Earth's gravity, enoted by g, refers to the attractive force that the Earth exerts on objects on or near its surface. Its strength is usually quoted in terms of the acceleration produced by it (acceleration due to gravity), which in SI units is measured in m/s² (metres per second per second, equivalently written as m·s−2). It has an approximate value of 9.8 m/s², which means that, ignoring air resistance, the speed of an object falling freely near the Earth's surface increases by about 9.8 metres per second every second.