When an object falls through the air, it does not endlessly accelerate. Instead, it eventually reaches a constant speed known as terminal velocity. This concept explains why raindrops fall at a steady pace and why a skydiver’s descent eventually stabilizes.
How Terminal Velocity Works
Two primary forces govern a falling object’s movement through the atmosphere. Gravity continuously pulls the object downwards, causing it to accelerate. As the object gains speed, a second force emerges: air resistance, also known as drag. This drag force acts in the opposite direction of motion, pushing upwards against the falling object.
Air resistance increases as the object’s speed increases. Eventually, the upward force of air resistance becomes equal to the downward force of gravity. At this point, the net force on the object becomes zero. The object stops accelerating and continues to fall at its constant terminal velocity.
What Influences Terminal Velocity
Several factors determine an object’s terminal velocity. The object’s mass plays a role; a heavier object experiences a greater gravitational pull. This larger gravitational force requires a correspondingly larger drag force to achieve balance, meaning heavier objects generally attain higher terminal velocities.
The shape and size of an object also influence its terminal velocity. Objects with a larger cross-sectional area, or those that are less streamlined, experience more air resistance. This increased drag causes them to reach terminal velocity at a lower speed compared to objects with smaller, more aerodynamic profiles. Scientists use a “drag coefficient” to quantify how much an object’s shape and surface resist air flow.
The density of the fluid through which the object falls is another important factor. Air density, for instance, affects the magnitude of air resistance. Denser air creates more drag, leading to a lower terminal velocity for a given object. This means an object would fall slower in thick fog than in thin, dry air, assuming all other conditions are equal.
A Penny’s Journey to Terminal Velocity
A penny, due to its small mass and relatively flat shape, quickly reaches its terminal velocity after being dropped. This speed is typically in the range of 25 to 65 miles per hour (about 11 to 29 meters per second). The exact speed can vary depending on the penny’s orientation as it falls, as its flat profile often causes it to flutter like a leaf, increasing air resistance.
A penny generally achieves this constant speed after falling only about 50 feet (approximately 15 meters). Its small weight of approximately 2.5 grams means little drag is required to balance the gravitational pull, preventing further acceleration.
The Harmless Penny Myth
The idea that a penny dropped from a tall building could be lethal is a widespread urban legend. However, this is untrue. A penny quickly reaches its terminal velocity of 25 to 65 miles per hour, preventing it from accelerating to dangerous speeds.
At this speed, a falling penny poses minimal threat to a person on the ground. Its impact force is comparable to that of a pebble thrown by hand. Scientific tests, including those by the television show “MythBusters,” have shown that a penny falling at its terminal velocity cannot penetrate human skin or skull. This prevents it from becoming a deadly projectile.