  thumb|terminal velocity where the downward force of gravity (fg) equals the air resistance/force of drag (fd) and buoyancy, velocity becomes asymptotic to a constant value with net acceleration = 0thumb|200px|right|the downward force of gravity (fg) equals the restraining force of drag (fd). the net force on the object is then zero, and the result is that the velocity of the object remains constant. the terminal velocity of an object is the velocity of the object when 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. ==examples== based on wind resistance, for example, the terminal velocity of a skydiver in a belly-to-earth (i.e., face down) free-fall position is about 195&nbsp;km/h (122&nbsp;mph or 54&nbsp;m/s). this velocity is the asymptotic limiting value of the acceleration process, because the effective forces on the body balance each other more and more closely as the terminal velocity is approached. in this example, a speed of 50% of terminal velocity is reached after only about 3 seconds, while it takes 8 seconds to reach 90%, 15 seconds to reach 99% and so on. higher speeds can be attained if the skydiver pulls i
Fluid_dynamics