Short answers, pulled from the story.
Kinetic energy is the form of energy an object possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating body equals half the product of its mass and the square of its speed. It equals the work needed to accelerate the object from rest to its speed.
William Thomson, later Lord Kelvin, is credited with coining the term kinetic energy around 1849 to 1851. William Rankine, who introduced the term potential energy in 1853, later noted that Thomson and Peter Tait substituted the word kinetic for the earlier phrase actual energy.
The adjective kinetic has its roots in the Greek word kinesis, meaning motion. The dichotomy between kinetic energy and potential energy can be traced back to Aristotle's concepts of actuality and potentiality.
Willem 's Gravesande provided experimental evidence in 1722 by dropping weights from different heights into a block of clay. He determined that their penetration depth was proportional to the square of their impact speed. Emilie du Chatelet recognized the implications and published an explanation.
Kinetic energy increases with the square of the speed, so an object doubling its speed has four times as much kinetic energy. As a consequence, a car traveling twice as fast needs four times the distance to stop under a constant braking force, and it takes four times the work to double the speed.
The SI unit of energy, including kinetic energy, is the joule, where mass is measured in kilograms and speed in metres per second. The English unit of energy is the foot-pound.
Because kinetic energy is a function of velocity, it depends on the observer's frame of reference. A bullet passing an observer has kinetic energy in that observer's frame, but the same bullet has zero kinetic energy for an observer moving alongside it at the same velocity.