Velocity: speed with a direction
Average velocity is displacement divided by the time interval.
Out and back
Two athletes each take 4 s. One runs 40 m forward from O. The other starts 40 m from O and runs back to O.
Both ran at the same pace. How could we tell their motions apart with numbers?
What this lesson covers
The idea
Average velocity = displacement ÷ time interval, vav = s/t: how fast position changes and in which direction, its direction being that of the displacement ('+' or '–'); like average speed, its SI unit is m s–1, or km h–1.
Out and back
Two athletes each take 4 s. One runs 40 m forward from O. The other starts 40 m from O and runs back to O.
Both ran at the same pace. How could we tell their motions apart with numbers?
- By giving the direction of the motion too
- By giving the colour of their clothes
- We cannot tell them apart
Start, end and time
Set the start, the end and the time interval. Read the average velocity.
Displacement over time
From 0 m to 40 m in 4 s the displacement was +40 m, so the average velocity was +10 m s⁻¹. Going back from 40 m to 0 m in the same 4 s gave a displacement of −40 m and an average velocity of −10 m s⁻¹. The sizes are equal but the directions are opposite.
Average velocity = displacement ÷ time interval, vav = s/t. It tells how fast the position changes and in which direction; its direction is that of the displacement, shown by + or −. Its SI unit is m s⁻¹, the same as for average speed (also measured in km h⁻¹).
Notes
Average velocity = displacement ÷ time interval (vav = s/t), with the direction of the displacement. Unit: m s⁻¹.
Check yourself
An athlete goes from 0 m to 40 m in 4 s. What is her average velocity?
Answer: 10 m s⁻¹
vav = s/t = 40 m ÷ 4 s = +10 m s⁻¹.
Another athlete goes from 40 m back to 0 m in 4 s. What is her average velocity?
Answer: -10 m s⁻¹
s = 0 m − 40 m = −40 m, so vav = −40 m ÷ 4 s = −10 m s⁻¹.
How does average velocity differ from average speed?
What is the SI unit of average velocity?
- It is measured in different SI units. Both have the same SI unit, metre per second.
- Average velocity has a direction, shown by a plus or minus sign — correct. Yes!
- It is calculated from the total distance travelled. That describes average speed. Average velocity is calculated from the displacement.
- m s⁻². m s⁻² is the unit of acceleration.
- m. The metre is the unit of displacement and distance, not of velocity.
- m s⁻¹ — correct. Yes!