
decreases. The balloon will stop rising when its weight matches the upthrust.
To understand how the upthrust changes as a balloon goes up, we need to consider the principle of buoyancy. Buoyancy is the upward force exerted on an object immersed in a fluid, such as air or water. According to Archimedes' principle, the buoyant force is equal to the weight of the fluid displaced by the object.
As a balloon rises, it moves into higher altitudes where the air density decreases. Air density refers to how much mass is present in a given volume of air. At higher altitudes, there is less air above the balloon, which means less air mass that can be displaced.
Since the upthrust or buoyant force is directly proportional to the weight of the air displaced, as the balloon goes up, the weight of air displaced becomes less and less. Therefore, the upthrust decreases as the balloon rises.
This decrease in upthrust also affects the overall weight of the balloon. The weight of the balloon (including the gas inside) remains constant, but the buoyant force decreases, reducing the overall force supporting the balloon. If the buoyant force becomes less than the weight of the balloon, it will no longer rise and will reach a point of equilibrium called the "float altitude" where it neither rises nor falls.