When something is dropped and is pulled by gravity towards the Earth, it accelerates as it falls until it reaches a velocity from which it can accelerate no further. This is called its terminal velocity. Terminal velocity happens when the force of the air resistance against the falling object increases to the point where it equals the force of gravity pulling the object. The terminal velocity of an object depends on how much air resistance it experiences. This is not affected by its weight but by its surface area and the streamlining of its shape.
Terminal velocity is achieved, therefore, when the speed of a moving object is no longer increasing or decreasing; the object’s acceleration (or deceleration) is zero. The force of air resistance is approximately proportional to the speed of the falling object, so that air resistance increases for an object that is accelerating, having been dropped from rest until terminal velocity is reached. At terminal velocity, air resistance equals in magnitude the weight of the falling object. Because the two are oppositely directed forces, the total force on the object is zero, and the speed of the object has become constant.
Terminal velocity, steady speed achieved by an object freely falling through a gas or liquid. A typical terminal velocity for a parachutist who delays opening the chute is about 150 miles (240 kilometres) per hour. Raindrops fall at a much lower terminal velocity, and a mist of tiny oil droplets settles at an exceedingly small terminal velocity. An object dropped from rest will increase its speed until it reaches terminal velocity; an object forced to move faster than its terminal velocity will, upon release, slow down to this constant velocity.
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