Free fall and terminal velocity

A fluid is a substance that can flow: both liquids and gases are fluids. An object falling through a fluid experiences resistance to its motion, called drag. In air, this is usually called air resistance.

Consider a skydiver beginning to fall. Weight acts downwards, while air resistance acts upwards, opposing the downward motion. The weight remains approximately constant during the fall, but air resistance increases as the skydiver speeds up.

The resultant force is the overall force after the forces have been combined. When weight is greater than air resistance, the resultant force is downwards, so the skydiver accelerates downwards.

How terminal velocity develops

The fall can be understood in three stages:

1. At first, resistance is small. The skydiver is moving slowly, so air resistance is much smaller than weight. There is a large downward resultant force and the skydiver speeds up.
2. As speed increases, resistance increases. The upward air resistance becomes larger, while weight stays approximately constant. The downward resultant force becomes smaller, so the acceleration decreases. The skydiver is still speeding up, but gains less speed each second.
3. Eventually, the forces balance. Air resistance becomes equal to weight. The resultant force is now zero, so the acceleration is zero. The skydiver continues falling at a constant velocity called terminal velocity.

At terminal velocity, neither force has disappeared: weight still pulls downwards and air resistance still acts upwards. Their effects balance. This explains why the skydiver continues moving rather than stopping.

The same sequence applies to an object falling through a liquid: it initially accelerates, resistance increases as it speeds up, and it eventually reaches terminal velocity when the resultant force becomes zero.

• Initially: weight exceeds resistance → downward resultant force → acceleration.
• As speed rises: resistance increases → resultant force and acceleration decrease.
• At terminal velocity: forces balance → zero resultant force → zero acceleration → constant downward velocity.

Weight stays approximately constant; resistance changes with speed.