When an apple and a feather are dropped from the same height, the apple consistently reaches the ground first. This observation challenges intuition, as one might expect heavier objects to fall more quickly.
The Constant Pull of Gravity
Despite common perception, gravity accelerates all objects at the same rate regardless of their mass. In a vacuum, a feather and an apple would accelerate downwards equally, increasing their speed by approximately 9.8 meters per second every second. This value, known as the acceleration due to gravity (g), is constant near Earth’s surface.
Galileo Galilei famously demonstrated this principle. He conducted an experiment from the Leaning Tower of Pisa, dropping objects of different masses simultaneously. His observations suggested that all objects fall with the same acceleration when other forces are absent.
Air Resistance
Air resistance, also known as drag, significantly influences how objects fall through Earth’s atmosphere. This force opposes an object’s motion through the air, becoming more pronounced as its speed increases.
Air resistance depends on several factors. Surface area is significant; objects with larger surface areas exposed to motion encounter more resistance. A flat feather, with its broad shape, presents a much larger surface area relative to its weight compared to a compact apple. The object’s shape also plays a role, with less aerodynamic shapes experiencing greater drag.
The density of an object is another important consideration. A feather has a very low density, meaning it has little mass distributed over a relatively large volume. In contrast, an apple is much denser. This combination of low density and large surface area for a feather results in substantial air resistance relative to its gravitational pull. As an object falls, its speed increases, and so does the air resistance acting upon it. Eventually, a falling object reaches a speed where the upward force of air resistance precisely balances the downward force of gravity. At this point, the net force on the object becomes zero, and it stops accelerating, continuing to fall at a constant speed known as terminal velocity. Because of its shape and low density, a feather reaches a much lower terminal velocity very quickly, while an apple, being denser and more compact, needs to fall much faster before air resistance can equal its greater gravitational force.
Falling Without Air: The Vacuum Experiment
The definitive demonstration that air resistance is the primary reason for differing fall rates involves conducting an experiment in a vacuum. In a vacuum chamber, all the air is removed, eliminating the force of air resistance entirely. When a feather and a heavier object, such as a bowling ball or an apple, are dropped simultaneously within this airless environment, they fall at the exact same rate.
This outcome powerfully illustrates that in the absence of any opposing forces, gravity accelerates all objects uniformly, regardless of their mass or composition. The feather, no longer impeded by air resistance, accelerates alongside the heavier object, striking the ground at precisely the same moment. This experiment provides compelling evidence that the perceived difference in falling speeds in our everyday environment is not due to gravity acting differently on objects of varying masses, but solely to the varying effects of air resistance on their specific shapes, sizes, and densities. Therefore, while gravity consistently pulls all objects downwards, the presence of air resistance creates the observable differences in their falling speeds.