Terminal velocity is the highest velocity attainable by an object in free fall. It occurs once the sum of the drag force (Fd) and buoyancy equals the downward force of gravity (FG) acting on the object. Since the net force on the object is zero, the object has zero acceleration.
In fluid dynamics, an object is moving at its terminal velocity if its speed is constant due to the restraining force exerted by the fluid through which it is moving.
As the speed
of an object increases, the drag force acting on the object, resultant
of the substance (e.g., air or water) it is passing through, increases.
At some speed, the drag or force of resistance will equal the
gravitational pull on the object (buoyancy is considered below). At this
point the object ceases to accelerate and continues falling at a
constant speed called terminal velocity (also called settling velocity).
An object moving downward with greater than terminal velocity (for
example because it was thrown downwards or it fell from a thinner part
of the atmosphere or it changed shape) will slow down until it reaches
terminal velocity. Drag depends on the projected area, and this is why
objects with a large projected area relative to mass, such as
parachutes, have a lower terminal velocity than objects with a small
projected area relative to mass, such as bullets.
