Although we cannot see the air, it is still made of atoms and molecules, just like everything else. When an object passes through the air, these molecules push against it, causing a force of friction called air resistance.
Imagine dropping an apple from the top of a skyscraper and watching it fall. Intuitively, you might guess that it will be falling at a faster speed right before hitting the ground than at the moment you released it. In other words, the apple sped up while falling. However, is there a way to quantitatively describe an object’s motion through air? If we were to drop the same apple from an airplane, would it ever reach a maximum downward velocity?
Air resistance is the force that air exerts on objects moving through it. Scientists often refer to this force as drag or drag force, a term we’ll use interchangeably throughout the lesson. Note that in many cases, ‘drag’ can refer to any type of fluid, not just air. However, for the purposes of our discussion, we’ll always refer to air as the fluid.
Typically, this force is directed opposite to the object’s motion, thereby slowing it down. If you’ve ever held your hand out of a speeding car’s window, you’ve noticed how the air pushes your hand in a direction opposite to that of the car’s movement. When a piece of paper is dropped to the ground, air resistance slows down its fall.
Although we’ll only be focusing on the force that air molecules exert on solid objects, it’s important to mention that this force may exist between different fluid layers in addition to existing between a solid object and a fluid.
Picture Credit : Google