In everyday use and kinematics, the speed of an object is the amount of change in its position; therefore, it is a scalar quantity. The average speed of an object in an interval of time is the distance traveled by an object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as the duration of the time interval approaches zero.

Speed is distance divided by time. The SI unit of speed is meter per second, but the most common unit of speed in everyday use is kilometres per hour, or, in the United States and the United Kingdom, miles per hour. For travel by air and sea, a knot is usually used.

According to special relativity, the maximum possible speed at which energy or information can move is the speed of light in a vacuum, c = 299792458 meters per second (approximately 10.79 billion km/h or 671 million miles per hour). Matter cannot reach the speed of light, as it will require an infinite amount of energy. In the physics of relativity, the concept of rapidity replaces the classical concept of speed. Unlike instantaneous speed, the average speed is defined as the total distance covered divided by a time interval. For example, if a distance of 80 kilometres is covered in 1 hour, the average speed is 80 kilometres per hour. Likewise, if you cover 320 kilometres in 4 hours, the average speed will also be 80 kilometres per hour. If the distance in kilometres (km) is divided by the time in hours (hours), the result is in kilometres per hour (km/h).

Average speed does not describe speed fluctuations that may have occurred at shorter intervals (since this is the total distance traveled divided by the whole travel time), and therefore the average speed is often very different from the instantaneous speed. If the average speed and travel time are known, the distance traveled can be calculated by changing the definition.

In graphical terms, the slope of the tangent line at each point in the distance and time graph represents the instantaneous speed at that moment, and the slope of the chord line in the same graph is the average speed over the time interval covered by the chord. The average speed of the object Vav = s÷t.

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