First Law of Thermodynamics

Essentially a restatement of energy conservation, it states that the change in the internal energy of a system is equal to the heat added plus the work done on the system.

Force

A push or a pull that causes an object to accelerate.

Free-body diagram

Illustrates the forces acting on an object, drawn as vectors originating from the center of the object.

Frequency

The number of cycles executed by a system in one second. Frequency is the inverse of period, f = 1/T. Frequency is measured in hertz, Hz.

Frictional force

A force caused by the roughness of two materials in contact, deformations in the materials, and a molecular attraction between the materials. Frictional forces are always parallel to the plane of contact between two surfaces and opposite the direction that the object is being pushed or pulled.                                                                                                                                                

Gravitational constant

The constant of proportionality in Newton’s Law of Gravitation. It reflects the proportion of the gravitational force and , the product of two particles’ masses divided by the square of the bodies’ separation. N · m²/kg².

Gravitational Potential Energy

The energy associated with the configuration of bodies attracted to each other by the gravitational force. It is a measure of the amount of work necessary to get the two bodies from a chosen point of reference to their present position. This point of reference is usually chosen to be a point of infinite distance, giving the equation . Objects of mass m that are a height h above the surface of the earth have a gravitational potential energy of .

Half-life

The amount of time it takes for one-half of a radioactive sample to decay.

Heat

A transfer of thermal energy. We don’t speak about systems “having” heat, but about their “transferring” heat, much in the way that dynamical systems don’t “have” work, but rather “do” work.

Hooke’s Law

For an oscillating spring, the restoring force exerted by the spring is directly proportional to the displacement. That is, the more the spring is displaced, the stronger the force that will pull toward the equilibrium position. This law is expressed mathematically as F = –kx, where F is the restoring force and x is the displacement. The constant of proportionality, –k, is the spring constant.

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.

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.

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.

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.

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.

Kepler’s First Law

The path of each planet around the sun is an ellipse with the sun at one focus.

Kepler’s Second Law

If a line is drawn from the sun to the planet, then the area swept out by this line in a given time interval is constant.

Kepler’s Third Law

Given the period, T, and semimajor axis, a, of a planet’s orbit, the ratio is the same for every planet.

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