P

Pascals
The unit for measuring pressure. One Pascal is equal to one Newton per meter squared, 1 Pa = 1 N/m2.
Pendulum
A pendulum consists of a bob connected to a rod or rope. At small angles, a pendulum’s motion approximates simple harmonic motion as it swings back and forth without friction.
Period
The time it takes a system to pass through one cycle of its repetitive motion. The period, T, is the inverse of the motion’s frequency, f = 1/T.
Phase
Two oscillators that have the same frequency and amplitude, but reach their maximum displacements at different times, are said to have different phases. Similarly, two waves are in phase if their crests and troughs line up exactly, and they are out of phase if the crests of one wave line up with the troughs of the other.
Phase change
When a solid, liquid, or gas changes into another phase of matter.
Photoelectric effect
When electromagnetic radiation shines upon a metal, the surface of the metal releases energized electrons. The way in which these electrons are released contradicts classical theories of electromagnetic radiation and supports the quantum view according to which electromagnetic waves are treated as particles.
Photoelectron
The name of an electron released from the surface of a metal due to the photoelectric effect.
Photon
A small particle-like bundle of electromagnetic radiation.
Pitch
Another word for the frequency of a sound wave.
Planck’s constant
A constant, J · s, which is useful in quantum physics. A second constant associated with Planck’s constant is .
Polarization
A process that aligns a wave of light to oscillate in one dimension rather than two.
Potential energy
Energy associated with an object’s position in space, or configuration in relation to other objects. This is a latent form of energy, where the amount of potential energy reflects the amount of energy that potentially could be released as kinetic energy or energy of some other form.
Power
Defined as the rate at which work is done, or the rate at which energy is transformed. P is measured in joules per second (J/s), or watts (W).
Pressure
A measure of force per unit area. Pressure is measured in N/m2 or Pa.
Principal axis
The straight line that runs through the focal point and the vertex of a mirror or lens.
Proton
A positively charged particle that, along with the neutron, occupies the nucleus of the atom.
Pulley
A pulley is a simple machine that consists of a rope that slides around a disk or block.