Distance, Speed and Velocity

Distance and speed are scalar quantities, as they are fully described by magnitude with no reference to direction. ... Velocity can be thought of as speed with direction. The formula for speed is change in distance divided by change in time. The formula for velocity is change in displacement divided by change in time.

• Speed is directly proportional to distance when time is constant: v ∝ s (t constant)
• Speed is inversely proportional to time when distance is constant: v ∝ ⅟t (s constant)

Combining these two rules together gives the definition of speed in symbolic form.

v =	s
	t

Speed is the rate of change of distance with time.

Velocity is a vector quantity that refers to "the rate at which an object changes its position."

Velocity is a vector quantity.

The average speed during the course of a motion is often computed using the following formula:

In contrast, the average velocity is often computed using this formula

Average Speed versus Instantaneous Speed

Since a moving object often changes its speed during its motion, it is common to distinguish between the average speed and the instantaneous speed. The distinction is as follows.

• Instantaneous Speed - the speed at any given instant in time.
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• Average Speed - the average of all instantaneous speeds; found simply by a distance/time ratio.

Speed and velocity are both measured using the same units. The SI unit of distance and displacement is the meter. The SI unit of time is the second. The SI unit of speed and velocity is the ratio of two — the meter per second.