6.9: Drag Force and Terminal Speed
An interesting force in everyday life is the force of drag on an object when it is moving in a fluid. Like friction, the drag force always opposes the motion of an object. Unlike simple friction, the drag force is proportional to some function of the velocity of the object in that fluid. This functionality is complicated and depends upon the shape of the object, its size, its velocity, and the fluid it is in. For most large objects, such as cyclists, cars, and baseballs, that are not moving too slowly, the magnitude of the drag force is proportional to the square of the speed of the object.
Athletes, as well as car designers, seek to reduce the drag force to lower their race times. The aerodynamic shaping of an automobile can reduce the drag force and so increase a car's gas mileage. At highway speeds, over 50% of the power of a car is used to overcome air drag. The most fuel-efficient cruising speed is about 70–80 km/h. For this reason, during the 1970s oil crisis in the United States, maximum speeds on highways were set at about 90 km/h
While falling under the influence of gravitational force, the maximum velocity achieved by any object is called the terminal velocity. For example, a skydiver with a mass of 75 kg achieves a terminal velocity of about 350 km/h while traveling in a pike (head first) position, minimizing the area and the drag. In a spread-eagle position, that terminal velocity may decrease to about 200 km/h as the area increases. This terminal velocity becomes much smaller after the parachute opens.
The effect of size of the object that is falling through the air presents another interesting aspect of air drag. If a person falls from a 5 meter high tree branch, they will likely get hurt and possibly fracture a bone. However, small squirrels do this all the time without getting hurt. This is because the person does not reach terminal velocity in such a short distance but the squirrel does.
