Our everyday experience suggests heavier items descend faster than lighter ones, but this intuition is not entirely accurate. A hammer and a feather can fall at the exact same rate under specific conditions. Understanding why this occurs requires examining the fundamental principles of gravity and the influence of air resistance.
Gravity’s Constant Acceleration
Gravity imparts the same acceleration to all objects, irrespective of their mass. Near Earth’s surface, this uniform acceleration, denoted as ‘g’, is approximately 9.8 meters per second squared (m/s²). This means an object’s downward velocity increases by about 9.8 meters per second each second, assuming no other forces are acting upon it.
While gravity exerts a greater force on more massive objects, this larger force acts on a proportionally larger mass. For instance, a hammer has more mass than a feather, so gravity pulls on the hammer with a greater force. However, according to Newton’s second law of motion (Force = mass × acceleration, or F=ma), a larger mass also requires a greater force to achieve the same acceleration. The increased gravitational force on a heavier object precisely balances its increased resistance to acceleration. All objects accelerate downwards at the same rate due to gravity, a principle true in the absence of external factors.
Understanding Air Resistance
Air resistance, also known as aerodynamic drag, is a force that opposes an object’s motion through the air. This force arises from collisions between the falling object and air molecules, effectively slowing the object down. The magnitude of air resistance depends on the object’s shape, its cross-sectional area, and its velocity.
Objects with larger surface areas or less streamlined shapes experience greater air resistance. As an object’s speed increases, the air resistance it encounters also increases significantly. A feather, with its large surface area and low mass, is highly susceptible to air resistance, which exerts a substantial opposing force relative to its weight. In contrast, a dense object like a hammer has a much smaller surface area relative to its mass, so air resistance has a minor effect. This difference explains why, in Earth’s atmosphere, a hammer appears to fall much faster than a feather.
The Vacuum Experiment: A Clear Demonstration
To observe gravity’s true effect without the interference of air resistance, experiments are conducted in a vacuum. A vacuum is an environment where air has been largely removed. In such conditions, gravity is the only significant force acting on falling objects. When air resistance is eliminated, objects of different masses and shapes fall at the same rate, reaching the ground simultaneously.
A compelling demonstration occurred during the Apollo 15 mission on the Moon in 1971. Lunar Module Commander David Scott dropped a 1.32-kilogram hammer and a falcon feather (weighing between 0.3 and 3 grams) from approximately 1.6 meters. Because the Moon has virtually no atmosphere, both objects fell unimpeded and struck the lunar surface at the same instant. This iconic experiment confirmed that in the absence of air, all objects fall at the same rate due to gravity.