AP Physics 1 Key Terms & Vocabulary

113 essential AP Physics 1 terms, defined — aligned to the College Board CED.

A
absolute pressure: Total pressure including the atmosphere, P = P₀ + ρgh, where P₀ is surface (atmospheric) pressure and ρgh is the depth contribution.
acceleration: The rate of change of velocity, a = Δv/Δt (m/s²). A nonzero acceleration means speed OR direction is changing — an object can have zero velocity yet nonzero acceleration, like a ball at the top of a toss.
amplitude: The maximum displacement from equilibrium. In SHM it does NOT affect the period, but total energy scales with A² (E = ½kA²).
angular acceleration: The rate of change of angular velocity, α = Δω/Δt (rad/s²). It's what net torque produces, just as net force produces linear acceleration.
angular displacement: The angle through which an object rotates (θ), measured in radians for AP work. 3 full turns = 6π rad, not 1080° — plugging degrees into rotational equations silently corrupts the answer.
angular impulse: The change in angular momentum from a torque acting over time, ΔL = τΔt. The rotational version of impulse FΔt.
angular momentum: The rotational analog of linear momentum, L = Iω. A point mass moving in a straight line also has L = mvr⊥ about a point.
angular velocity: How fast something rotates, ω = Δθ/Δt (rad/s). Every point on a rigid body shares the same ω, no matter its distance from the axis.
apparent weight: The reduced weight an object seems to have in a fluid: actual weight minus the buoyant force. A scale reads mg − F_b for a submerged object.
apparent weightlessness: Astronauts feel weightless because they and their station are in free fall together — continuously falling toward Earth while moving sideways fast enough to keep missing it.
arc length: The distance traveled along a circular path, s = rθ (θ in radians). Links the rotational angle to the actual linear distance covered.
Archimedes' principle: The buoyant force equals the weight of the fluid displaced. Two objects displacing the same volume feel the same buoyant force, regardless of their own weights.
atmospheric pressure: The pressure from the column of air above you. It decreases with altitude because there's less air weighing down from above.
average velocity: Total displacement divided by total time, v_avg = Δx/Δt. It describes the whole trip, not any single instant — you can average 60 km/h yet hit 95 km/h along the way.
B
Bernoulli's equation: Conservation of energy per unit volume in a flowing fluid: P + ½ρv² + ρgy = constant — pressure energy, kinetic energy, and gravitational PE trade off.
Bernoulli's principle: Along a horizontal streamline, faster-moving fluid has LOWER pressure. It comes from energy conservation: P + ½ρv² + ρgy = constant.
buoyant force: The upward force a fluid exerts on a submerged/floating object, F_b = ρ_fluid·V_displaced·g — equal to the weight of fluid displaced, independent of the object's own weight.
C
center of mass: The single point that moves as if all of a system's mass were concentrated there. Internal forces (like a diver curling up) can't change its path — only external forces can.
centripetal acceleration: The center-pointing acceleration of circular motion, a_c = v²/r. Even at constant speed the direction keeps changing, so there's always acceleration.
centripetal force: The net inward force that keeps an object moving in a circle, F_c = mv²/r. It's not a new force — it's provided by tension, gravity, friction, or normal force pointing toward the center.
centripetal force in orbit: Gravity supplies the centripetal force that continuously bends an orbiting object's straight-line path into a circle, F = GMm/r².
coefficient of friction: The dimensionless number μ relating friction to normal force. Static μ_s is larger than kinetic μ_k, which is why it's harder to start a crate sliding than to keep it moving.
conservation of angular momentum: When net external torque is zero, total angular momentum stays constant. Pulling mass inward lowers I, so ω rises to keep L = Iω fixed.
conservation of energy: Total energy is never created or destroyed, only transferred or transformed. When a block loses mechanical energy to friction, that energy becomes heat — it isn't gone.
conservation of momentum: When the net external force on a system is zero, its total momentum stays constant. It holds in BOTH elastic and inelastic collisions, because internal forces come in canceling third-law pairs.
conservative force: A force like gravity or the spring force whose work depends only on start and end points, not the path taken. This is why gravitational PE depends only on height, no matter the route up.
continuity equation: For an incompressible fluid, volume flow rate is constant: A₁v₁ = A₂v₂. Where the pipe narrows, the fluid must speed up.
