When a person steps out of an aircraft, they immediately accelerate downward due to gravity. As the body gains speed, it encounters air resistance, known as drag. This upward force opposes the downward pull of gravity. The interaction between these two opposing forces governs the physics of the fall.
Defining Terminal Velocity
Terminal velocity is the constant, maximum speed a falling object can achieve through a fluid, such as air. This speed is reached when the downward force of gravity is perfectly balanced by the upward force of air resistance. At this point of equilibrium, the net force acting on the body becomes zero.
Because the net force is zero, the body stops accelerating, and its speed stabilizes. The object continues to fall, but its velocity remains constant. For most practical purposes in freefall, the smaller buoyant force is disregarded because its effect is negligible compared to drag and gravity.
The Time and Distance Required to Reach Terminal Velocity
A human achieves stable terminal velocity in a standard body position relatively quickly. On average, a person falling belly-to-earth reaches terminal velocity in approximately 12 seconds. During this initial acceleration phase, the body covers about 1,500 feet (450 meters).
Once equilibrium is reached, the average speed for a human in this face-down posture is around 120 mph (195 kilometers per hour). The body does not suddenly hit this maximum speed. Velocity increases rapidly at first, reaching about 50% of its final terminal velocity in just the first three seconds.
The speed approaches terminal velocity asymptotically, meaning it gets closer without ever fully achieving it mathematically. A falling body typically reaches 90% of its maximum constant speed in about eight seconds. It takes roughly 15 seconds to achieve 99% of the terminal velocity, which is considered the stable speed for the remainder of the fall.
Key Factors Influencing the Descent
The time and speed required to reach terminal velocity are not fixed values; they fluctuate based on several physical variables, including the body’s orientation and the surrounding atmosphere. The most significant factor is the body’s projected surface area, which directly affects the amount of air resistance. A skydiver who spreads their limbs out in a belly-to-earth position maximizes their surface area, creating more drag.
This increased drag results in a lower terminal velocity, typically around 120 mph, and takes a slightly longer time to achieve. Conversely, if a person adopts a head-first or vertical dive position, they minimize their cross-sectional area and reduce the drag coefficient. This streamlined posture results in a much higher terminal velocity, with experienced skydivers reaching speeds around 200 mph.
The density of the air through which the person is falling also plays a significant role in determining the final speed. Air density decreases as altitude increases, which means the air is “thinner.” Since air resistance is directly proportional to air density, less dense air provides less drag force.
A fall initiated from a very high altitude will result in a higher terminal velocity because the body is accelerating through less-resistant air. As the person descends to lower altitudes where the air is denser, the drag force increases, and the terminal velocity will gradually decrease until it stabilizes at the lower-altitude value.