AP Physics Glossary: Key Terms with FRQ-Ready Definitions

Free-Body Diagram (FBD)

A diagram showing all forces acting on a single object, drawn as arrows from a single point (or the object's center of mass), with each force labeled by its physical source (F_grav, F_N, F_T, F_f, F_app).

On the FRQ: Every arrow needs a labeled source. An unlabeled arrow, or a "net force" arrow, loses the FBD point.

Newton's Second Law

The net force on an object equals its mass times its acceleration: ΣF = ma. Vector equation — apply component by component in a chosen coordinate system.

On the FRQ: Name Newton's second law by name and write ΣF = ma before substituting. Justification points require this.

Conservation of Energy

In an isolated system, total mechanical energy plus energy dissipated equals a constant. In problems: KE_i + PE_i + W_nc = KE_f + PE_f, where W_nc is work done by non-conservative forces (friction, applied forces outside the system).

On the FRQ: Define your system before invoking conservation. Non-conservative forces must be accounted for explicitly.

Conservation of Momentum

The total momentum of a system is conserved if the net external force is zero. Especially useful in collisions: m₁v₁ᵢ + m₂v₂ᵢ = m₁v₁f + m₂v₂f (for a 1D two-body collision).

On the FRQ: Justify by naming "isolated system" or "no net external force during the collision." Skipping the justification loses the setup point.

Moment of Inertia (I)

The rotational analog of mass: I = Σmr² for a system of particles, or ∫r² dm for a continuous object. Depends on both the mass distribution and the axis of rotation.

On the FRQ: On Physics C: Mechanics, deriving I via integration is a standard task. Show the integrand, limits, and the dm relation.

Torque (τ)

The rotational equivalent of force: τ = r × F, or |τ| = rF sinθ. Net torque produces angular acceleration: Στ = Iα.

On the FRQ: State "net torque about [chosen axis] equals Iα" before substituting. Choice of axis matters — pick one that eliminates unknown forces.

Electric Field (E)

The force per unit positive charge at a point in space: E = F/q. Points away from positive source charges and toward negative source charges. Superposition applies.

On the FRQ: On field-line FRQs, draw lines from + to −, never crossing, with density proportional to field strength.

Gauss's Law

The electric flux through any closed surface equals the enclosed charge divided by ε₀: ∮E·dA = Q_enc/ε₀. Powerful when the charge distribution has high symmetry (spherical, cylindrical, planar).

On the FRQ: Justify choosing Gauss's law by citing the symmetry. Choose a Gaussian surface where E is either constant on the surface or perpendicular to it.

Kirchhoff's Rules

Junction rule: the sum of currents into a junction equals the sum out. Loop rule: the sum of voltage changes around any closed loop is zero. Together they solve any DC circuit.

On the FRQ: State the rule by name before writing the equation. Choose a loop direction and stick to it.

Faraday's Law

The induced EMF in a loop equals the negative rate of change of magnetic flux through it: ε = −dΦ_B/dt. The sign (Lenz's law) says the induced current opposes the change in flux.

On the FRQ: Always cite Lenz's law when stating the direction of the induced current. Diagram the flux change first.

Impulse (J)

The change in momentum: J = Δp = ∫F dt. On a force-vs-time graph, impulse is the area under the curve.

On the FRQ: On graph-based FRQs, cite "area under the F-vs-t curve" as the justification for computing impulse from the graph.

Simple Harmonic Motion (SHM)

Motion where the restoring force is proportional to displacement from equilibrium: F = −kx. Period T = 2π√(m/k) for a mass-spring; T = 2π√(L/g) for a small-angle pendulum.

On the FRQ: Justify SHM by identifying the linear restoring force. If the restoring force is nonlinear, SHM only applies to small displacements.

Definitions that earn points.

Cramapple's per-criterion grading tells you when your definition earns the rubric point and when it doesn't.