Air resistance is the opposing (drag) force that air molecules exert on an object moving through air. It always points opposite the object's velocity, grows with speed, and acts as a nonconservative external force that removes mechanical energy and momentum from a system.
Air resistance is the force air exerts on anything moving through it, always pushing against the direction of motion. Think of it as friction's airborne cousin. Instead of two surfaces rubbing, you have billions of air molecules colliding with the front of a moving object and slowing it down.
Two things make air resistance different from the friction you meet in Topic 2.6. First, it depends on speed. The faster an object moves, the harder the air pushes back, which is why a falling object can reach terminal velocity (the speed where drag balances gravity and acceleration drops to zero). Second, it depends on shape and cross-sectional area, which is why a flat sheet of paper falls slower than the same paper crumpled into a ball. Because air resistance is nonconservative, the work it does converts mechanical energy into thermal energy, and it never gives that energy back.
Air resistance lives in Unit 2 (Force and Translational Dynamics, Topic 2.6) and Unit 4 (Topic 4.3). In Unit 2, it's a force you put on free-body diagrams and feed into Newton's second law. A skydiver speeding up, drag growing, and acceleration shrinking toward zero is the classic second-law reasoning chain. In Unit 4, air resistance is the textbook example of a net external force. Learning objective 4.3.B says the momentum of a system changes only when a net external force acts on it. If your system is just the projectile and you include air resistance, momentum is NOT constant, because momentum is being transferred to the air (the environment). Air resistance is also why real projectiles lose mechanical energy. It explains the gap between idealized physics answers and lab data, which is exactly the kind of reasoning experimental-design FRQs reward.
Keep studying AP Physics 1 Unit 4
Friction (Unit 2)
Both are resistive forces that oppose motion, but kinetic friction is roughly constant while air resistance grows with speed. That speed dependence is the whole reason terminal velocity exists for falling objects but not for a block sliding on a frictional floor.
Constant Velocity (Unit 2)
Terminal velocity is just Newton's second law with zero net force. When air resistance grows until it equals gravity, acceleration hits zero and the object falls at constant velocity. Constant velocity does not mean no forces; it means balanced forces.
Total Mechanical Energy (Unit 4)
Air resistance is nonconservative, so it does negative work and drains mechanical energy into thermal energy. If a ball comes back down slower than it left, air resistance is where that kinetic energy went.
Surface Area (Unit 2)
Air resistance scales with the cross-sectional area facing the airflow. Same mass, bigger area, more drag. That's why a parachute works and why crumpling a paper changes how it falls even though its mass is unchanged.
Most of the time, the exam tells you to assume air resistance is negligible so the clean kinematics and energy equations apply, as in the 2022 short FRQ (clay and rubber sphere thrown horizontally) and the 2023 long FRQ (cart released on a ramp). Your job is to know what that assumption buys you and what breaks when it's removed. Expect MCQ stems asking how a velocity-time graph changes with air resistance (acceleration decreases over time, approaching terminal velocity), why an object lands with less speed than energy conservation predicts, or why measured acceleration in an experiment is smaller than the theoretical value. In FRQ paragraph responses, the winning move is to name air resistance as an external, nonconservative force, state its direction (opposite velocity), and trace the consequence: momentum transferred to the environment (LO 4.3.B) or mechanical energy converted to thermal energy.
Kinetic friction between solid surfaces is approximately constant (μk times the normal force) regardless of speed. Air resistance increases with speed, depends on shape and cross-sectional area, and has no coefficient-times-normal-force formula on the AP equation sheet. The big consequence: an object falling through air can reach terminal velocity, while kinetic friction just produces constant deceleration. On a free-body diagram, both point opposite the motion, but only air resistance changes magnitude as the object speeds up or slows down.
Air resistance is a drag force that always points opposite an object's velocity and increases as the object moves faster.
Terminal velocity happens when air resistance grows large enough to balance gravity, making the net force and acceleration zero.
Air resistance is nonconservative, so it does negative work and converts mechanical energy into thermal energy that you can't get back.
If air resistance acts on your chosen system, it is an external force, so the system's momentum is not conserved (LO 4.3.B).
When a problem says to neglect air resistance, it's telling you projectile acceleration is exactly g and mechanical energy is conserved.
In lab-based FRQs, air resistance is a common physical reason why measured acceleration or final speed comes out lower than the idealized prediction.
Air resistance is the opposing force air molecules exert on an object moving through the air. It points opposite the velocity, grows with speed, and depends on the object's shape and cross-sectional area.
No. Air resistance depends on speed, shape, and cross-sectional area, not mass directly. Mass matters indirectly because a heavier object needs more drag to balance its weight, so it reaches a higher terminal velocity than a lighter object of the same shape.
Kinetic friction is roughly constant (μk·N) no matter how fast the object slides, while air resistance increases with speed. That's why falling objects reach terminal velocity but sliding blocks just decelerate at a constant rate.
Not for the object alone. Air resistance is a net external force, so per LO 4.3.B the object's momentum changes and momentum is transferred to the air. If you expanded the system to include the air, total momentum would still be conserved.
Neglecting air resistance makes projectile acceleration exactly g and keeps mechanical energy conserved, so the standard kinematics and energy equations work cleanly. Released FRQs like the 2022 clay-and-sphere question and the 2023 cart-on-a-ramp experiment use this assumption so you can apply the idealized models.