Weight (W = mg)
Forces in Action - OCR A-Level Physics
- Weight ($W = mg)$: the gravitational force acting on a mass, always directed towards the centre of the Earth
- Normal contact forceThe perpendicular force exerted by a surface on an object resting on it, preventing the object from passing through.: the force perpendicular to a surface, arising when an object presses against it. It is an electromagnetic force at the atomic level
- FrictionA resistive force that opposes the relative motion (or attempted motion) between two surfaces in contact.: a force acting parallel to a surface, opposing relative motion or the tendency to slide. Static friction prevents motion; kinetic friction opposes motion
- TensionThe pulling force transmitted through a string, rope, cable, or similar when pulled tight by forces at each end.: a pulling force transmitted through a string, rope, cable or rod. Acts along the direction of the string
- 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).): a resistive force opposing motion through a fluid. Increases with speed (often proportional to v or \(v^{2}\))
- UpthrustThe upward force exerted on an object submerged in a fluid, equal to the weight of fluid displaced (Archimedes' principle).: the upward force on an object submerged (fully or partially) in a fluid, equal to the weight of fluid displaced
Examiner Tips and Tricks
- In free body diagramA diagram showing all the forces acting on a single object, drawn as arrows from the centre of the object. questions, label every arrow with both the name of the force AND what object exerts it.
- For example, 'Weight (gravitational pull of the Earth)' not just 'W'.
- Examiners often withhold marks for unlabelled arrows.