Free body diagram
Scalars & Vectors - OCR A-Level Physics
Key Definition
Free body diagram
A diagram showing all the forces acting on a single object, drawn as arrows from the centre of the object.
A diagram showing all the forces acting on a single object, drawn as arrows from the centre of the object.
- Each arrow represents one force. The length is proportional to the magnitude.
- Label every force with its name and value (if known).
- Common forces: weight (always downward), normal contact forceThe perpendicular force exerted by a surface on an object resting on it, preventing the object from passing through. (perpendicular to surface), tensionThe pulling force transmitted through a string, rope, cable, or similar when pulled tight by forces at each end. (along a string), frictionA resistive force that opposes the relative motion (or attempted motion) between two surfaces in contact. (opposing motion), drag/air resistanceThe opposition to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow. The ratio of potential difference to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).. Measured in ohms (Ω). (opposing motion).
- Only draw forces acting ON the object, not forces the object exerts on others.
- Do NOT draw the resultant force on a free body diagramA diagram showing all the forces acting on a single object, drawn as arrows from the centre of the object. -- only the individual forces.
Common Mistake
MEDIUM
Wrong: Including a 'forward force' on an object moving at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. on a frictionless surface.
Right: If there is no friction and the object moves at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹., there is no horizontal force at all. Newton's first lawAn object remains at rest or moves with constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. unless acted on by a resultant force.: no resultant force is needed to maintain constant velocity.
Right: If there is no friction and the object moves at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹., there is no horizontal force at all. Newton's first lawAn object remains at rest or moves with constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹. unless acted on by a resultant force.: no resultant force is needed to maintain constant velocity.