What is a reference framework?
Define a reference framework
What is a reference frame?
A reference frame is an object (or a set of objects) relative to which we choose to study the motion of another object. It is our ‘point of view’ from which we observe.
To define a reference frame, we often associate it with:
- a reference object (the ground, a car, a train, etc.)
- imaginary axes attached to this object, to identify positions
- a clock to measure the passage of time
The most commonly used reference frames in secondary school
| Reference frame | Reference object | Used for... |
|---|---|---|
| Earth-centred | The ground, a building | Studying a car, a pedestrian, a falling object |
| Vehicle-centred | The car, the train, the boat | Studying what happens inside the vehicle |
| Geocentric | The centre of the Earth, axes pointing towards the stars | Studying the motion of the Moon or satellites |
| Heliocentric | The centre of the Sun | Studying the motion of the planets |
Method for describing motion
To describe a motion correctly, you must always follow three steps:
- Choose the reference frame (relative to what am I observing?)
- Choose the object under study (which object am I watching move?)
- Describe the motion in this reference frame (path, speed, etc.)
Example: “In the Earth-centred reference frame, the car is moving in a straight line at a constant speed.”
A common pitfall to avoid
Never simply say “this object is moving”: this is an incomplete statement in physics! You must always specify the reference frame: “this object is moving relative to the ground”, for example. The same object can be at rest in one reference frame and in motion in another, without any contradiction.