The common sight of a bowling ball plummeting while a feather drifts slowly downward suggests that heavier objects fall faster. Understanding why some objects fall slower than others requires looking beyond mere weight and exploring the fundamental forces at play. The answer lies in a dynamic balance between the universal force that pulls everything toward the Earth (gravity) and the opposing force exerted by the air we breathe (air resistance). This dynamic interaction determines the unique path and speed of every object dropped in our atmosphere.
Gravity: The Constant Pull
The primary force driving all falling objects is gravity, which is a constant and predictable acceleration for everything on Earth. Near the planet’s surface, this gravitational influence causes any object to gain speed at a uniform rate of approximately 9.8 meters per second every second.
This principle dictates that, in a perfect vacuum where no air is present, all objects fall at precisely the same rate, regardless of their mass or composition. The Italian scientist Galileo Galilei first proposed this concept centuries ago, suggesting that a heavy cannonball and a light pebble would land simultaneously if air resistance were removed. His experiments provided evidence for this constant rate of acceleration.
This theoretical idea was visibly confirmed in 1971 during the Apollo 15 mission when astronaut David Scott dropped a hammer and a feather on the Moon. Since the Moon has virtually no atmosphere to impede the objects, the heavy hammer and the light feather struck the lunar surface at the exact same moment. This demonstration confirmed that mass does not influence the rate of fall under gravity alone.
Air Resistance: The Opposing Force
The reason objects fall differently in the Earth’s atmosphere is due to the presence of air, which introduces a counter-force known as air resistance, or drag. This force is a type of friction that arises when a falling object collides with the countless air molecules in its path. Air resistance always acts in the direction opposite to the object’s motion, working directly against the downward pull of gravity.
The magnitude of this opposing force is not constant; instead, it increases as the object’s speed increases. As an object starts to fall, it accelerates rapidly due to gravity, but the growing speed causes it to encounter more air molecules per second. This results in a progressively stronger drag force pushing upward on the object.
This relationship means that the net force acting on the object is the difference between the constant downward pull of gravity and the ever-increasing upward force of air resistance. Initially, gravity is much stronger, causing acceleration. As the object speeds up, the drag force grows until it determines the object’s overall motion.
How Physical Properties Determine Drag
The amount of air resistance an object experiences is highly dependent on its specific physical characteristics, which explains why a feather is slowed more than a hammer.
Cross-Sectional Area and Shape
One of the most significant factors is the object’s cross-sectional area, which is the area facing the direction of motion. An object with a large surface area, like a flat sheet of paper, pushes aside and interacts with a greater volume of air molecules than a compact object of the same weight, leading to a much larger drag force.
The object’s shape is also highly influential, quantified by a value called the drag coefficient. Streamlined or aerodynamic shapes, such as a pointed bullet, have a low drag coefficient and allow air to flow smoothly around them. Conversely, blunt or irregular shapes, like a parachute, have a high drag coefficient, creating turbulence and significantly increasing the opposing force.
The Mass-to-Area Ratio
The most crucial determinant of a fall rate is the ratio of the object’s mass to its cross-sectional area. A heavy object, such as a dense metal ball, has a large gravitational force pulling it down relative to its surface area, meaning the air resistance has less effect on its overall acceleration. A light object, like a piece of foam, has a small gravitational force relative to its surface area, allowing the air resistance to quickly overcome the downward pull, dramatically slowing its fall.
Terminal Velocity: The Speed Limit of Falling Objects
The difference in fall rates is ultimately explained by the concept of terminal velocity, which represents the highest constant speed an object can achieve while falling through a fluid like air. As an object accelerates downward, the drag force opposing its motion continues to increase. Eventually, the upward drag force becomes equal in magnitude to the constant downward force of gravity.
At this moment, the net force on the object becomes zero, and the object stops accelerating. It continues to fall, but at a steady, constant speed, which is its terminal velocity. This speed limit is unique for every object and is determined by the balance between its weight and its specific drag characteristics.
Objects that are very light or have a large surface area relative to their mass, such as a snowflake or a skydiver with an open parachute, reach a low terminal velocity very quickly. Conversely, dense objects with a small surface area, like a golf ball or a skydiver in a tuck position, must reach a much higher speed before the air resistance is large enough to balance the strong gravitational pull.