Air resistance (drag) is a resistive force that air exerts on an object moving through it, always pointing opposite the velocity and growing with speed. In AP Physics C, it's a nonconservative force that converts mechanical energy into thermal energy and leads to terminal velocity in falling objects.
Air resistance, usually called drag in Physics C, is the force air molecules exert on anything moving through them. It always points opposite the object's velocity, and unlike kinetic friction, its magnitude depends on how fast you're going. The two standard models you'll see are linear drag (F = bv, good for slow motion) and quadratic drag (F = cv², good for fast motion). It also depends on the object's cross-sectional area, shape (the drag coefficient), and the density of the air.
The two big consequences define how the term shows up in this course. First, in dynamics, drag grows as an object speeds up until it balances gravity, producing terminal velocity. Setting this up means writing Newton's second law as a differential equation, which is signature Physics C work. Second, in energy (Topic 3.3), drag is a nonconservative force. It does negative work on the object, so mechanical energy is not conserved. That "lost" mechanical energy doesn't vanish; it becomes thermal energy in the object and the surrounding air.
Air resistance maps to Topic 3.3, Conservation of Energy, and it's basically the reason that topic needs the word "conservation" qualified. The cleanest version of energy conservation (ΔK + ΔU = 0) only works when nonconservative forces do no work. The moment a problem includes drag, you switch to the full statement, where the work done by nonconservative forces equals the change in mechanical energy. Recognizing when that switch is required is one of the most common conceptual moves on the exam.
It also matters because Physics C is a calculus-based course, and velocity-dependent drag is the classic excuse to make you actually use calculus. F = -bv turns Newton's second law into a separable differential equation whose solution is an exponential approach to terminal velocity. If you can sketch v(t) for a falling object with drag and explain the asymptote, you understand the concept the way the exam wants you to.
Keep studying AP Physics C: Mechanics Unit 3
Nonconservative Force (Unit 3)
Air resistance is the textbook nonconservative force alongside friction. The work it does depends on the path taken, not just the endpoints, so you can never assign it a potential energy function. Any energy problem with drag must account for the negative work it does.
Terminal Velocity (Unit 2)
Terminal velocity is what happens when drag catches up to gravity. As a falling object speeds up, drag grows until it exactly balances mg, the net force hits zero, and velocity stops changing. Solving for it is just setting bv = mg (or cv² = mg) and solving for v.
Thermal Energy (Unit 3)
The mechanical energy that drag removes isn't destroyed. It's converted to thermal energy, heating the object and the air. This is the bookkeeping move that saves total energy conservation even when mechanical energy conservation fails.
Work-Energy Theorem (Unit 3)
The work-energy theorem still holds with drag because it counts the work of ALL forces, conservative or not. A skydiver at terminal velocity has zero net work being done (drag's negative work cancels gravity's positive work), which is exactly why kinetic energy stays constant.
Air resistance shows up two ways. In most FRQs, it's explicitly neglected ("assume air resistance is negligible"), and that phrase is your green light to use clean energy conservation or constant-acceleration kinematics, like in the 2019 FRQ on a pendulum-and-block system. When drag is NOT neglected, expect one of three tasks: (1) write Newton's second law as a differential equation with a -bv or -cv² term and solve or sketch v(t), (2) find terminal velocity by setting net force to zero, or (3) explain qualitatively how adding air resistance changes a result, such as why a projectile's range shrinks or why a returning ball lands slower than it launched. For energy questions, the move is always the same. Drag does negative work, so final mechanical energy is less than initial, and the difference went to thermal energy. Justification points often hinge on saying that clearly.
Both oppose motion and both are nonconservative, but kinetic friction has a roughly constant magnitude (μN) that doesn't care how fast you slide, while air resistance grows with speed (proportional to v or v²). That difference is huge mathematically. Friction problems give constant acceleration; drag problems give differential equations and exponential approaches to terminal velocity.
Air resistance (drag) always points opposite an object's velocity and increases with speed, modeled as F = bv at low speeds or F = cv² at high speeds.
Drag is a nonconservative force, so when it acts, mechanical energy is not conserved and the energy removed becomes thermal energy.
Terminal velocity occurs when drag grows to exactly balance gravity, making the net force zero and the velocity constant.
With linear drag, Newton's second law becomes a separable differential equation, and velocity approaches terminal velocity exponentially rather than instantly.
When a problem says to neglect air resistance, you can use clean conservation of mechanical energy; when drag is included, you must subtract the work it does.
A projectile with air resistance has a shorter range, a lower peak, and lands moving slower than it launched, because drag drains kinetic energy the whole flight.
It's the resistive force (drag) that air exerts on a moving object, pointing opposite its velocity and increasing with speed. In the energy unit, it's treated as a nonconservative force that converts mechanical energy into thermal energy.
No. The work drag does depends on the path traveled, so you can't define a potential energy for it. That's exactly why mechanical energy isn't conserved when air resistance acts.
Kinetic friction has a roughly constant magnitude (μN) regardless of speed, while drag depends on velocity (bv or cv²). Friction gives constant-acceleration problems; drag gives differential equations and terminal velocity.
No, energy is still conserved overall. Drag converts mechanical energy (kinetic plus potential) into thermal energy in the object and the air, so mechanical energy decreases but total energy doesn't.
Neglecting drag makes mechanical energy conserved and acceleration constant, so the math stays clean. When the exam wants you to include drag, it usually hands you a force law like F = -bv and asks for a differential equation, a v(t) sketch, or terminal velocity.