1,800+ MCAT terms, defined.

Static friction is the force parallel to the contact surface that keeps two surfaces from sliding past each other while they are at rest relative to each other. It adjusts to match the applied force up to a maximum, fs ≤ μsN. Equality ho...

A concave mirror curves inward like the inside of a spoon and converges light to a focal point at f = r/2, so f is positive. Beyond f its image is real and inverted; inside f it is virtual, upright and enlarged.

Uniform circular motion is travel around a circle at constant speed. The velocity still changes direction, so the object accelerates toward the center (a = v²/r), which requires a net inward force. Constant speed does not mean zero accel...

The laws of thermodynamics describe how energy and entropy behave. The first says energy is conserved: ΔU = Q − W. The second says the total entropy of a system plus its surroundings increases in a spontaneous process, so heat flows spon...

Fluorescence is the emission of a photon when an excited electron drops back to the ground state within nanoseconds of absorbing light. The emitted light typically has lower energy and a longer wavelength than the absorbed light, because...

A resultant wave is the single wave formed when two or more waves overlap in the same medium. At each point its displacement is the sum of the individual displacements (superposition). In-phase waves give a larger amplitude, and waves 18...

Coulomb's constant k ≈ 9.0 × 10⁹ N·m²/C² is the proportionality constant in F = kq₁q₂/r², and k = 1/(4πε₀). Its huge size means small charges exert large forces. In a dielectric medium the force is reduced by the dielectric constant κ.

"Observer's" is a fragment, not a full MCAT term. The exam's usual context is the observer (detector) in the Doppler effect, where the observer's measured frequency differs from the source's when they move relative to each other. Approac...

A source charge is the charge that creates the electric field and potential being analyzed, as opposed to the small test charge placed there to probe it. The field and potential at a point depend on the source charge and the distance, no...

Young's double-slit experiment passes coherent light through two narrow slits so the overlapping waves form alternating bright and dark fringes on a screen, showing that light behaves as a wave. Bright fringes occur where the path differ...

A diffraction grating is a surface with many equally spaced parallel slits or lines that produces sharp bright maxima where d sinθ = mλ. Because the angle depends on wavelength, it splits white light into colors, with longer wavelengths...

Speed of sound is how fast a sound wave travels through a medium. The medium's stiffness and density set it, not the source. It is generally greatest in solids and least in gases, and when sound changes media its frequency stays fixed wh...

A non-conservative force does work that depends on the path taken, so no potential energy can be defined for it. Friction and air drag are the standard examples; they typically convert mechanical energy to thermal energy, so mechanical e...

Spectra (plural of spectrum) show how light is spread over wavelength or frequency. In MCAT physics the key case is atomic line spectra: electrons moving between quantized energy levels emit or absorb photons with ΔE = hf, so each elemen...

In optics, light rays converge when they bend toward one another and meet at a point. Converging elements (convex lenses in air, concave mirrors) have positive focal length f. Testable point: they can form a real image or a virtual one,...

Static pressure is the pressure a fluid exerts on its surroundings through random molecular collisions, the P term in Bernoulli's equation. For an ideal fluid in steady flow at constant height, static pressure is lower where the fluid mo...

Air resistance is the drag force air exerts on a moving object, directed opposite its motion relative to the air and growing with speed. Most testable: a falling object speeds up until drag equals its weight, then falls at constant termi...

Constructive interference is the superposition of two or more waves whose displacements add to give a larger amplitude. It occurs when waves meet in phase, so path difference is a whole number of wavelengths (0, λ, 2λ, …).

Two quantities are inversely proportional when their product stays constant, so multiplying one by a factor divides the other by that same factor. The MCAT tests this by scaling one variable and asking for the other, such as doubling pre...

An open pipe is a tube open at both ends, so a standing sound wave has a displacement antinode at each end. Its resonant frequencies are fn = nv/2L for every integer n (all harmonics), unlike a pipe closed at one end, which gives only od...

The harmonic number n is the positive integer that labels a standing-wave mode, with n = 1 the fundamental. The nth harmonic has a frequency n times the fundamental. Strings and pipes open at both ends allow every integer n; a pipe close...

Vector addition combines quantities that have magnitude and direction (displacement, velocity, force) into one resultant, either head-to-tail graphically or by adding x and y components. Most testable: the resultant's magnitude is usuall...

Adhesive forces are attractive intermolecular forces between unlike substances, such as a liquid and its container wall. If adhesion exceeds the liquid's cohesion, the meniscus is concave and the liquid rises in a narrow tube; if cohesio...

Dynamic pressure is the kinetic energy per unit volume of a moving fluid, ½ρv², measured in pascals. In Bernoulli's equation for ideal flow, faster fluid has higher dynamic pressure and, at constant height, lower static pressure.

An equipotential line (a surface in three dimensions) joins points at the same electric potential, so the field does no net work on a charge moved along it. It is perpendicular to the electric field lines, which point toward lower potent...

