MCAT Physics

Right-Hand Rule

A common method used to determine the direction of a vector created as the product of two vectors. The thumb points in the direction of the first vector, the fingers point in the direction of the second vector, and the palm points in the direction of the resultant.

Power

The rate at which work is done, given by the equation  P=WΔt , where W is work and t is time (in seconds). The SI unit for power is the watt (W).

Work-Energy Theorem

States that the net work performed on an object is related to its change in energy. In most applications, the work-energy theorem is used to relate work and kinetic energy.

Work

One of the two forms of energy transfer in which a force is applied to change the energy of a system. In mechanics, commonly calculated by the equation W = Fd cos θ, where F is the magnitude of the applied force, d is the magnitude of the displacement, and θ is the angle between these two vectors.  The SI unit of work is the joule (J).

Velocity

A vector quantity describing an object's displacement over the elapsed time, expressed as  v=ΔxΔt.  The SI unit of velocity is meters per second (m/s).

Vector

A quantity that has both magnitude and direction.

Translational equilibrium

State where the sum of the forces acting on an object is zero, giving it no net acceleration. An object may be in rotational equilibrium, translational equilibrium, or both simultaneously.

torque

A force creating rotation about an axis; measured as the lever arm (the distance between the fulcrum and the applied force) times the magnitude of the force times the sine of the angle between them: τ = rF sin θ.  The SI unit of torque is the newton meter (N⋅m).

Speed

A scalar quantity describing the distance traveled divided by the time required to travel that distance. The SI unit of speed is meters per second (m/s).

Scalar

A quantity that has magnitude but no direction.

Rotational Equilibrium

State where the sum of the torques acting on a body is zero, giving it no net angular acceleration. An object may be in rotational equilibrium, translational equilibrium, or both simultaneously.

Normal Force

Perpendicular component of the force caused when two surfaces push against each other, denoted by N.

Nonconservative Force

A force that dissipates mechanical energy from a system. As such, the energy dissipated depends on the path taken from initial to final position. Examples include friction, air resistance, and viscous drag.

Newton's Third Law

States that if one object exerts a force on another, the other object exerts a force on the first that is equal in magnitude but opposite in direction; the law of "action and reaction."

Newton's Second Law

States that an object will accelerate in proportion to the net force acting on it: Fₙₑₜ = ma.

Newton's First Law

States that if no net force acts on an object, its velocity is constant.

Mass

A scalar quantity used as a measure of an object's inertia.  The SI unit of mass is the kilogram (kg).

Kinetic Energy

The energy of an object in motion, calculated by the equation  K=12mv2 and given in the SI unit of joules (J).

Inverse Relationship

A relationship between variables such that an increase in one variable is associated with a decrease in the other: AB = constant.

Gravity

A ubiquitous attractive force existing between any two objects, with magnitude directly proportional to the product of the two masses observed and inversely proportional to the square of their distance from each other:  Fg=Gm1m2r2.  As gravity is a force, it is measured in newtons (N).

Gravitational Potential Energy

The energy of an object due to its height above a given datum, calculated by the equation U = mgh and given in the SI unit of joules (J).

Frictional Force

An antagonistic force that points parallel and opposite in direction to the direction of movement (or attempted movement) of an object. Related to a coefficient of friction and the normal force: Static friction :  0 ≤ fs ≤ μsN.

Force

A vector quantity describing the push or pull on an object. The SI unit for force is the newton (N).

First Law of Thermodynamics

States that the change in internal energy of a system is equal to the heat transferred into the system minus the work done by the system: ΔU = Q - W. An extension of the law of conservation of energy.

Elastic Potential Energy

The energy associated with stretching or compressing a spring, calculated by the equation  U=1/2 kx^2 and given in the SI unit of joules (J).

Displacement

A vector quantity describing the straight-line distance between an initial and a final position of some particle or object.

Direct Relationship

A relationship between variables such that an increase in one variable is associated with an increase in the other:  A/B=constant.

Conservative Force

A force that does not cause dissipation of mechanical energy from a system. As such, the work performed is independent of the path taken. Examples include gravity and electrostatic forces. Elastic forces are nearly conservative.

