Real objects do not fall forever faster. Drag grows with speed until it balances the weight, the resultant force vanishes, and the fall settles to a steady terminal velocity. The same idea explains why a feather and a parachute drift while a stone plummets.
Frictional, viscous and drag forces oppose motion and grow with speed. An object released in air first accelerates at g while the drag is small. As it speeds up the drag rises until it equals the weight; the resultant force is then zero, the acceleration is zero, and the object continues at a constant terminal velocity.
Drop an object and watch the two arrows. The weight is fixed; the drag grows as the speed grows. When the drag arrow matches the weight arrow, the velocity-time graph flattens at the terminal value. Change the mass and the air resistance and see the terminal velocity shift.
| Stage | Forces | Motion |
|---|---|---|
| just released | drag ≈ 0, so resultant ≈ weight | acceleration ≈ g (maximum) |
| speeding up | drag grows, resultant falls | still accelerating, but less and less |
| terminal velocity | drag = weight, resultant = 0 | constant velocity, zero acceleration |
Drag depends on speed and on the size and shape of the object. A larger surface area gives more drag at a given speed, which lowers the terminal velocity; more weight raises it. At terminal velocity, drag = weight.
Four quick checks on drag and terminal velocity. Each correct answer earns XP and lights this skill on your star map.
When a falling object has reached terminal velocity, the resultant force on it is:
As a falling object speeds up through the air, the drag force on it:
At the very instant an object is released from rest in air, its acceleration is:
Two balls have the same size and shape but different masses. Falling through air, the heavier ball has a terminal velocity that is:
Terminal velocity is a constant velocity, which means the acceleration is zero, not that there are no forces. The weight and the drag are both still acting; they are simply balanced. A second trap: when a parachute opens the diver slows down but does not stop and does not move upward. The drag exceeds the weight only while the diver is moving fast, so it decelerates the diver to a new, lower terminal velocity. Saying the diver "moves back up" is wrong.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and 2 standard.
A sphere of weight 6.0 N falls through a liquid where the drag force is given by D = 0.50v, with D in newtons and v in m s⁻¹. Its terminal velocity is:
Which velocity-time graph best describes an object dropped from rest and reaching terminal velocity in air?
A skydiver falling at terminal velocity opens a parachute. Immediately afterward, the diver:
Two identical-shaped paper shapes are dropped: one flat sheet and the same sheet crumpled into a ball. The crumpled ball reaches the floor first mainly because it has:
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