Free body diagram
Scalars & Vectors - OCR A-Level Physics
Key Definition
Free body diagram
A diagram showing every force that acts ON one chosen object, drawn as arrows starting at a single point (usually the centre of the object). Forces the object exerts on other things are NOT shown.
A diagram showing every force that acts ON one chosen object, drawn as arrows starting at a single point (usually the centre of the object). Forces the object exerts on other things are NOT shown.
Diagram pending
Box on a horizontal floor being pulled right by a rope. Single dot represents the box. Four labelled arrows from the dot: weight $W$ down, normal $N$ up, tension $T$ pointing along the rope, friction $f$ pointing left (opposing motion). Arrow lengths roughly proportional to magnitudes. No "resultant" arrow shown.
Will be replaced with a GeoGebra SVG in stream 2.
- Each arrow represents one force. The arrow's length is roughly proportional to the magnitude; its direction is the direction of the force.
- Label every force with its name and (if known) its value. Examiners deduct marks for unlabelled arrows.
- Common forces in OCR problems: weight (always vertically downward, $W = mg$), normal contact forceThe perpendicular reaction force exerted by a surface on an object resting on it. $N$ (perpendicular to the surface), tensionThe pulling force transmitted through a string, rope, or cable that is pulled tight. $T$ (along a string, pulling away from the object), frictionA resistive force that opposes relative motion or attempted motion between two surfaces in contact. $f$ (opposing motion), drag or air resistance (opposing motion through a fluid).
- Only draw forces acting ON the object. Forces the object exerts on other things belong on a different free body diagram for those things.
- Do NOT draw the resultant force on a free body diagram. The resultant is calculated from the individual forces; it is not itself a separate force.
Common Mistake
MEDIUM
Wrong: Including a "forward force" on an object moving at constant velocity on a frictionless surface, or drawing a separate "resultant" arrow alongside the individual forces.
Right: With no friction and constant velocity, there is no horizontal force at all. By Newton's first lawAn object remains at rest or moves at constant velocity unless acted on by a resultant force., no resultant force is needed to maintain constant velocity. The resultant is what you calculate from the diagram, not an extra arrow on it.
Right: With no friction and constant velocity, there is no horizontal force at all. By Newton's first lawAn object remains at rest or moves at constant velocity unless acted on by a resultant force., no resultant force is needed to maintain constant velocity. The resultant is what you calculate from the diagram, not an extra arrow on it.
Examiner Tips and Tricks
- Draw the object as a dot (or a small box) and start every arrow at that dot. Examiners mark the diagram if forces start anywhere else.
- For each named force, ask: "what is touching the object?" Weight is the only force that acts without contact (other than electromagnetic and gravitational in field problems).
- If the object is on a slope, draw the weight vertical (NOT perpendicular to the slope) and the normal contact force perpendicular to the slope. The decomposition comes after.