Motion

Level 1

What is motion?

An object is in motion when its position changes over time relative to another object chosen as a reference. This object of comparison is called the frame of reference.

  Examples : 
- A car drives along the road: the road is chosen as the frame of reference. 
- A plane flies in the sky: the ground is chosen as the frame of reference.

The path

The path is the set of all positions occupied by an object as it moves. It traces a line followed by the moving object. 

It can be: 

  • Straight: shaped like a straight line  
  • Circular: shaped like a circle
  • Curved: any other curved shape 
 
The ball moves — its path is drawn in blue

How speed changes.

 
Each dot = position at equal time intervals
  • Motion is uniform if the speed stays constant
    • (e.g. a car driving steadily at 50 km/h).
  • Motion is speeding up if the speed increases
    • (e.g. a car accelerating).
  • Motion is slowing down if the speed decreases
    • (e.g. a car braking).

Average speed

To describe motion, we use its speed. Average speed is given by the formula:

$$v = \frac{d}{t}$$

where: 

  • \(d\) is the distance travelled
  • \(t\) is the time taken.

  Example: If a car travels 120 km in 2 h, then $$v = \frac{120}{2} = 60 \ \text{km/h}$$

Units of speed

The official unit of speed in the International System of Units is the metre per second (m/s). In everyday life, the kilometre per hour (km/h) is often used.

Conversion: $$1 \ \text{m/s} = 3.6 \ \text{km/h}$$