Question

Air resistance, or drag, is a function of fluid density, velocity of the object, and the reference area.

Why doesn't an object's density affect its air resistance? If a tennis ball and a metal ball of same size are thrown in the air - doesn't the tennis ball experience greater air resistance?

Answer 1

Cause only the body surface is in contact with the fluid. The density of the body doesn't affect the drag because it is an internal property, not concerning the surface.

Answer 2

The particular surface of the tenis ball makes the boundary layer become turbulent. It has more drag, but more energy too, remaining the boundary layer bonded on the body surface. This allows the tennis ball to reach longer distances.

Answer 3

Suppose that the tennis ball's surface were as smooth as the metal ball's.

Air resistance is proportional to the square of the speed of the object. Therefore, it is roughly equal to bv^2 where b is a constant determined by the surface in the direction of motion.

The air resistance for both objects is the same, but mg, the gravitational force, is much larger for the metal ball, so it will have a greater terminal velocity.

Answer 4

Perfect explanations - thank you! :)