D
density: Mass per unit volume, ρ = m/V (kg/m³). Same material means same density regardless of size — it sets whether an object sinks or floats.
displacement: The straight-line change in position from start to finish, Δx (a vector). Walk 5 m north then 3 m south and your displacement is 2 m north — it can be smaller than the distance traveled.
distance: The total path length traveled (a scalar, always positive). Walk 5 m north then 3 m south and your distance is 8 m even though you end up only 2 m from start.
E
elastic collision: A collision where total kinetic energy is conserved as well as momentum. Billiard balls clicking apart with the same total KE before and after is the classic example.
elastic potential energy: Energy stored in a stretched or compressed spring, PE = ½kx². Because x is squared, tripling the stretch stores nine times the energy.
energy conservation in SHM: Total mechanical energy stays constant, continuously trading between KE and spring PE. All KE at equilibrium, all PE at the endpoints.
energy distribution in rolling: Gravitational PE splits between translation and rotation based on I/(mr²). A solid disk (smaller I) reaches the bottom faster than a hoop because more energy goes to motion.
equilibrium position: The center point of oscillation where net force is zero. Speed is maximum here and acceleration is zero.
F
floating condition: A floating object sits so the buoyant force equals its weight (F_b = mg). The submerged fraction equals the ratio of object density to fluid density.
force: A push or pull measured in newtons (N) that can change an object's motion. Forces are vectors, so direction matters and only the NET force determines acceleration.
free fall: Motion under gravity alone, with air resistance ignored. Every object accelerates downward at g ≈ 9.8 m/s² regardless of mass, so a dropped phone and textbook hit the ground together.
free-body diagram: A sketch showing every external force acting ON one object as an arrow from a single point. It includes only real forces (gravity, normal, tension, friction) — never a leftover 'force of motion' or velocity.
frequency: The number of cycles per second (f, in hertz). The reciprocal of the period: f = 1/T.
G
gauge pressure: Pressure measured relative to atmospheric, P_gauge = ρgh. Depends only on depth, fluid density, and g — not on container shape.
gravitational force: The attractive force between any two masses, F = Gm₁m₂/r². It follows an inverse-square law, so tripling the distance cuts the force to one-ninth.
gravitational potential energy: Stored energy from height in a gravity field, PE = mgh. Only CHANGES matter, so you can place the zero height wherever is convenient and still get the right answer.
H
hydrostatic pressure: Pressure in a static fluid increases with depth, P = P₀ + ρgh. At equal depth the pressure is the same regardless of container shape.
I
impulse: The change in momentum, J = FΔt (N·s). A longer collision time means a smaller force for the same momentum change — why airbags and bent knees work.
impulse-momentum theorem: The net impulse on an object equals its change in momentum, J = Δp. For constant mass it reduces to Newton's second law, since FΔt = mΔv leads to F = ma.
incompressible fluid: A fluid whose density stays essentially constant under pressure (water is treated this way). This assumption underlies continuity and Bernoulli.
inelastic collision: A collision where momentum is conserved but kinetic energy is not — some becomes heat, sound, or deformation. Most real crashes are inelastic.
inertia: An object's resistance to changes in its motion, measured by its mass. More mass means it's harder to start, stop, or turn.
inertial reference frame: A frame that is not accelerating, where Newton's laws hold true. A coin in a steadily moving car obeys them; in a hard-braking car it appears to lurch forward because that frame is accelerating.
instantaneous velocity: The velocity at one specific moment — the slope of the position-vs-time graph at that point. The tighter the time interval you measure over, the closer you get to the true instantaneous value.
isolated system: A system with no net external force, so its total momentum is conserved. Internal collisions and explosions can shuffle momentum between parts but can't change the total.
J
joule: The SI unit of energy and work, equal to one newton-meter (N·m). Work is in joules, not newtons, because it combines force WITH the distance over which it acts.
K
kinematic equations: The set of equations for constant acceleration, like v = v₀ + at, x = x₀ + v₀t + ½at², and v² = v₀² + 2aΔx. Pick the one whose variables match what you know and want — they only work when acceleration is constant.
kinetic energy: The energy of motion, KE = ½mv² (a scalar, joules). Because speed is squared, doubling the speed quadruples the kinetic energy.
kinetic friction: The friction on a sliding object, f_k = μ_k·N, opposing the motion. It depends on the normal force and surface pair, not on the contact area or speed.
