Tangential Velocity - For an object orbiting around another object, the sideways component of velocity; that is, the component of velocity parallel to the second object's surface and thus perpendicular to the line joining the centers of the two objects.

Telescope - Optical instrument that forms enlarged images of very distant objects.

Temperature - The property of a material that tells how warm or cold it is with respect to some standard.

Terminal Speed - The speed at which the acceleration of a falling object terminates because friction balances the weight.

Terminal Velocity - Terminal speed together with the direction of motion (down for falling objects).

Terrestrial Radiation - Radiant energy emitted from the earth after having been absorbed from the sun.

Theory - A synthesis of a large body of information that encompasses well-tested and verified hypotheses about certain aspects of the natural world.

Thermal Contact - The state of two or more objects or substances in thermal contact when they have reached a common temperature

Thermal Equilibrium - The state of two or more objects or substances in thermal contact when they have reached a common temperature

Thermonuclear Fusion - A nuclear fusion reaction brought about by extremely high temperatures.

Thermostat - A type of valve or switch that responds to changes in temperature and that is used to control the temperature of something.

Time Dilation - An observable stretching, or slowing, of time in a frame of reference moving past the observer at a speed approaching the speed of light.

Torque - The tendency of a force to cause rotation about an axis; the product of the force and the lever arm; measured in Newton-meters.

Total Internal Reflection - The 100% reflection (with no transmission) of light that strikes the boundary between two media at an angle greater than the critical angle.

Transformer - A device for increasing or decreasing voltage by means of electromagnetic induction.

Transmutation - The conversion of an atomic nucleus of one element into an atomic nucleus of another element through a loss or gain in the number of protons

Transparent - Term applied to materials that allow light to pass through them in straight lines.

Transverse Waves - A wave in which the vibration is at right angles to the direction in which the wave is traveling.

Trough - One of the places in a wave where the wave is lowest or the disturbance is greatest in the opposite direction from the crest.

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Ultrasonic - Term applied to sound frequencies above 20,000 hertz, the normal upper limit of human hearing.

Ultraviolet - Electromagnetic waves of frequencies higher than those of violet light.

Umbra - The darker part of a shadow where all the light is blocked.

Unstable Equilibrium - The state of an object balanced so that any small rotation lowers its center of gravity

Universal Constant of Gravitation - The constant G in the equation for Newton's law of universal gravitation; changes the units of mass and distance on the right side of the equation to the units of force on the left side.

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- The symbol for volt. Also, when in lowercase italic, the symbol for speed or velocity. When in upper-case italic, the symbol for voltage.

Vector - An arrow whose length represents the magnitude of a quantity and whose direction represents the direction of the quantity.

Vector Quantity - A quantity in physics, such as force or velocity, that has both magnitude and direction.

Velocity - Speed together with the direction of motion.

Vibration - A “wiggle in time”; a repeating, to-and-fro motion of something (such as a pendulum or the particles of an elastic body or a fluid) when displaced from the position of equilibrium.

Virtual Image - An image formed through reflection or refraction that can be seen by an observer but that cannot be projected on a screen because light from the object does not actually from to a focus.

Volt - The SI unit of electric potential. One volt (symbol V) is the electric potential. At which one coulomb of charge would have one joule of potential energy.

Voltage - (a) Electric potential; measured in volts. (b) Potential difference; measured in volts.

Voltage Source - A device, such as a dry cell, battery, or generator, that provides a potential difference.

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- The symbol for watt. Also, when in italic, the symbol for work.

Watt - The SI unit of power. One watt of power is expended when one joule of work is done in one second.

Wave - A “wiggle in space and time”; a disturbance that repeats regularly in space and time and that is transmitted progressively from one particle or region in a medium to the next with no actual transport of matter.

Wave Front - The crest, trough, or any continuous portion of a two-dimensional or three-dimensional wave in which the vibrations are all the same way at the same time.

Wavelength - The distance from the top of a crest of a wave to the top of the following c rest, or equivalently, the distance between successive identical parts of the wave.

Weight - The force on a body of matter due to the gravitational attraction of another body (commonly the earth).

Weight Density - The weight of a substance divided by its volume.

White Light - Light, such as sunlight, that is a combination of all the colors. Under white light, white objects appear white and colored objects appear in their individual colors.

Work - The product of the force on an object and the distance through which the object is moved (when the force is constant and the motion takes place in a straight line in the direction of the force); energy expended when the speed of something is increased or when something is moved against the influence of an apposing force; measured in joules.