A skydiver does not get faster and faster forever. The air pushes back harder the quicker they fall, until that push exactly matches their weight. From that exact moment the speed holds perfectly steady, and they have reached terminal velocity.
The Key Idea
An object falling through a fluid has a constant weight downward and a drag (resistance) upward that grows with speed. When drag grows to equal the weight, the resultant force is zero, so there is absolutely no acceleration and the object falls at a constant terminal velocity.
SECTION 01
Drag catches up with weight.
Release the skydiver and watch the two arrows. Weight stays the same. Drag starts at zero and grows as they speed up. Once terminal velocity is reached, the "Deploy Parachute" button will unlock. Click it to dramatically alter the surface area and watch the forces suddenly flip.
SECTION 02
Three stages of a fall.
Just released: drag is extremely small, so weight is much larger; the resultant force is large and downward, and the object accelerates at close to g.
Speeding up: as the speed rises, drag rises too, so the resultant force shrinks and the acceleration slowly falls; the velocity-time curve bends over.
Terminal velocity: drag has grown to perfectly equal the weight, the resultant force is strictly zero, the acceleration is strictly zero, and the object falls at a constant velocity.
Constant velocity means zero resultant force
At terminal velocity the object is still moving, and moving incredibly fast, yet the resultant force is zero. Drag equals weight, so they completely cancel each other out. A common mistake is to think a moving object must have a forward (or downward) resultant force. It does not: constant velocity and completely balanced forces go together. This is exactly the same idea you will meet for a car cruising on a highway at a steady speed.
Worked Example
A skydiver opens their parachute. Describe and explain the immediate effect on their velocity and the forces acting on them.
Step 1 : Immediate Force Balance
Opening the parachute greatly increases the surface area, which immediately causes a massive increase in air resistance (drag). The drag is now much larger than the weight.
Step 2 : The Resultant Force
Because drag is acting upwards and is larger than the downward weight, the resultant force is suddenly upwards.
Step 3 : Effect on Motion
An upward resultant force on a falling object causes it to decelerate. The skydiver slows down until the drag shrinks back to equal the weight again, reaching a new, much safer terminal velocity.