A physics term, velocity describes the motion of objects. Velocity measures the movement of objects based on their speed and direction. The speed of an object measures how much distance it covers over a specific period of time. Speed is a scalar measurement since it only defines the magnitude of how fast an object is moving. Velocity is a vector quantity since it describes both speed and direction.
An object with a constant velocity does not change in speed or direction. The only objects that qualify as moving at a constant velocity are those that move in a straight line at a steady speed. An object outside a solar system, in interstellar space, that is not under the influence of outside forces might be described as an object that moves with a constant velocity.
Objects with changing velocity exhibit a change in speed or direction over a period of time. Changes in the velocity of objects are measured as acceleration. Objects with a constant speed and a changing direction are also accelerating. Comets and asteroids within the solar system are examples of objects with changing velocity since their speed or direction is influenced by gravity.
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Mathematics of Acceleration
Acceleration measures changing velocity as a result of changes in direction or speed. Mathematically, acceleration is equal to the change in velocity divided by a specific amount of time. A car that increases its velocity by 10 miles per hour every two seconds is accelerating at 5 miles per hour every second. Changes in the direction of an object also constitute acceleration and are usually shown using a graph.
Instant velocity is a method for determining how quickly an object is changing its speed or direction at a given point in time. Instant velocity is determined by reducing the period of time used to measure acceleration to such a small quantity that the object does not accelerate during the given period of time. This method of measuring velocity is useful for producing graphs measuring a series of changes in velocity.
Terminal velocity is a term used to describe the movement of an object falling freely through the atmosphere. Objects falling to the ground in a vacuum will constantly accelerate until they reach the ground. An object falling through the atmosphere, however, will eventually stop accelerating due to increasing amounts of air resistance. The point at which air resistance equals the acceleration caused by gravity -- or whatever force is acting on the object -- is known as terminal velocity.