When contemplating a fall, questions often arise about the speed a person might reach and what limits that speed. Understanding the science behind falling involves exploring the fundamental forces at play that determine the velocity of any object, including a human, as it moves through the atmosphere. This exploration reveals why objects accelerate and what eventually prevents them from speeding up indefinitely.
The Unseen Pull: How Gravity Accelerates Us
The primary force driving any fall is gravity, an invisible pull that constantly accelerates objects toward Earth. This acceleration means that as an object falls, its speed continuously increases. Centuries ago, Galileo Galilei demonstrated that in a vacuum, all objects fall at the same rate, regardless of their mass. This principle highlights gravity’s uniform effect, causing objects to gain speed equally under ideal conditions.
Near Earth’s surface, the acceleration due to gravity, symbolized as ‘g’, has a standard value of approximately 9.8 meters per second squared (m/s²) or about 32 feet per second squared (ft/s²). This means that for every second an object falls, its downward velocity increases by 9.8 m/s. If a human were to fall in a complete vacuum, their speed would continue to increase without limit.
The Invisible Brake: The Role of Air Resistance
As an object falls through Earth’s atmosphere, it encounters an opposing force known as air resistance, or drag. This force arises from the constant collisions between the falling object and air molecules. Air resistance acts as a brake, directly opposing the downward motion caused by gravity.
The magnitude of air resistance is not constant; it increases significantly as the falling object gains speed. This is because a faster-moving object collides with more air molecules per unit of time, creating a greater resistive force. Air resistance is also influenced by the object’s cross-sectional area and the density of the air itself. A larger cross-sectional area presents more surface for air molecules to push against, increasing the drag force.
The Speed Limit: Understanding Terminal Velocity
The interplay between the constant downward pull of gravity and the increasing upward force of air resistance leads to a specific speed limit for falling objects, known as terminal velocity. Terminal velocity is achieved when the force of air resistance exactly balances the force of gravity, resulting in zero net acceleration. At this point, the object stops speeding up and continues its descent at a constant, maximum velocity.
For a human, terminal velocity can vary significantly depending on body position. In a typical “spread-eagle” or belly-to-earth skydiving position, a human’s terminal velocity is usually around 120 miles per hour (mph), which is approximately 54 meters per second (m/s) or 193 kilometers per hour (km/h). However, by adopting a more streamlined, head-first or “pencil” dive position, a person reduces their cross-sectional area and can reach higher speeds, often ranging from 150 to 180 mph (240 to 290 km/h), and potentially exceeding 200 mph (322 km/h). Highly streamlined positions can even approach 300 mph (480 km/h).
Several factors influence a human’s specific terminal velocity. Body mass plays a role; a more massive person, assuming the same shape and orientation, will generally have a higher terminal velocity because a greater gravitational force requires a larger air resistance force to achieve equilibrium. The density of the air also affects terminal velocity; at higher altitudes where the air is less dense, there are fewer air molecules to collide with, leading to less air resistance and consequently a higher terminal velocity.