- The symbol for joule.

Joule - The SI unit of work and of all other forms of energy as well. One joule (symbol J) of work is done when a force of one newton is exerted on an object that moves a distance of one meter in the direction of the force.


- The symbol for kelvin. Also, when in lower case, the symbol for the prefix -kilo.

kcal - The symbol for kilocalorie.

Kelvin - The SI unit of temperature. A temperature measured in kelvins (symbol K) indicates the number of units above absolute zero. Since the divisions on the Kelvin scale and the Celsius scale are the same size, a change in temperature of one kelvin is equal to a change in temperature of 1 Celsius.

Kelvin Scale - A temperature scale calibrated in terms of energy itself as well as in terms of the freezing and boiling points of water. Absolute zero (-273 degrees Celsius) is taken as 0 K. There are no negative temperatures on the Kelvin scale.

kg - The symbol for kilogram.

kilocalorie - A unit of heat. One kilocalorie equals 1000 calories, or the amount of heat required to raise the temperature of one kilogram of water by 1 degree Celsius.

kilogram - The fundamental SI unit of mass. One kilogram (symbol kg) is the amount of mass in one liter of water at 4°C.

Kinetic Energy - The energy of motion. It is equal to half the mass multiplied by the square of the speed.

km - The symbol for kilometer.


L - The symbol for liter.

Laser - An optical instrument that produces a beam of coherent light, that is, a beam in which the waves all have the same frequencyphase, and direction.

Law - A general hypothesis or statement about the relationship of natural quantities that has been tested over and over again and has not been contradicted. Also known as a principle.

Law of Conservation of Angular Momentum - The statement that an object or system of objects will maintain a constant angular momentum unless acted upon by an unbalanced external torque. 

Law of Conservation of Energy - The statement that energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes.

Law of Conservation of Momentum - The statement that in the absence of a net external force, the momentum of an object or system of objects remains unchanged.

Law of Inertia - The statement that every body continues in its state of rest, or of motion in a straight line at constant speed, unless it is compelled to change that state by a net force exerted upon it. Also known as Newton's First Law.

Law of Reflection - The statement that when a wave strikes a surface, the angle of incidence is equal to the angle of reflection. It holds true for both partially and totally reflected waves. 

Law of Universal Gravitation - The statement that for any pair of objects, each object attracts the other object with a force that is directly proportional to the mass of each object, and inversely proportional to the square of the distance between their centers of mass.

Lens - A piece of glass (or other transparent material) that can bend parallel rays of light so that they cross, or appear to cross, at a single point.

Lever - A simple machine, made of a bar that turns about a fixed point.

Lever Arm - For a force that tends to cause rotation about an axis and that is perpendicular to the line between the point of contact and the axis, the length of that line.

Lift - In the application of Bernoulli's principle, the net upward force produced by the difference between upward and downward pressures. When the lift equals the weight, horizontal flight is possible.

Light Year - The distance traveled by light in one year.

Line Spectrum - The pattern of distinct lines of color, corresponding to particular wavelengths, that are seen in the spectroscope when a hot gas is viewed.

Linear Momentum - The product of the mass and the velocity of an object. Also called simply momentum.

Linear Speed - The distance moved per unit of time. Also called simply speed.

Longitudinal Wave - A wave in which the vibration is in the same direction as that in which the wave is traveling, rather than at right angles to it.

Lunar Eclipse - The cutoff of light from the full moon when the earth is directly between the sun and the moon, so that the earth's shadow is cast on the moon.

- The symbol for meter, Also, when in italic (meter) the symbol for mass.

Machine - A device for multiplying (or decreasing) forces or simply changing the direction of forces. 

Magnetic Domain- A microscopic cluster of atoms with their magnetic fields aligned.

Magnetic Field - A force field that fills the space around every magnet or current-carrying wire. Another magnet or current-carrying wire introduced into this region will experience a magnetic force acting on itself.

Magnetic Pole - One of the regions on a magnet that produce magnetic forces.

Mass - A measure of the quantity of matter a body contains; may also be considered a measure of the inertia of an object.

Mechanical Advantage - The ratio of output force to input force for a machine.

Mechanical Energy - The energy due to the position or the movement of something; potential or kinetic energy (or a combination of both).

Mirage - A floating image that appears in the distance and is due to the refraction of light in the earth's atmosphere.

Molecule - Two or more atoms of the same or different elements joined to form a larger particle.

Momentum - The product of the mass and the velocity of an object. Has direction as well as size. Also called linear momentum.

Monochromatic - Having a single color or frequency.


- The symbol for newton.

Natural Frequency - A frequency at which an elastic object naturally tends to vibrate, so that minimum energy is required to produce a forced vibration or to continue vibration at that frequency.

Neap Tide - A tide that occurs when the moon is halfway between a new and full moon, in either direction. The tides due to the sun and the moon partly cancel, so that the high tides are lower than average and the low tides are not as low as average.

Nearsighted - Term applied to a person who can see nearby objects clearly but not distant objects.

Net Force - The combination of all the forces that act on an object.

Neutral Equilibrium - The state of an object balanced so that any small rotation neither raises nor lowers its center of gravity.

Neutron - An electrically neutral particle that is one of the two kinds of particles found in the nucleus of an atom.

Newton - The SI unit of force. One newton (symbol N) is the force that will give an object of mass one kilogram an acceleration of one meter per second squared.

Newton's First Law See law of inertia

Newton's Law of Cooling - The statement that the rate of cooling of an object--whether by conduction, convection, or radiation--is approximately proportional to the temperature difference between the object and its surroundings.

Newton's Second Law - The statement that the acceleration produced by a net force on a body is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the body.

Newton's Third Law - The statement that whenever one body exerts a force on a second body, the second body exerts an equal and opposite force on the first.

Node  - Any part of a standing wave that remains stationary.

Normal - A line that is perpendicular to a surface.

Normal Force - For an object resting on a horizontal surface, the upward force that balances the weight of the object. Also called the support force.

Nuclear Fission - The splitting of an atomic nucleus, particularly that of a heavy element such as uranium-235, into two main parts, accompanied by the release of much energy.

Nuclear Fusion - The combining of nuclei of light atoms, such as hydrogen, into heavier nuclei, accompanied by the release of much energy.

Nucleon - The principal building block of the nucleus; a neutron or a proton.

Nucleus - The positively charged center of an atom, which contains protons and neutrons and has almost all the mass of the entire atom but only a tiny fraction of the volume.


Objective Lens - In an optical device using compound lenses, the lens closest to the object observed.

Ohm - The SI unit of electrical resistance. One ohm (symbol Ω) is the resistance of a device that draws a current of one ampere when a voltage of one volt is impressed across it.

Ohm's Law - The statement that the current in a circuit is directly proportional to the voltage impressed across the circuit and inversely proportional to the resistance of the circuit.

Opaque - Term applied to materials that absorb light without re-emission and thus do not allow light through them.

Optical Fiber - A transparent fiber, usually of glass or plastic, that can transmit light down its length by means of total internal reflection.

Out of Phase - Term applied to two waves for which the crest of one wave arrives at a point at the same time as a trough of the second wave arrives. Their effects cancel each other.