Conservation of Mechanical Energy

States that when only conservative forces act on an object and work is done, energy is conserved and described by the equation: ΔE = ΔU + ΔK = 0.

Centripetal Acceleration

The acceleration of an object traveling in a circle that points toward the center of the circle. In uniform circular motion, it is equal in magnitude to the velocity squared divided by the radius of the circle traversed:  ac=v^2/r.

Center of Mass

The point on some object or body where all of its mass is considered to be concentrated. In a uniform gravitational field, this is also the center of gravity.

Acceleration

A vector quantity describing a change in velocity over the elapsed time during which that change occurs, expressed as  a=Δv/Δt.  The SI unit of acceleration is m/s^2

Standard Temperature and Pressure (STP)

273 Kelvin (0° Celsius) and 1 atmosphere (760 mmHg).

Partial Pressure

The pressure contribution of a single gas in a container holding a mixture of gases, as given by: Pₐ = Xₐ Pₜₒₜₐₗ, where Xₐ is the mole fraction of gas A and Pₜₒₜₐₗ is the total pressure of the mixture.

Kinetic Molecular Theory of Gases

A series of assumptions used to account for the behavior of ideal gases. The theory describes gases as volumeless particles in constant, random motion that exhibit no intermolecular attractions and undergo completely elastic collisions with each other and the walls of their container.

Ideal Gas Law

A unification of Boyle's law, Charles's law, Gay-Lussac's law, and Avogadro's principle into the formula that describes the behavior of ideal gases: PV = nRT.

Ideal Gas

A hypothetical gas containing particles with zero volume and with no attractive intermolecular forces.

Henry's Law

States that the partial pressure of a gas dissolved in a solution is directly proportional to the partial pressure of this gas above the solution.

Graham's Law

Law stating that the rates at which two different gases effuse or diffuse are inversely proportional to the square root of their molar masses: r1/r2=sqrt (M2/M1).

Gay-Lussac's Law

States that at a constant volume, the pressure of an ideal gas is directly proportional to its temperature.

Effusion

The movement of gas from one compartment to another through a small opening under pressure.

Diffusion

Passive dispersion of a gas or solute throughout a medium by means of random motion.

Charles's Law

States that at a constant pressure, the volume of an ideal gas is directly proportional to its temperature.

Boyle's Law

States that at a constant temperature, the volume of an ideal gas is inversely proportional to its pressure.

Avogadro's Principle

States that the number of moles of a gas present is proportional to its volume, assuming constant pressure and temperature.

Viscosity

The measure of internal friction in a fluid, often denoted by η. Viscosity is responsible for creating viscous drag, a nonconservative force analogous to air resistance.

Turbulent Flow

Type of liquid flow that occurs when the linear flow speed in a tube exceeds the critical speed v_c. The motion of the fluid that is not adjacent to the container walls is highly irregular, forming vortices and a high resistance.

Pressure

The force per unit area:  P=F/A . May be provided as absolute (hydrostatic) pressure, which is the pressure below the surface of a fluid that depends on gravity and surface pressure, calculated by P = P₀ + ρgz, where P is the absolute pressure, P₀ is the pressure at the surface, ρ is the density of the fluid, g is acceleration due to gravity, and z is depth.  The SI unit for pressure is the pascal (Pa).

Pascal's Principle

States that when a pressure is applied to one point of an enclosed fluid, that pressure is transmitted in equal magnitude to all points within that fluid and to the walls of its container. This principle forms the basis of the hydraulic lift.

Laminar Flow

The smoothest type of liquid flow through a tube wherein thin layers of liquid slide over one another. Occurs as long as the linear flow speed remains below a critical speed v_c. Laminar flow can be represented by roughly parallel streamlines—lines that trace the path of water particles as they flow in a tube without ever crossing each other.

Gauge Pressure

The pressure above and beyond atmospheric pressure. When the pressure at the surface is atmospheric pressure, the gauge pressure is given by ρgz, where ρ is the density of the fluid, g is acceleration due to gravity, and z is depth.  The SI unit of pressure is the pascal (Pa).

Density

A scalar quantity defined as mass per unit volume, often denoted by ρ. Density of an object may be compared to water as a unitless quantity known as specific gravity.