L
lever arm: The perpendicular distance from the axis of rotation to the line of action of a force (the moment arm). Only the perpendicular force component creates torque.
M
mass: The amount of matter in an object (kg), which sets both its inertia and its weight. Mass doesn't change with location — only weight does.
maximum acceleration in SHM: Occurs at the endpoints (maximum displacement) where the restoring force is largest, since a = -kx/m.
maximum speed in SHM: Occurs at the equilibrium position where all energy is kinetic. It doubles when amplitude doubles, since KE scales with v².
mechanical energy: The sum of kinetic and potential energy, KE + PE. It stays constant when only conservative forces act, but friction converts some into heat.
momentum: The quantity of motion, p = mv (a vector, kg·m/s). Direction matters, so two equal-size momenta pointing opposite ways sum to zero, not double.
N
net force: The vector sum of all forces on an object, ΣF. If it's zero the object keeps constant velocity; if it's nonzero the object accelerates in that direction.
Newton's first law: An object at rest stays at rest and an object in motion stays in constant-velocity motion unless acted on by a net force. This is inertia — no force is needed to KEEP something moving, only to change its motion.
Newton's second law: Net force equals mass times acceleration, ΣF = ma. For the same net force a lighter object accelerates more, and acceleration always points in the direction of the net force.
Newton's second law for rotation: Net torque equals rotational inertia times angular acceleration, Στ = Iα. The rotational version of ΣF = ma.
Newton's third law: For every force, there's an equal and opposite force on the OTHER object — the action-reaction pair. The pair acts on different objects, so they never cancel and the system can still accelerate.
normal force: The support force a surface pushes perpendicular to itself. It is NOT always equal to mg — on an incline it's mg·cosθ, and in an accelerating elevator it changes.
O
orbital velocity: The speed needed to maintain a circular orbit, found by setting gravity equal to the centripetal force: v = √(GM/r). Smaller orbits require faster speeds.
P
parallel axis theorem: Finds rotational inertia about an axis offset from the center of mass: I′ = I_cm + Md². Spinning about any axis other than the center always increases I.
Pascal's principle: Pressure applied to a confined fluid transmits undiminished throughout it. The basis for hydraulic systems that multiply force.
perfectly inelastic collision: A collision where the objects stick together and move as one afterward. It conserves momentum and loses the maximum possible kinetic energy, like two clay lumps merging.
period: The time for one complete cycle of circular or repeating motion, T (seconds). If a point makes 2 revolutions per second, the period is 0.5 s.
period of a mass-spring system: T = 2π√(m/k). More mass lengthens the period; a stiffer spring (larger k) shortens it. Amplitude has no effect.
period of a pendulum: T = 2π√(L/g) for small angles. Depends only on length and gravity — the mass of the bob doesn't matter.
position-vs-time graph: A graph whose slope is velocity. A straight slanted line means constant velocity; a curve that steepens means the object is speeding up.
potential energy: Stored energy due to position or configuration of a system (joules). It belongs to a system, not a lone object — a single rock in deep space has no gravitational PE by itself.
power: The rate of doing work or transferring energy, P = W/t = Fv (watts). Two elevators that do the same work but in different times deliver different power — the faster one is more powerful.
pressure: Force per unit area, P = F/A (pascals). A small contact area concentrates the same force into much higher pressure.
projectile motion: Motion of an object under gravity alone after launch, with constant-velocity horizontal motion and free-fall vertical motion handled separately. A ball launched horizontally and one simply dropped hit the ground at the same time because their vertical motions are identical.
R
reference frame: The point of view from which motion is measured. A dropped phone falls straight down to you on a moving train but arcs forward to someone on the platform — both are correct for their frame.
relative velocity: An object's velocity as seen from a particular frame, found by adding or subtracting the frame's velocity. Walk 2 m/s toward the front of a 15 m/s bus and the road sees you at 17 m/s; walk toward the back and it sees 13 m/s.
restoring force: The force that always points back toward equilibrium, F = -kx. The minus sign is why the motion oscillates instead of running away.
resultant: The single vector you get by combining components, with magnitude from the Pythagorean theorem. A 3 m/s east plus 4 m/s north velocity has a resultant speed of 5 m/s.
rigid body: An object that holds its shape while rotating, so every point shares the same ω and α even though their linear speeds differ.
