Drag is the frictional force experienced by an object travelling through a fluid. Its magnitude depends on several factors, including the speed of the object, the shape (cross-sectional area) of the object, the roughness or texture of the object, and the density of the fluid through which it travels. The drag an object experiences when travelling through air is called air resistance.
Terminal velocity is the maximum velocity that an object falling in a uniform gravitational field can reach, i.e. when its drag equals its weight.
- The instant an object starts to fall, there is no drag force on it so the resultant force equals its weight. Therefore, it accelerates downwards (with an acceleration of g, the acceleration of free fall).
- As the object falls, its speed increases, and so the opposing drag force increase. This means that the resultant force on the object decreases and so its instantaneous acceleration decreases (becomes less than g).
- Eventually the object reaches terminal velocity – the drag force acting upon it equals is equal and opposite to its weight. There is no resultant force. At terminal velocity, the object has no acceleration so its speed is constant.
You can determine terminal velocity in fluids with light gates/motion sensors. The terminal velocity is reached when the velocity no longer increases. Using a pulley, the falling object can be attached to a light polystyrene ball by a thin thread; the light ball’s motion is identical to that of the falling object but easier to measure.