Continuity Equation

States that the mass flow rate of fluid must remain constant from one cross-section of a tube to another, given by A₁ v₁ = A₂ v₂.

Cohesion

A type of attractive force felt by liquid molecules toward each other. Cohesion is responsible for surface tension.

Bernoulli's Equation

Equation describing the conservation of energy in fluid flow, given by:P1 + 1/2ρv12 + ρgh1 = P2 + 1/2ρv22 + ρgh2. According to the Venturi effect, for a given depth, linear flow speed and pressure are inversely related.

Archimedes' Principle

States that a body that is fully or partially immersed in a liquid will be buoyed upwards by a force that is equal to the weight of the liquid displaced by the body: F_buoy = (ρ_fluid)(V_submerged)g.

Adhesion

A type of attractive force that molecules of a liquid feel toward molecules of another substance, such as in the adhesion of water droplets to a glass surface.

Electrolyte

An ion in water that is capable of conducting electricity in that solution.

Reduction Potential

A measure of the tendency of a species to be reduced, commonly used in identifying the anode and cathode of an electrochemical cell.

Nernst Equation

An equation used to determine a cell's electromotive force (emf) when conditions are not standard. Ecell=Ecello -0.0592/n logQ  where E_cell is the emf,  Ecello is the emf under standard conditions, n is the number of electrons transferred in the oxidation-reduction reaction, and Q is the reaction quotient.

Half-Cell

An electrode immersed in an electrolytic solution that is the site of either oxidation or reduction in an electrochemical cell.

Galvanic Cell

An electrochemical cell powered by a spontaneous oxidation-reduction reaction that produces an electric current; also called a voltaic cell.

Faraday's Constant

Denoted by F, it equals  9.65 × 10^4 C/mol e-. Commonly used in the formula It = nF, where I is current, t is time (in seconds), and n is moles of electrons transferred.

Electrolytic Cell

An electrochemical cell that uses an external electric source to drive a nonspontaneous (unfavorable) oxidation-reduction reaction.

Electrochemical Reaction

A chemical reaction that either is driven by or produces electricity.

Cathode

The electrode at which reduction occurs during a cell's oxidation-reduction reaction. Electrons always flow toward the cathode in an electrochemical cell.

Anode

The electrode where oxidation occurs during a cell's oxidation-reduction reaction. Electrons always flow from the anode in an electrochemical cell.

Resistivity

Intrinsic property of a conductor used to measure its resistance in the equation R=ρL/A , where R is the resistance, ρ is the resistivity, L is the length of the conductor, and A is its cross-sectional area.  The SI unit of resistivity is the ohm-meter (Ω⋅m).

Resistance

The natural tendency of a conductor to block current flow to a certain extent resulting in loss of energy or potential. Resistance is equal to the ratio of the voltage applied to the resulting current.  The SI unit for resistance is the ohm (Ω).

Potential Difference

The difference in electric potential between two points in an electric field, also called the voltage (ΔV).

Ohm's Law

Law stating that the voltage drop across a resistor is proportional to the current flowing through it, given by the equation V = IR.  Voltage drop is measured in volts (V).

Kirchhoff's Loop Rule

States that the sum of the voltage sources in a circuit loop is equal to the sum of voltage drops along that loop.

Kirchhoff's Junction Rule

States that the sum of currents directed into a node or junction point in a circuit equals the sum of the currents directed away from that point.

Insulator

A material in which electrons cannot move freely.

Equipotential Lines

Concentric circles emanating from a source charge that cross its electric field lines perpendicularly. No work is required for a test charge to travel along the circumference of an equipotential line, because the potential at every point along that line is the same.

Electromotive Force

The impetus for current flow created by a voltage source, such as a battery or outlet.  Electromotive force is measured in volts (V).

Electric Power

The rate at which the energy of flowing charges through a resistor or other device is dissipated, given by the equation P = IV.  The SI unit of power is the watt (W).

