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

Harmonic series

The series of standing waves supported by a string with both ends tied down. The first member of the series, called the fundamental, has two nodes at the ends and one anti-node in the middle. The higher harmonics are generated by placing an integral number of nodes at even intervals over the length of the string. The harmonic series is very important in music.


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.

Heat engine

A machine that operates by taking heat from a hot place, doing some work with that heat, and then exhausting the rest of the heat into a cool place. The internal combustion engine of a car is an example of a heat engine.

Heat transfer

A transfer of thermal energy from one system to another.

Hertz (Hz)

The units of frequency, defined as inverse-seconds (1 Hz = 1 s–1). “Hertz” can be used interchangeably with “cycles per second.”

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.


The longest side of a right triangle, opposite to the right angle.