
Q–T
Q
 Quark
 The building blocks of all matter, quarks are the constituent parts of protons, neutrons, and mesons.
R
 Radian
 A unit for measuring angles; also called a “rad.” 2π rad = 360º.
 Radiation
 Heat transfer via electromagnetic waves.
 Radioactive decay
 The process by which unstable nuclei spontaneously release particles and/or energy so as to come to a more stable arrangement. The most common forms of radioactive decay are alpha decay, beta decay, and gamma decay.
 Radioactivity
 An object is called radioactive if it undergoes radioactive decay.
 Radius of curvature
 With spherical mirrors, the radius of the sphere of which the mirror is a part.
 Rarefaction
 An area of high air pressure that acts as the wave trough for sound waves. The spacing between successive rarefactions is the wavelength of sound, and the number of successive areas of rarefaction that arrive at the ear per second is the frequency, or pitch, of the sound.
 Real image
 An image created by a mirror or lens in such a way that light does actually come from where the image appears to be. If you place a screen in front of a real image, the image will be projected onto the screen.
 Reflect
 A wave on a string that is tied to a pole at one end will reflect back toward its source, producing a wave that is the mirrorimage of the original and which travels in the opposite direction.
 Reflected ray
 The ray of light that is reflected from a mirror or other reflecting surface.
 Reflection
 The phenomenon of light bouncing off a surface, such as a mirror.
 Refracted ray
 The ray of light that is refracted through a surface into a different medium.
 Refraction
 The bending of light as it passes from one medium to another. Light refracts toward the normal when going from a less dense medium into a denser medium and away from the normal when going from a denser medium into a less dense medium.
 Restoring force
 The force that causes simple harmonic motion. The restoring force is always directed toward an object’s equilibrium position.
 Righthand rule
 A means of defining the direction of the cross product vector. To define the direction of the vector , position your right hand so that your fingers point in the direction of A, and then curl them around so that they point in the direction of B. The direction of your thumb shows the direction of the cross product vector.
 Rigid body
 An object that retains its overall shape, meaning that the particles that make up the rigid body stay in the same position relative to one another.
 Rotational kinetic energy
 The energy of a particle rotating around an axis.
 Rotational motion
 Occurs when every point in the rigid body moves in a circular path around a line called the axis of rotation.
 Rutherford nuclear model
 The model of the atom according to which negatively charged electrons orbit a positively charged nucleus. This model was developed by Ernest Rutherford in light of the results from his gold foil experiment.
S
 Scalar
 A quantity that possesses a magnitude but not a direction. Mass and length are common examples.
 Second Law of Thermodynamics
 There are a few versions of this law. One is that heat flows spontaneously from hot to cold, but not in the reverse direction. Another is that there is no such thing as a 100% efficient heat engine. A third states that the entropy, or disorder, of a system may increase but will never decrease spontaneously.
 Significant digits
 The number of digits that have been accurately measured. When combining several measurements in a formula, the resulting calculation can only have as many significant digits as the measurement that has the smallest number of significant digits.
 Simple harmonic oscillator
 An object that moves about a stable equilibrium point and experiences a restoring force that is directly proportional to the oscillator’s displacement.
 Sine
 In a right triangle, the sine of a given angle is the length of the side opposite the angle divided by the length of the hypotenuse.
 Snell’s Law
 Relates the angle of incidence to the angle of refraction: .
 Sound
 Waves carried by variations in air pressure. The speed of sound waves in air at room temperature and pressure is roughly 343 m/s.
 Specific heat
 The amount of heat of a material required to raise the temperature of either one kilogram or one gram of that material by one degree Celsius. Different units may be used depending on whether specific heat is measured in s of grams or kilograms, and joules or calories.
 Spectroscope
 A device that breaks incoming light down into spectral rays, so that one can see the exact wavelength constituents of the light.
 Speed
 A scalar quantity that tells us how fast an object is moving. It measures the rate of change in distance over time. Speed is to be contrasted with velocity in that there is no direction associated with speed.
 Spring
 Objects that experience oscillatory or simple harmonic motion when distorted. Their motion is described by Hooke’s Law.
 Spring constant
 Indicates how “bouncy” or “stiff” a spring is. More specifically, the spring constant, k, is the constant of proportionality between the restoring force exerted by the spring, and the spring’s displacement from equilibrium. The greater the value of k, more resistant the spring is to being displaced.
 Standing wave
 A wave that interferes with its own reflection so as to produce oscillations which stand still, rather than traveling down the length of the medium. Standing waves on a string with both ends tied down make up the harmonic series.
 Static friction
 The force between two surfaces that are not moving relative to one another. The force of static friction is parallel to the plane of contact between the two objects and resists the force pushing or pulling on the object.
 Strong nuclear force
 The force that binds protons and neutrons together in the atomic nucleus.
 Sublimation
 The process by which a solid turns directly into gas, because it cannot exist as a liquid at a certain pressure.
 Superposition
 The principle by which the displacements from different waves traveling in the same medium add up. Superposition is the basis for interference.
 System
 A body or set of bodies that we choose to analyze as a group.
T
 Tail
 In the graphical representation of vectors, the tail of the arrow is the blunt end (the end without a point).
 Tangent
 In a right triangle, the tangent of a given angle is the length of the side opposite the angle divided by the length of the side adjacent to the triangle.
 Temperature
 A measure of the average kinetic energy of the molecules in a system. Temperature is related to heat by the specific heat of a given substance.
 Tension force
 The force transmitted along a rope or cable.
 Thermal energy
 The energy of the molecules that make up an object. It is related to heat, which is the amount of energy transferred from one object to another object that is a different temperature.
 Thermal equilibrium
 Two materials are in thermal equilibrium if they are at the same temperature.
 Third Law of Thermodynamics
 An object cannot be cooled to absolute zero.
 Threshold frequency
 A property of a metal, the minimum frequency of electromagnetic radiation that is necessary to release photoelectrons from that metal.
 Tip
 In the graphical representation of vectors, the tip of the arrow is the pointy end.
 Torque
 The effect of force on rotational motion.
 Total internal reflection
 The phenomenon by which light traveling from a high n to a low n material will reflect from the optical interface if the incident angle is greater than the critical angle.
 Transformer
 A device made of two coils, which converts current of one voltage into current of another voltage. In a stepup transformer, the primary coil has fewer turns than the secondary, thus increasing the voltage. In a stepdown transformer, the secondary coil has fewer turns than the primary, thus decreasing the voltage.
 Translational kinetic energy
 The energy of a particle moving in space. It is defined in s of a particle’s mass, m, and velocity, v, as (1/2)mv^{2}.
 Translational motion
 The movement of a rigid body’s center of mass in space.
 Transverse waves
 Waves in which the medium moves in the direction perpendicular to the propagation of the wave. Waves on a stretched string, water waves, and electromagnetic waves are all examples of transverse waves.
 Traveling waves
 A wave with wave crests that propagate down the length of the medium, in contrast to stationary standing waves. The velocity at which a crest propagates is called the wave speed.
 Trough
 The points of maximum negative displacement along a wave. They are the opposite of wave crests.
