Describing motion

Learning objectives

• Calculate average speed.
• Distinguish speed from velocity.
• Read the slope of a distance–time graph.

The big idea

Distance counts the length of a path. Displacement joins the starting and finishing positions and includes a direction. Average speed uses total distance divided by total elapsed time.

Velocity describes speed with direction. An object can change velocity by turning even if its speed stays constant. A steeper distance–time graph means greater speed.

1. Follow the whole journey

Walking 3 m east and then 3 m west gives a total distance of 6 m but zero displacement. Distance follows the route; displacement compares the start with the finish and includes direction. Decide which is relevant before selecting a formula.

2. Speed is a rate

If the walk takes 4 s, average speed is 6/4 = 1.5 m/s. Include stopped time in the total journey time. This average does not mean every part of the journey happened at 1.5 m/s. Velocity adds direction, so turning can change velocity without changing speed.

3. Read a graph as a story

On a distance–time graph, a steeper section means faster movement and a horizontal section means a stop. The line is not a picture of a hill or the shape of the path. On a velocity–time graph, a horizontal line instead means constant velocity. Always read the axes first.

A method you can reuse

1. Mark the start, finish and time interval.
2. Find total distance or directed displacement.
3. Divide the appropriate quantity by elapsed time.
4. Check the graph axes before describing a slope.

Equations and relationships

Investigate and explain

Mark a short level route and time a walking toy over it. Repeat before comparing average speeds.

Track position over equal time intervals and compare distance with displacement. Projectile motion also includes acceleration; do not assume its speed is constant.

Open the interactive investigation

Check your understanding