The question of whether heavier objects fall faster than lighter ones is often rooted in everyday observation. When dropping a bowling ball and a feather, the bowling ball hits the ground first. However, this observation is deceptive and does not reflect the fundamental law of gravity. In a perfect scenario, where only gravity acts on the objects, a heavy object and a light object accelerate downward at precisely the same rate. This counter-intuitive principle is a foundational concept in the science of motion.
The Definitive Answer: Acceleration Due to Gravity
The scientific baseline for understanding free fall is acceleration due to gravity, denoted by the symbol $g$. This value represents the constant rate at which any object accelerates toward the Earth’s center during free fall. Near the surface of the Earth, this acceleration is approximately $9.8 \text{ meters per second squared}$ ($9.8 \text{ m/s}^2$).
This rate of acceleration is independent of the object’s mass. For every second an object falls, its downward speed increases by $9.8 \text{ meters per second}$. Assuming no other forces intervene, a bowling ball and a small marble, despite their different weights, will increase their velocity at the exact same rate. This means they will travel the same distance in the same amount of time and strike the ground simultaneously.
Galileo’s Insight: Why Mass Is Irrelevant
The understanding that all objects fall at the same rate overturned the long-held belief put forth by Aristotle that heavier objects fall proportionally faster. The Italian physicist Galileo Galilei challenged this ancient view in the early 1600s through experiments and thought experiments.
Galileo realized that mass cancels out in free fall due to a perfect counterbalance between two physical properties: gravitational force and inertia. A heavier object experiences a greater gravitational force pulling it toward the Earth. However, that same object also possesses proportionally greater inertia, which is its resistance to acceleration.
According to Newton’s Second Law of Motion, the acceleration of an object is equal to the force acting on it divided by its mass. Because the gravitational force is directly proportional to the object’s mass, the greater force exerted on the heavier object is perfectly divided by its greater mass, resulting in the same net acceleration ($g$) for all objects. This principle was demonstrated in a vacuum on the Moon during the Apollo 15 mission, where astronaut David Scott dropped a hammer and a feather, which hit the lunar surface simultaneously.
The Real-World Factor: Air Resistance
The reason the ideal physics principle seems incorrect in daily life is the presence of air resistance, also known as aerodynamic drag. This external force opposes the motion of an object as it moves through the atmosphere, complicating the gravity-only free-fall scenario. Air resistance depends on the object’s speed, its cross-sectional area, and its overall shape.
As an object falls, its speed increases, and the upward drag force increases with the square of the object’s velocity. This upward drag force eventually balances the downward gravitational force, leading to zero net acceleration. The object then stops speeding up and continues falling at a constant velocity known as terminal velocity.
Lighter objects, like a feather, have a low mass relative to their large surface area. The force of air resistance quickly becomes large enough to balance the feather’s weight, causing it to reach a very low terminal velocity almost immediately.
Conversely, a dense object, such as a cannonball, has a small cross-sectional area relative to its mass. The cannonball needs a much higher speed to generate enough drag force to balance its greater weight, meaning it accelerates for a longer time and reaches a much higher terminal velocity. Therefore, the difference in falling time on Earth is due to the highly variable effect of atmospheric drag on objects of different shapes and densities, not gravity treating mass differently.