When an object falls, it accelerates rapidly due to gravity. This acceleration does not continue indefinitely because the object is moving through air, a fluid medium. The maximum, constant speed an object can achieve during a fall is known as its terminal velocity. Understanding the physics behind this limit reveals the maximum speed a human can reach and how a person can actively change their rate of descent.
The Physics of Freefall: What is Terminal Velocity?
Terminal velocity is the point where the downward force of gravity is perfectly balanced by the upward force of air resistance, also called drag. Gravity acts as a constant downward pull, causing any object in freefall to accelerate toward the Earth. As the falling speed increases, the opposing force of air resistance grows stronger. This drag force depends on air density, the object’s speed squared, and its cross-sectional area. When the drag force exactly matches the gravitational force, the net force becomes zero, and acceleration stops, allowing the object to continue falling at this steady, constant maximum speed.
The Speed Limit: How Fast Can a Human Fall?
The terminal velocity for a human is not a fixed number, but a range determined by body orientation and air density. The most common and stable orientation for skydivers is the “belly-to-earth” position, where the body is spread out to maximize air resistance. In this position, the terminal velocity for an average human is approximately 120 miles per hour (193 kilometers per hour). This speed is typically reached after about 12 seconds of freefall. While mass plays a role, the body’s shape and resulting cross-sectional area are far more significant in determining the final speed.
Manipulating the Fall: Factors that Change Terminal Velocity
Skydivers regularly manipulate their terminal velocity by intentionally changing their body position to alter the drag force. The primary factor controlled is the cross-sectional area, the amount of body surface presented to the oncoming air. Spreading the limbs wide, as in the belly-to-earth position, increases this area and results in the lower terminal velocity of around 120 mph. Conversely, streamlining the body drastically increases the rate of descent by minimizing surface area. By tucking the arms and legs close to the body, a person can achieve a much higher terminal velocity, often ranging from 150 to 180 mph.
Air Density and Altitude
The density of the air itself also affects the maximum speed attainable. Air density decreases as altitude increases, meaning there are fewer air molecules to create resistance at higher elevations. Because of this thinner air, an object falling from an extreme altitude experiences less drag and reaches a higher terminal velocity. This results in significantly faster speeds during the initial phase of the fall before the skydiver descends into the denser air closer to the ground.