Acceleration is the rate of change of velocity per unit of time. In physics, the delta symbol (\(\Delta\)) represents change so the formula for acceleration becomes \(\vec{a} = { \Delta \vec{v} \over t }\)

The Periodic Table of the Elements categorizes elements primarily by the number of protons in their nucleus (atomic number) and secondarily by the characteristics they exhibit.

An air mass is a large body of air that has similar moisture (density) and temperature characteristics. A front is a transition zone between two air masses.

Secondary consumers (carnivores) subsist mainly on primary consumers. Omnivores are secondary consumers that also eat producers. Examples are rats, fish, and chickens.

The four planets closest to the Sun (Mercury, Venus, Earth, and Mars) are called terrestrial (Earth-like) planets because, like the Earth, they're solid with inner metal cores covered by rocky surfaces.