| English | Free body diagram |
|---|---|
| Bahasa Melayu | Gambar rajah jasad bebas |
| 中文 | 受力图 |
Definition
A diagram that shows all the forces acting on a single object, drawn as arrows from the object.
In exam terms
A free body diagram is a sketch that shows every force acting on one chosen object, drawn as arrows starting from the object and pointing in the direction of each force. The length of each arrow can represent the size of the force, and each arrow is labelled (for example weight, normal reaction, tension, friction).
It is a tool for analysis rather than a physical quantity, so it has no unit.
The key rule is to include only the forces acting on the object, not the forces the object exerts on other things. For example, a book resting on a table has just two forces: its weight acting downward and the normal reaction from the table acting upward.
Don't confuse it with a whole-system diagram
Do not confuse a free body diagram with a diagram of a whole system that shows several objects and all their interactions. A free body diagram isolates one object.
A related mistake is drawing Newton's third law pairs on the same body, action and reaction act on different objects, so only one of each pair belongs on a given free body diagram.
In Paper 2 this is usually asked with 'Draw…' or 'Label the forces acting on…'. Draw arrows from the object, name each force, and let opposing arrow lengths reflect whether the object is balanced or accelerating.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)