What is Motion?
Motion is all around us — from a car driving down a highway to a football soaring through the air. In science, we describe motion precisely using measurements and mathematics so that we can predict, compare, and understand how objects move.
A change in the position of an object over time, usually described relative to a reference point.
A fixed location used to describe the position or movement of an object. For example, 'the car is 50 m north of the traffic light' uses the traffic light as a reference point.
To fully describe motion, we use several key quantities:
- Distance — how far an object has travelled in total
- Displacement — the straight-line distance and direction from start to finish
- Speed — how fast an object is moving
- Velocity — speed in a specific direction
- Acceleration — how quickly speed or direction is changing
Think of walking from your classroom to the cafeteria. The total length of the path you walked is the distance. But the displacement is just the straight-line distance between your classroom door and the cafeteria door — a much shorter number!
Speed and Velocity
Speed tells us how quickly an object covers distance, but it says nothing about direction. Velocity adds direction to that measurement.
The rate at which an object covers distance. It is a scalar quantity (magnitude only).
The rate of change of displacement. It is a vector quantity (magnitude AND direction).
The standard unit for both speed and velocity is metres per second (m/s), although km/h is also commonly used.
Example: Calculating Speed
A cyclist travels 600 m in 40 seconds. What is their speed?
Step 1: Write the formula:
Step 2: Substitute values:
Step 3: Calculate:
The cyclist's speed is 15 m/s.
Speed and velocity are often used interchangeably in everyday language, but in science they are different! A car travelling in a circle at constant speed is not travelling at constant velocity, because its direction keeps changing.
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