A common misunderstanding about falling is the idea that an object will continue to accelerate faster and faster until it hits the ground. When a human falls from a great height, the speed increases rapidly at first, but this acceleration does not continue indefinitely. The speed eventually levels off, creating a maximum limit to how fast they can fall. Understanding this limit requires looking at the forces at play beyond gravity, which ultimately control the final speed of any descent through the atmosphere.
The Force Behind the Fall: Initial Acceleration
The primary force initiating a fall is gravity, which pulls any object with mass toward the Earth’s center. This gravitational pull creates a constant rate of acceleration near the planet’s surface, denoted as g. This rate is approximately 9.8 meters per second squared (32 feet per second squared). This constant means that in every second an object falls, its downward speed increases by 9.8 meters per second. After two seconds, the speed would be 19.6 m/s, assuming no other forces are acting on it. If a human were to fall through a vacuum, this acceleration would continue uninterrupted until impact. However, on Earth, the presence of air introduces a counteracting factor that fundamentally changes the physics of the descent.
The Counteracting Force: Understanding Air Resistance
The atmosphere introduces a force known as air resistance, or drag, which acts in the opposite direction of the falling motion. As a human accelerates downward, they collide with air molecules, and this friction creates an upward push that opposes gravity. Initially, when the speed is low, the force of air resistance is very small, allowing the human to accelerate at nearly the full gravitational rate.
The magnitude of this drag force is not constant; it depends on several factors that change during a fall. The most significant factors are the falling object’s speed and its cross-sectional area, which is the surface area pushing against the air. As the speed of the fall increases, the air resistance force increases dramatically, specifically with the square of the velocity.
The air density and the object’s shape, quantified by its drag coefficient, also play major roles. A falling human can change their shape by spreading their limbs, which increases their cross-sectional area and drag. By manipulating their body position, a skydiver can actively control the amount of drag they create, which directly affects their rate of descent.
The Maximum Speed: Defining Terminal Velocity
Terminal velocity is the point where the upward force of air resistance exactly balances the downward force of gravity. Once these two opposing forces are equal, the net force on the falling human becomes zero. This balance means the object stops accelerating and continues to fall at a constant, maximum speed.
For a human in a standard, belly-to-earth position, terminal velocity is typically around 120 miles per hour (mph). This posture maximizes the surface area and drag, resulting in a lower maximum speed. Reaching this speed usually takes only about 10 to 15 seconds of freefall.
A person can significantly increase this speed by adopting a more streamlined shape, such as a head-first or head-down dive. By minimizing the cross-sectional area, a skydiver reduces the drag force, causing the balance of forces to occur at a much higher velocity. In this highly aerodynamic position, terminal velocity can range from 150 mph to over 180 mph, with some specialized speed divers exceeding 300 mph.