The refractive index n of a material is the speed of light in vacuum divided by its speed in the material (n = c/v), so n is 1 for vacuum and greater than 1 for ordinary materials. Light entering a higher-n medium slows and bends toward...

Like charges are charges of the same sign, both positive or both negative, and they repel each other. The force follows Coulomb's law, F = k|q₁q₂|/r², so doubling the separation cuts the force to one-fourth.

Sound level (intensity level) is a logarithmic measure of a sound's intensity compared with the threshold of hearing, given in decibels (dB). Each tenfold increase in intensity adds 10 dB, and doubling intensity adds about 3 dB.

The freezing point is the temperature at which a liquid and its solid coexist in equilibrium at a given pressure; for a pure substance it equals the melting point. While a pure substance freezes, heat leaves but its temperature stays con...

Natural frequency is the frequency at which a system oscillates by itself when displaced and released. It is set by the system's own properties, such as mass, stiffness, length or tension. A driving force at this frequency produces reson...

SI units are the international standard metric units: meter, kilogram, second, ampere, kelvin, mole and candela. Other units, such as the newton and joule, are combinations of these. Testable point: convert to SI before calculating, espe...

Doppler ultrasound sends high-frequency sound into the body and measures the frequency shift of echoes from moving reflectors, usually red blood cells, to find their speed. Motion toward the transducer raises the echo frequency, and moti...

Rotational equilibrium means the net torque on an object is zero, so its angular acceleration is zero: it either does not rotate or spins at constant angular velocity. Most testable point: about a chosen pivot, set clockwise torques equa...

The angle of reflection is the angle between a reflected ray and the normal, the line perpendicular to the surface at the point of contact. By the law of reflection it equals the angle of incidence, and both are measured from the normal,...

An electric dipole is a pair of equal and opposite charges, +q and −q, separated by a small distance d. Its dipole moment is p = qd, pointing from − to +. In a uniform field it has zero net force but feels a torque that turns it toward a...

The index of refraction, n = c/v, compares the speed of light in vacuum with its speed in a material, so n ≥ 1. Entering a higher-n medium, light slows and bends toward the normal (Snell's law: n₁ sinθ₁ = n₂ sinθ₂), while its frequency s...

The law of reflection says the angle of incidence equals the angle of reflection, both measured from the normal (the perpendicular to the surface), and the incident ray, reflected ray and normal lie in one plane. Measuring from the surfa...

Lens power is the reciprocal of a lens's focal length, P = 1/f, measured in diopters (1 D = 1 m⁻¹). A converging lens has positive power and a diverging lens has negative power. A shorter focal length means a larger magnitude of power.

Relative densities compare one material's density with another's, often as specific gravity (ρ/ρwater, unitless). The key test point: an object denser than the surrounding fluid sinks, a less dense one floats, and a floating object's sub...

Angular displacement (Δθ) is the net angle through which an object rotates about an axis, usually measured in radians. Counterclockwise is conventionally positive. The most testable link is s = rΔθ, which holds only when Δθ is in radians.

Chemical potential energy is energy stored in the arrangement of electrons and nuclei in a substance's bonds. It is released, often as heat, when a reaction forms products with lower potential energy. Breaking bonds takes in energy; form...

Circularly polarized light is a transverse wave whose electric field keeps constant magnitude and rotates around the direction of travel, made of two perpendicular components of equal amplitude 90° out of phase. Plane-polarized light is...

Electrolytic conduction is the flow of electric current through a solution or molten salt by the movement of ions, not electrons. Both cations and anions carry charge. More dissolved ions generally means higher conductivity, and heating...

The electrostatic force vector is the force one charge exerts on another, directed along the line joining them: away for like charges (repulsion), toward for opposite charges (attraction). Its magnitude is k|q₁||q₂|/r², so doubling the d...

Electric current is the rate at which net charge passes a point in a conductor, I = ΔQ/Δt, measured in amperes (C/s). Conventional current points the way positive charge would move, which in a metal wire is opposite to electron motion.

Radius of curvature (R) is the radius of the sphere of which a curved mirror or lens surface is a part. It is a physics optics term, not an organic chemistry one. Most testable: for a spherical mirror, focal length is half of R (f = R/2).

Plane polarized light is light whose electric field oscillates in a single plane, perpendicular to the direction of travel. Unpolarized light loses half its intensity at the first polarizer; a second polarizer at angle θ then transmits I...

In a right triangle, the square of the hypotenuse equals the sum of the squares of the two legs. In physics it gives the magnitude of a resultant from perpendicular vector components, R = √(Rx² + Ry²). It holds for right angles, so resol...

Spherical aberration is image blur from lenses or mirrors with spherical surfaces: rays far from the optical axis focus at a different point than rays near it, so a point source becomes a smeared spot. It can be reduced by limiting light...

A Venturi tube is a pipe with a narrow throat used to relate fluid speed and pressure. By continuity the fluid moves faster in the throat, and by Bernoulli's principle the pressure there is lower (for a horizontal pipe and an ideal fluid...