The everyday concept of speed arises when we consider how quick or slow a body moves. Somehow we relate the displacement of the body with the time spent in such displacement. In this section, we are going to define what is meant in physics by average speed and its difference with average velocity.

## Average speed

Average speed of a body that moves from point P1 to point P2 is defined as the ratio of the distance traveled and the corresponding elapsed time. It is given by the expression:

where:

• ${V}_{\mathrm{avg}}$ : Average speed in the interval studied. The unit of measurement in the International System (S.I.) is meter per second (m/s)
• $∆s$ : Distance traveled in the interval considered. Measured over the trajectory. The unit of measurement in the International System (S.I.) is the meter (m)
• $∆t$ : Time taken by the body to go from P1 to P2. The unit of measurement in the International System (S.I.) is the second (s)
• s,s2 : Distance traveled by the body on the trajectory from the beginning of the motion to P1 (s1), and from the beginning of the motion to P2 (s2). The unit of measurement in the International System (S.I.) is the meter (m)
• t1t2 : Time in which the body is found in the initial point P1 and in the final point P2 respectively. The unit of measurement in the International System (S.I.) is the second (s)

Average speed is a scalar magnitude as opposed to average velocity which is a vector. The unit of measurement in the International System (S.I.) of the average speed is the meter per second (m/s). On the other hand, unlike average velocity, which depended on the vectors of the initial and final positions of the motion, the average speed depends on the distance traveled over the trajectory. Therefore, a body in motion will always have an average speed greater than 0. In general, the ${V}_{\mathrm{avg}}\ge \left|\stackrel{\to }{{v}_{\mathrm{avg}}}\right|$  is satisfied, and both values will be the same in the case of rectilinear motion without change of direction.

Experiment and Learn

Data

Average speed

The graph shows the trajectory followed by a body, its initial and final positions and the distance traveled (Δs) between the two points.

Drag the points closer or farther apart and observe as the distance traveled decreases or increases respectively. Furthermore, you can change the time the body takes to go from one point to the other (Δt) and notice the change in the average speed.

What happens if you do not change the position but decrease the time?