When an object falls through the air, it accelerates due to gravity. However, this acceleration does not continue indefinitely. Instead, a maximum speed is reached where the forces acting on the object balance each other. This constant speed is known as terminal velocity, representing the fastest an object can fall through a fluid like Earth’s atmosphere.
Understanding Terminal Velocity
As an object falls, gravity causes it to accelerate, increasing its speed. This increased speed leads to greater air resistance, or drag, which grows stronger as the object moves faster.
Terminal velocity is reached when the upward force of air resistance precisely counteracts the downward pull of gravity. At this equilibrium, the net force on the object becomes zero, and it stops accelerating. The object then continues to fall at a steady, constant speed.
Factors Influencing Terminal Velocity
Several factors significantly affect a human’s terminal velocity. Body position plays a considerable role, as it dictates the amount of surface area exposed to the air. A spread-eagle or belly-down position, for instance, maximizes air resistance, resulting in a lower terminal velocity, typically around 120 miles per hour (54 meters per second). Conversely, a head-first, streamlined dive minimizes air resistance, allowing for a higher terminal velocity that can exceed 200 miles per hour (89 meters per second).
The mass of the individual also contributes, with heavier objects generally achieving a slightly higher terminal velocity, though air resistance has a more pronounced effect for humans. Air density is another important variable; thinner air at higher altitudes offers less resistance, leading to a greater terminal velocity compared to denser air at lower altitudes. The object’s shape and surface area are primary determinants of the drag generated, directly influencing the final speed.
The Time to Reach It
For a human falling through the atmosphere, it takes approximately 10 to 12 seconds to reach terminal velocity. During this period, a falling person generally covers about 1,500 feet (450 meters). These are averages, varying with factors like body orientation and atmospheric conditions.
The process involves an initial rapid acceleration due to gravity. As speed builds, air resistance increases, causing the rate of acceleration to gradually decrease. The body continuously approaches this maximum speed, becoming effectively constant for practical purposes after about 12 seconds.
Real-World Scenarios
Skydiving provides a common real-world illustration of humans reaching terminal velocity. Skydivers frequently manipulate their body positions to control their descent rate. Adopting a stable, belly-to-earth position allows them to reach a relatively consistent terminal velocity, typically around 120 miles per hour. By altering their posture, such as tucking into a head-down dive, they can reduce their surface area and increase their speed, sometimes exceeding 200 miles per hour.
Upon reaching terminal velocity, skydivers experience a sensation of steady pressure from the air pushing upwards against their body. The deployment of a parachute drastically increases air resistance, slowing descent to a safe landing speed.