How can a basketball weigh 7.5 to 8.5 pounds when blown up but much less when deflated? What is it filled with when deflated?
I will answer your question in two parts. First, the actual weight of a basketball is dominated by its skin and, which weighs about 22 ounces (about 1.4 pounds). The air inside a properly inflated basketball weighs only about 0.03 pounds. Of that 0.03 pounds of air, only about 0.01 are measurable on a scale because buoyant effects due to the surrounding air support the other 0.02 pounds of air. That’s because the first 0.02 pounds of air put into the basketball simply fill it so that it’s spherical– air has gone from outside the basketball to inside the basketball and the scale won’t notice this change in location. Once you pump extra air into the ball, packing the air more tightly than normal and stiffening the ball’s surface, that additional air will appear on the scale’s weight measurement. A properly inflated basketball has about 0.01 pounds of extra air in it, so it’ll weigh an extra 0.01 pounds on a scale.
So what is the 7.5 to 8.5 pounds that a basketball is supposed to contain? Or the 13 pounds that a football is supposed to contain? Those aren’t weights at all. In fact, they are careless abbreviations for a different physical quantity: pressure. They should actually be written “7.5 to 8.5 pounds-per-square-inch” and “13-pounds-per-square inch” respectively.
Fluids such as air have pressures — the forces they exert on each unit of surface area they contact. For example, air that is listed as having a pressure of 7.5 pounds per square inch exerts a force of 7.5 pounds on each square inch of surface it touches. That means that the air in a properly inflated basketball pushes outward with a force of 7.5 to 8.5 pounds on each square inch of the inner surface of that ball. That outward push stretches the ball tight and gives it its feel and bounciness. Similarly, a properly inflated football has a pressure of 13 pounds per square inch and thus the air inside it exerts an outward force of 13 pounds on each square inch of surface inside the ball. Again, this outward push stretches the ball taut and gives it its bounciness and feel.
An underinflated basketball or football weighs just slightly less than a properly inflated ball because its skin hasn’t changed and the weight of the air it contains is so insignificant. But the decrease in outward forces on the skin of the ball significantly changes its feel and bounciness.