Drag is a force that opposes or retards the motion of an object as it moves through a fluid—a substance that flows, such as air or water. You experience drag forces due to the air—sometimes called air resistance—whenever you jog, bicycle, or drive your car. In biology, drag forces are extremely important for microorganisms moving in water. The drag force $\vec{F}{}_{\text{drag}}$

• Is opposite the direction of motion, as shown in FIGURE 6.18.

• Increases in magnitude as the object’s speed increases.

FIGURE 6.18 The drag force is opposite the direction of motion.

Drag is more complex than friction because there are two different physical causes of drag. For a relatively large object, such as a ball that has been thrown, most of the drag—the force that resists the ball’s motion—occurs because the moving ball has to push away the air in front of it. This drag is said to be caused by inertial forces, forces that arise because a fluid’s mass makes the fluid difficult to move. In contrast, for a small particle, such as dust that is slowly sinking through a liquid, most of the drag is due to viscous forces from the fluid molecules sticking to one another and to the particle.

Fortunately, in many situations only one of these causes of drag is significant; that is, the drag is almost entirely due to inertial forces or almost entirely due to viscous forces. Fairly simply models will allow us to characterize drag in these cases. We can tell whether the drag is inertial or viscous by examining a quantity called the Reynolds number.