Ideal gas law

An equation, PV = nRT, that relates the pressure, volume, temperature, and quantity of an ideal gas. An ideal gas is one that obeys the approximations laid out in the kinetic theory of gases.
Impulse
A vector quantity defined as the product of the force acting on a body multiplied by the time interval over which the force is exerted.
Incident ray
When dealing with reflection or refraction, the incident ray is the ray of light before it strikes the reflecting or refracting surface.
Inclined plane
A wedge or a slide. The dynamics of objects sliding down inclined planes is a popular topic on SAT II Physics.
Index of refraction
The index of refraction n = c/v of a substance characterizes the speed of light in that substance, v. It also characterizes, by way of Snell's Law, the angle at which light refracts in that substance.
Induced current
The current induced in a circuit by a change in magnetic flux.
Inelastic collision
A collision in which momentum is conserved but kinetic energy is not.
Inertia
The tendency of an object to remain at a constant velocity, or its resistance to being accelerated. Newton’s First Law is alternatively called the Law of Inertia because it describes this tendency.
Inertial reference frame
A reference frame in which Newton’s First Law is true. Two inertial reference frames move at a constant velocity relative to one another. According to the first postulate of Einstein’s theory of special relativity, the laws of physics are the same in all inertial reference frames.
Instantaneous velocity
The velocity at any given instant in time. To be contrasted with average velocity, which is a measure of the change in displacement over a given time interval.
Internal energy
The energy stored in a thermodynamic system.
Inversely proportional
Two quantities are inversely proportional if an increase in one results in a proportional decrease in the other, and a decrease in one results in a proportional increase in the other. In a formula defining a certain quantity, those quantities to which it's inversely proportional will appear in the denominator.
Isolated system
A system that no external net force acts upon. Objects within the system may exert forces upon one another, but they cannot receive any impulse from outside forces. Momentum is conserved in isolated systems.
Isotope
Atoms of the same element may have different numbers of neutrons and therefore different masses. Atoms of the same element but with different numbers of neutrons are called isotopes of the same element.