Describing motion

 

Speed

Speed is a measure of how fast an object moves. We need two things to calculate the average speed of an object:

 

• distance travelled.
• time taken.

 

To calculate the average speed of an object, we can use the following equation:

 

 

 

Where:

 

• S = Speed in metres per second (m/s)
• D = Distance in metres (m)
• T = Time in seconds (s)

 

 

Rearranging the speed equation

You might also need to find the distance or time, not just the speed. So, you need to know how to change the formula to get different answers.
The formula triangles below show you how to find speed, distance and time:

 

 

Plotting the speed of an object

The unit for speed is metres per second (m/s). This can be represented on a distance-time graph.

Distance-time graph

 

 

As you can see from the graph above, time (s) is plotted along the the x-axis and distance (m) is plotted along the the y-axis. The direction and slope of the lines provide information about the speed of the object.

 

Relative motion

When you are in a car on the motorway, you might see other cars pass by slowly, even though you know they are moving very fast. This is because of how they move compared to you.

How you find their relative speed depends on if they’re moving in the same direction or the opposite direction. The table below shows us how to do this:

Scenario	Formula for relative speed
Objects moving in opposite directions towards, or away from, each other.	Relative speed = speed of object 1 + speed of object 2
Objects moving in the same direction.	Relative speed = fastest speed – slowest speed

 

🏎️ Knowledge Check: Motion and Speed

Test your ability to calculate speed and interpret motion graphs based on the KS3 Motion guide.

1. Which of the following is the correct formula to calculate speed?

• Speed = Distance × Time
• Speed = Time ÷ Distance
• Speed = Distance ÷ Time

2. On a distance-time graph, what does a steeper gradient (slope) represent?

• The object is moving slower.
• The object is moving faster.
• The object has stopped moving.

3. If the line on a distance-time graph is perfectly horizontal, what is the object doing?

• Moving at a steady speed.
• Accelerating.
• Staying stationary.

4. What does a curved line on a distance-time graph indicate?

• The object is moving at a steady speed.
• The speed of the object is changing.
• The object is moving backwards.

5. A car travels 100 metres in 5 seconds. What is its average speed?

• 20 m/s
• 500 m/s
• 0.05 m/s

Click to Reveal Answers

1. Speed = Distance ÷ Time (Calculated by dividing distance by time taken).
2. The object is moving faster (Steeper slope = higher speed).
3. Staying stationary (Distance does not change as time increases).
4. The speed of the object is changing (Curvature indicates acceleration or deceleration).
5. 20 m/s (100m ÷ 5s = 20 m/s).

 

Summary:

 

• Speed is a measure of how fast an object moves.
• You can use this formula to find speed: speed = distance / time.
• You can also change the formula to find distance and time.