Acceleration

Acceleration is the rate of change in velocity. Remember that velocity is properly understood as a combination of speed and direction, so acceleration can refer to the rate of change in direction as well as change in speed, or some combination of both. For example, something orbiting at a constant speed is still accelerating. In the figure below, note that the length of the velocity vector (representing speed) is not changing, because the acceleration (due to gravity) is always orthogonal (perpendicular to) to the velocity.

That acceleration due to gravity is usually expressed by the constant, g = 9.8 meters per second per second. That number is approximate, and only valid near the surface of the planet, but it means that every second you're falling you'll be falling 9.8 m/s (22 mph) faster. In three seconds you'll be going highway speeds. The fastest production cars can just barely match that, doing 0-60 mph in just under three seconds. That's also about the acceleration that a cheetah can pull in a straight line. All of those things can accelerate at about 1 g. Impressive in a different way is the remarkable mantis shrimp. The mantis shrimp packs a punch that has it's foreleg accelerating at about 10,600 g! That's on the level of a bullet fired out of a gun. This motion is so tremendous that it can even cause cavitation for additional damage.