How Terminal Velocity Is Reached
A falling object does not usually keep accelerating at the same rate forever. As it speeds up, air resistance increases until it can balance the downward force of gravity. At that point the object continues falling at a constant speed called terminal velocity.
Weight starts the acceleration
When an object is released, its weight pulls downward. If it begins from rest, air resistance is initially small because the object is not moving quickly through the air. The downward force is larger than the upward resistance, so there is a downward resultant force.
A resultant force causes acceleration. The object's speed increases, but its mass and weight remain almost unchanged during an ordinary short fall. What changes strongly is the air resistance acting against the motion.
Air resistance comes from interactions with air particles and pressure differences around the object. It depends on speed, shape, surface area and air density. A broad flat object usually experiences more drag than a compact object of similar mass. A denser atmosphere can also produce more drag at the same speed, while thinner air usually produces less.
Drag grows as speed increases
As the object falls faster, it collides with and redirects more air each second. The upward drag force increases. The gap between weight and drag becomes smaller, so the downward resultant force and acceleration also become smaller.
Eventually drag becomes equal in size to weight. The forces are balanced, the resultant force is zero and acceleration stops. The object is still moving downward, but its velocity is now constant. Balanced forces do not mean the object must be stationary.
This constant falling speed is terminal velocity. It is not one universal value. A raindrop, a skydiver and a sheet of paper have different terminal velocities because their mass, shape and surface area differ. Terminal velocity is therefore a balance created by a particular object in particular conditions.
Changing shape changes the balance
A skydiver can alter drag by changing body position. Spreading arms and legs increases the area facing the airflow and lowers terminal velocity. Tucking into a compact shape reduces drag and allows a higher speed.
Opening a parachute creates a much larger surface area. Drag suddenly becomes greater than weight, producing an upward resultant force that slows the downward motion. As speed decreases, drag falls until it balances weight again at a new, lower terminal velocity.
Follow the force story:
- •Weight acts downward throughout the fall.
- •Drag acts upward against the motion.
- •Increasing speed produces increasing drag.
- •Equal weight and drag give zero acceleration.
- •A larger area usually gives a lower terminal velocity.
The takeaway
Terminal velocity occurs when air resistance balances an object's weight. Before that point, the object accelerates at a decreasing rate as drag grows. Track the two opposing forces rather than speed alone, and the stages of a fall become a clear example of how resultant force controls acceleration.