Objects dropped from the same height often reach the ground at different times. A feather drifts slowly downwards, while a stone plummets rapidly. This leads many to believe that heavier objects fall faster than lighter ones, but this intuition doesn’t fully capture the complexities of how objects move through the air.
The Universal Pull of Gravity
Gravity exerts a constant downward pull on all objects, regardless of their mass. In a perfect vacuum, a feather and a bowling ball dropped from the same height would accelerate at precisely the same rate and hit the ground simultaneously. Galileo Galilei famously demonstrated this phenomenon, theorizing that acceleration due to gravity is uniform for all objects near Earth’s surface. Earth’s gravity accelerates objects at approximately 9.8 meters per second squared, meaning an object’s downward speed increases by 9.8 meters per second every second it falls.
While the force of gravity acting on an object is directly proportional to its mass, this larger force on a more massive object is precisely what is needed to accelerate that greater mass at the same rate. Thus, gravity alone would cause all objects to fall at identical speeds if no other factors were involved.
How Air Resistance Changes Things
In the real world, falling objects encounter air, which creates a force known as air resistance or drag. This force acts opposite to the object’s motion, working against gravity. Air resistance is the primary reason why different objects fall at varying rates outside of a vacuum. Its magnitude depends on several factors related to the falling object and the surrounding air.
One factor is the object’s shape; a streamlined object experiences less air resistance than a blunt one. Its cross-sectional area also plays a role, as a larger area pushing through the air encounters more resistance. For example, a flat sheet of paper experiences more air resistance than a crumpled ball of the same paper due to its greater surface area. Air density also affects air resistance; objects fall slightly faster in less dense air, such as at higher altitudes.
Understanding Terminal Velocity
As an object falls, its speed increases, and consequently, the force of air resistance acting upon it also increases. This upward force continues to grow until it eventually balances the downward force of gravity. At this point, the net force on the object becomes zero, and it stops accelerating. The constant speed reached when air resistance equals gravity is known as terminal velocity.
For instance, a skydiver in a freefall will accelerate until they reach their terminal velocity, typically around 120 miles per hour (about 193 kilometers per hour) in a belly-to-earth position. A lighter object with a large surface area, like a snowflake, will have a much lower terminal velocity because it reaches equilibrium with air resistance at a significantly slower speed.
The Complete Picture of Falling Objects
The observed differences in how quickly objects fall are due to the varying degrees of air resistance they encounter. Air resistance introduces a counteracting force that is highly dependent on an object’s characteristics, including its shape, size, and how it presents itself to the air.
A heavy, dense object with a small cross-sectional area, such as a rock, experiences relatively little air resistance compared to the strong gravitational pull on its mass, allowing it to accelerate quickly to a high speed. Conversely, a light object with a large, spread-out form, like a parachute, generates substantial air resistance even at lower speeds. This significant drag quickly balances the smaller gravitational force, leading to a much slower descent. Therefore, the complete picture of falling objects involves a dynamic interplay between the constant acceleration due to gravity and the variable, opposing force of air resistance.