6.4 Drag force and terminal speed

Another interesting force in everyday life is the force of drag on an object when it is moving in a fluid (either a gas or a liquid). You feel the drag force when you move your hand through water. You might also feel it if you move your hand during a strong wind. The faster you move your hand, the harder it is to move. You feel a smaller drag force when you tilt your hand so only the side goes through the air—you have decreased the area of your hand that faces the direction of motion.

Drag forces

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 not moving too slowly, the magnitude of the drag force $F_{\text{D}}$ is proportional to the square of the speed of the object. We can write this relationship mathematically as $F_{\text{D}}\propto v^2.$ When taking into account other factors, this relationship becomes

where C is the drag coefficient, A is the area of the object facing the fluid, and $\rho$ is the density of the fluid. (Recall that density is mass per unit volume.) This equation can also be written in a more generalized fashion as $F_{\text{D}}=b{v}^2,$ where b is a constant equivalent to $0.5C\rho A.$ We have set the exponent n for these equations as 2 because when an object is moving at high velocity through air, the magnitude of the drag force is proportional to the square of the speed. As we shall see in Fluid Mechanics , for small particles moving at low speeds in a fluid, the exponent n is equal to 1.

Athletes as well as car designers seek to reduce the drag force to lower their race times ( [link] ). Aerodynamic shaping of an automobile can reduce the drag force and thus increase a car’s gas mileage.

The value of the drag coefficient C is determined empirically, usually with the use of a wind tunnel ( [link] ).

The drag coefficient can depend upon velocity, but we assume that it is a constant here. [link] lists some typical drag coefficients for a variety of objects. Notice that the drag coefficient is a dimensionless quantity. 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 (about 45–50 mi/h). For this reason, during the 1970s oil crisis in the United States, maximum speeds on highways were set at about 90 km/h (55 mi/h).