rolling without slipping: When the contact point doesn't slide, locking translation and rotation together: v_cm = rω. Static friction acts but does no work.
rotational equilibrium: When net torque is zero (Στ = 0), so angular velocity stays constant. Independent of translational equilibrium — a system can have one without the other.
rotational inertia: An object's resistance to angular acceleration (moment of inertia, I = Σmr²). Depends on mass AND how that mass is distributed from the axis — farther mass means harder to spin.
rotational kinematics: The rotational analogs of the linear motion equations: ω = ω₀ + αt and ω² = ω₀² + 2αθ mirror their linear counterparts with θ, ω, α replacing x, v, a.
rotational kinetic energy: The energy of a spinning object, KE_rot = ½Iω². A scalar (no direction), and it adds to translational KE for a rolling object.
rotational work: Work done by a torque, W = τΔθ (Δθ in radians). The rotational analog of W = Fd.
S
scalar: A quantity with magnitude only and no direction, like distance, speed, mass, or energy. Adding two scalars is just normal arithmetic — no signs or arrows to track.
simple harmonic motion: Oscillation driven by a restoring force proportional to displacement (F = -kx). Period is independent of amplitude.
small-angle approximation: A pendulum is only SHM at small angles, where the restoring torque is proportional to angular displacement. Large swings break the constant-period rule.
speed: How fast an object moves regardless of direction (a scalar). It is the magnitude of velocity, so it is never negative.
spring constant: The stiffness of a spring, k (N/m) — the slope of a force-vs-stretch graph. A larger k means a stiffer spring that needs more force per meter of stretch.
spring force: The restoring force of a spring, F = −kx (Hooke's law), always pointing back toward the relaxed position. Compress it and it pushes out; stretch it and it pulls in.
spring potential energy: Energy stored in a stretched or compressed spring, PE = ½kx². It grows with the SQUARE of x, so doubling the stretch stores 4× the energy.
static friction: The friction that prevents a stationary object from sliding, adjusting up to a maximum of μ_s·N. Below that max it exactly matches your push, so a 15 N push on a stuck box gives exactly 15 N of friction.
system: The object or set of objects you choose to analyze together. The choice matters: momentum is conserved only when no net EXTERNAL force acts on the system you picked.
T
tangential velocity: The linear speed of a point on a rotating body, v = rω. Points farther from the axis move faster even though ω is the same everywhere.
tension: The pulling force transmitted through a string, rope, or cable. In an ideal massless rope it's the same throughout, and two blocks joined by one string share the same tension.
torque: The rotational equivalent of force, τ = rF·sinθ (N·m). It depends on BOTH the force and the lever arm — pushing farther from the axis gives more torque.
total kinetic energy of rolling: A rolling object's energy splits two ways: KE = ½mv_cm² + ½Iω². Forgetting the spin term undercounts the energy.
V
vector: A quantity with both magnitude and direction, like displacement, velocity, acceleration, and force. You combine vectors by accounting for direction (signs in 1D, components in 2D), not by adding magnitudes.
vector components: The perpendicular (x and y) pieces a vector splits into, found with sine and cosine. Components let you handle each direction independently, which is the key to solving 2D motion.
velocity: The rate of change of position, v = Δx/Δt (a vector, m/s). The sign carries direction, so +20 m/s and −20 m/s are equal speeds moving opposite ways.
velocity-vs-time graph: A graph whose slope is acceleration and whose area under the line is displacement. A flat line means constant velocity; crossing zero means the object reverses direction.
volume flow rate: The volume of fluid passing per second, A·v (m³/s). Stays constant along a pipe for an incompressible fluid (continuity).
W
watt: The SI unit of power, equal to one joule per second. A watt rating tells you the RATE of energy delivery, not the total energy a device can supply.
weight: The gravitational force on an object, W = mg (a force, in newtons). An astronaut in orbit feels weightless yet still has gravity acting on her — she's just in free fall.
work: Energy transferred by a force over a displacement, W = Fd·cosθ (joules). A force perpendicular to the motion does zero work, which is why carrying a box across a level floor does no work on it.
work-energy theorem: The net work done on an object equals its change in kinetic energy, W_net = ΔKE. Do 50 J of net work on a resting cart and it gains 50 J of kinetic energy.

Where these terms show up

Every AP Physics 1 term below belongs to one of the units on the exam. Each unit page has sample questions that put the vocabulary to work:

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