Electric Potential Energy

The amount of work required to bring a test charge q₀ from infinity to a point within the electric field of some source charge Q, given by the equation U = q₀V.  The SI unit of electric potential energy is the joule (J)

Electric Potential

The amount of electric potential energy per unit charge; the work required to bring a positive test charge q₀ from infinity to within an electric field of another positive source charge, Q, divided by that test charge's magnitude. Calculated by the equation  V=kQ/r.  The SI unit of electric potential is the volt (V).

Electric Field

The electrostatic force that a source charge would exert on a positive test charge q₀ divided by the magnitude of that test charge:  E=F/qo.  Electric fields are represented by electric field lines—imaginary lines that show the direction in which a positive test charge is accelerated by the coulombic force due to the electric field of a source charge. The SI unit of electric field strength is N/C or V/m.

Electric Circuit

A conducting pathway that contains one or more voltage sources that drive an electric current along that pathway and through connected passive circuit elements (such as resistors).

Dielectric

An insulating material placed between the two plates of a capacitor; used to increase capacitance. If the circuit is plugged into a current source, more charge will be stored. If the circuit is not plugged in, the voltage of the capacitor will decrease, indirectly increasing its capacitance. The strength of a dielectric is measured by the dielectric constant.

Current

A flow of charge per time. The flow of charge is motivated by a potential difference (voltage). Current is denoted I and can be calculated as  Δq/Δt. Current is conventionally considered the theoretical movement of positive charge.  The SI unit of current is the ampere (A).

Coulomb's Law

The law describing the electrostatic force that exists between two charges, q₁ and q₂, that are separated by a distance r. Given by the equation  F = kq1q2/r2.

Conductor

A material in which electrons can move with relative ease.

Capacitor

An electric device used in circuits that is composed of two conducting plates separated by a short distance; these devices store electric charge.

Capacitance

A measure of a capacitor's ability to store charge at a given voltage; calculated by the ratio of the magnitude of charge on one plate to the voltage across the two plates: C=Q/V. The SI unit for capacitance is the farad (F).

Specific Rotation ([/α/])

A standardized measure of a compound's ability to rotate plane-polarized light.

Photon

A particle of light energy with a value equal to hf, where h is Planck's constant and f is the frequency of radiation.

Wavelength

A quantity equal to the distance between any two equivalent consecutive points along a wave, such as two consecutive crests (peaks) or two consecutive troughs (valleys); expressed as λ.  The SI unit of wavelength is the meter (m).

Wave Speed

The speed of a wave, which is related to its frequency and wavelength by the equation v = fλ.

Total Internal Reflection

The condition in which the incident angle of light traveling from a medium with a high /n/ to a medium with a low n is greater than the critical angle θ_c. This results in all of the light being reflected and none of it being refracted.

Spherical Mirror

Spherical mirrors have the appearance of a curved surface that is either concave or convex. A converging mirror is a concave mirror with a positive focal length, while a diverging mirror is a convex mirror with a negative focal length. Diverging mirrors always produce virtual images.

Speed of Light

The speed of electromagnetic waves traveling through a vacuum, given by the equation c = λf, where c is a constant equal to  3.00 × 10^8 m/s.

Sound Level

The loudness of a sound, measured in decibels (dB) and denoted by β. Given by the equation  β=10log(I/I0) , where I is the intensity of the sound and I₀ is a reference intensity of 10⁻¹²

Snell's Law

Equation describing the angle of refraction for a light ray passing from one medium to another, given by n₁ sin θ₁ = n₂ sin θ₂, where n represents the index of refraction in each medium.

Plane-Polarized Light

Light that has been passed through a polarizing filter, allowing only the transmission of waves containing electric field vectors parallel to the lines of the filter.

Plane Mirror

A mirror in which incident light rays remain parallel after reflection, always producing a virtual image that appears to be the same distance behind the mirror as the object is in front of the mirror.

Magnification

A dimensionless value denoted by m given by the equation: m=-i/o where i is image distance and o is object distance. A negative m denotes an inverted image, whereas a positive m denotes an upright image.

Lens

A transparent device with a curvature that causes light to bend (refract) as it passes through. May be converging or diverging. A lens with a thick center that converges light rays at a point where the image is formed is called a converging lens. A lens with a thin center that diverges light after refraction and always forms a virtual image is called a diverging lens.