| Glossary | ||||||||||||||
| reaction force
— one member of an action–reaction pair of forces that is equal in strength and opposite in direction to the action force that is its counterpart. real image — image formed by light rays that converge at the location of the image. A real image can be viewed or projected on a screen. reference frame — coordinate system assigning positions and times to events. Two different reference frames can be in motion with respect to each other. reflected ray — light ray that bounces off a surface or boundary between two materials. reflection — process by which waves (including light) bounce off or return by striking a surface or the boundary between two different materials. refraction — process by which waves (including light) change direction by traveling into or through a surface or boundary between two materials. refrigerator — device that uses a reverse Carnot cycle to do work to transfer thermal energy from a colder object to a warmer one. rem — unit of measure for ionizing radiation that estimates its impact on a human. Acronym for roentgen equivalent man. renewable energy — comes from natural sources of energy—such as solar, water from rainfall, wind, tides, or geothermal heat—that are replenished naturally and presumably can be used as an energy source indefinitely. reproducibility — ability to obtain the same results for experiments or observations by doing the same experiment or making the same observation in the same way. resistance — opposition that a conducting material or device offers to resist the flow of current. Measured in ohms (Ω). resistor — electrical device that resists the flow of electric current. resolution of forces — process of breaking a single force into the equivalent set of one or more component forces. resonance — condition where the frequency of a periodic force matches a natural frequency of an oscillating system. At resonance, energy accumulates over many cycles of the periodic force and oscillations can become very large, even when the causative force is relatively small. | rest energy
— energy equivalent to the mass of a particle or body at rest, i.e., with zero speed, given by the equation E = mc2. resultant vector — vector sum of two or more vectors. reversibility — property of an ideal thermodynamic process that can return to its original state by running the same series of steps backwards. revolution — spinning motion of an object about an axis outside of the object. The Earth, for example, revolves around the Sun. rods — light-sensitive cells in the eye that respond to differences in brightness of the light. rolling friction — friction force that occurs between two surfaces in rolling contact with each other. rotation — spinning motion of an object about an axis within the object. The Earth, for example, rotates about its axis every 24 hours. rotational energy — mechanical energy of rotation for an object, equal to one-half of the product of its moment of inertia and the square of angular velocity, or Er = ½Iω2. Rotational energy is measured in units of joules (J). rotational inertia — apparent resistance of an object to change its state of rotation. The measure of rotational inertia about a particular axis is called the moment of inertia. safety factor — multiplier typically between 2 and 20 that compares calculated forces and actual design specifications. For example, a safety factor of 10 means a structure that is calculated to support a force of 5 N is designed to withstand 50 N. satellite — natural or technological object that is gravitationally bound to, and travels in a repeating orbit around, a larger object such as a planet. scalar — quantity that can be completely described by a single value (without a direction). Mass, pressure, and temperature are scalars. | |||||||||||||
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