One of the most obvious aspect of an object in motion is how fast it is moving. Speed (v) is defined as the distance travelled by an object per unit of time. Speed is a scalar quantity. The SI unit for speed is meters per second or m/s.

Three classifications of speed exist: constant, average, and instantaneous speed. It is improtant to understand each and how all three speeds are related.

Constant speed

Constant speed occurs when the object travels the same distance in equal periods of time. It is an example of uniform motion.

The car travels at a rate of 2.0 m every 0.1 s. Since the car travels the same distance in equal periods of time the speed is said to be constant

A table is another way to show constant speed. As shown, the car travels 2.0 m every 0.1 s. The car therefore has a constant speed of 20 m/s.

Table 1:

Time (s)

Distance Traveled (m)

0.2

4.0

0.4

8.0

0.6

12

0.7

14

0.8

16

Average speed

Average speed (v_{av}) is defined as the total distance traveled by an object per unit time. Since speed is derived from two scalar quantities, distance and time, it is also a scalar quantity. The SI unit for speed is meters per second or m/s.

Example 1:

Jay went to visit his parents for the long weekend. He traveled a total distance of
346 km. His total trip took him 4.5 hours. What was the average speed for his drive home?

Jay's average speed throughout the trip was 77 km/hr. He probably traveled faster than this for part of his trip and at some point he probably traveled slower. He may even have stopped the car for a junk food break.

Example 2:

1. You will first need to convert the time into hours.

2. Now apply the equation.

Instantaneous speed

Instantaneous speed (v_{in}) is is the speed at a specific instant in time. It can be thought of as the speed that the speedometer reads at any given moment.