| Saturated air: | Air in which equilibrium exists between evaporation and condensation; the relative humidity is 100 percent. |
| Scalar Quantity: | A physical quantity, which is described completely by its magnitude. |
| Second's Pendulum: | A simple pendulum whose time period on the surface of earth is 2 seconds. |
| Semiconductors: | Elements whose electrical conductivity is intermediate between that of a conductor and an insulator. |
| Shear strain: | The ratio of the relative displacements of one plane to its distance from the fixed plane. |
| Shear stress: | The restoring force developed per unit area when deforming force acts tangentially to the surface of body producing change in the shape of the body without any change in volume. |
| Siemens: | The derived S.I. unit of electrical conductance, equal to the conductance of an element that has a resistance of 1 ohm, also written as ohm-1. |
| Simple harmonic motion: | The vibratory motion that occurs when the restoring force is proportional to the displacement from mean position and is directed opposite to the displacement. |
| Snell's law: | The ratio of sin i to sin r is a constant and is equal to the refractive index of the second medium with respect to the first. |
| Solenoid: | A cylindrical coil of wire that becomes electromagnetic when a current flows through it |
| Sonic boom: | Sound waves that pile up into a shock wave when a source is traveling at or faster than the speed of sound. |
| Specific heat: | The amount of heat energy required to increase the temperature of one gram of a substance by 1 . |
| Speed: | The distance traveled by a body per unit of time. |
| Spin quantum number: | From quantum mechanics model of the atom, one of four descriptions of the energy state of an electron wave; this quantum number describes the spin orientation of an electron relative to an external magnetic field. |
| Standing waves: | The waves formed due to superposition of two waves of same frequency and traveling in opposite directions with same speed. |
| Steam-point: | It is the temperature of steam over pure boiling water under 1 atm pressure. It is equal to 100 or 212 . |
| Stefan-Boltzmann law: | The amount of energy radiated per second per unit area of a perfectly black body is directly proportional to the fourth power of the absolute temperature of the surface of the body. |
| Superconductors: | Some materials in which, under certain conditions, the electrical resistance approaches zero. |
| Surface tension: | The property of a liquid due to which its surface behaves like a stretched membrane. |
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