Motion & Speed
Learn to describe and calculate motion using the speed formula, interpret distance-time graphs, and apply these skills to real Australian contexts.
Describing Motion
Motion is a change in position of an object over time. To describe motion precisely, scientists use three quantities: distance, time, and speed.
Distance
How far an object travels. Unit: metres (m) or kilometres (km).
Time
How long the journey takes. Unit: seconds (s) or hours (h).
Speed
How fast an object moves. Unit: m/s or km/h.
Australian Curriculum Connection
This lesson aligns with AC9S7U04: "Forces can change the motion of objects; the relationship between force, mass, and acceleration can be described mathematically; speed is defined as distance/time."
The Speed Formula
Speed is calculated by dividing the distance travelled by the time taken. All three quantities are related by a single formula.
s = d ÷ t
speed = distance ÷ time
Find Speed
s = d ÷ t
Find Distance
d = s × t
Find Time
t = d ÷ s
Units matter! If distance is in metres and time is in seconds, speed is in m/s. If distance is in km and time is in hours, speed is in km/h.
Distance-Time Graphs
A distance-time graph plots distance (y-axis) against time (x-axis). The gradient (slope) of the line equals the speed. The steeper the line, the greater the speed.
A Cyclist's Journey — Distance-Time Graph
Steep Line
Large gradient = high speed. The object is moving quickly.
Horizontal Line
Zero gradient = stationary. The object is not moving (at rest).
Gentle Slope
Small gradient = low speed. The object is moving slowly.
Key Vocabulary
Speed
The rate of change of distance; how fast an object is moving. Calculated as distance ÷ time. Unit: m/s or km/h.
Average Speed
Total distance divided by total time for a journey; accounts for variations in speed during the trip.
Gradient
The slope of a line on a distance-time graph; equals the speed of the object at that point.
Motion
A change in position of an object with respect to time; can be described using distance, time, and speed.
Worked Examples
A red kangaroo runs 200 m in 25 seconds. Calculate its speed in m/s.
Formula: s = d ÷ t
Substitute: s = 200 m ÷ 25 s
Answer: s = 8 m/s
A train travels from Sydney to Melbourne (880 km) at an average speed of 110 km/h. How long does the journey take?
Rearrange formula: t = d ÷ s
Substitute: t = 880 km ÷ 110 km/h
Answer: t = 8 hours
A peregrine falcon dives at 15 m/s for 6 seconds. How far does it travel?
Rearrange formula: d = s × t
Substitute: d = 15 m/s × 6 s
Answer: d = 90 m
Knowledge Check
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Key Concepts Summary
- ✓ Speed = distance ÷ time (s = d ÷ t); units are m/s or km/h.
- ✓ Rearranged: d = s × t and t = d ÷ s.
- ✓ On a distance-time graph: steep line = fast; horizontal line = stationary; gentle slope = slow.
- ✓ The gradient (slope) of a distance-time graph equals the speed.
- ✓ Always check units: if distance is in km and time in hours, speed is in km/h, not m/s.