Friction at a surface is one example of a resistive force, a force that opposes or resists motion. Resistive forces are also experienced by objects moving through fluids—gases and liquids. The resistive force of a fluid is called drag, with symbol $\vec{F}{}_{\text{drag}}$. Drag, like kinetic friction, points opposite the direction of motion. FIGURE 5.9 shows an example.

FIGURE 5.9 Air resistance is an example of drag.

Drag can be a significant force for objects moving at high speeds or in dense fluids. Hold your arm out the window as you ride in a car and feel how the air resistance against it increases rapidly as the car’s speed increases. Drop a lightweight object into a beaker of water and watch how slowly it settles to the bottom.

For objects that are heavy and compact, that move in air, and whose speed is not too great, the drag force of air resistance is fairly small. To keep things as simple as possible, you can neglect air resistance in all problems unless a problem explicitly asks you to include it.