Tension, Normal force, Upthrust & Friction (OCR A Level Physics): Revision Note

Tension, Normal Force, Upthrust & Friction

• Tension:

  The force experienced by a cable, rope, or string when pulled, hung, rotated or supported

• This is normally labelled as T on free body diagrams

Tension always acts away from the mass

• Normal Contact Force:

  The force arising when an object rests against another object acting at a 90° angle to the plane of contact

• It is sometimes also referred to as the reaction force

• This is normally labelled as N or R on free body diagrams

• This force arises from Newton's Third Law

Normal contact force always acts perpendicular to the surface

• Upthrust:

  The upward buoyancy force acting on an object when it is in a fluid

• Upthrust can occur in liquids and gases

Upthrust always acts upwards

• Friction:

  The force that arises when two surfaces are in contact with each other

• Friction always opposes the motion

• This is normally labelled as F or Fr on free body diagrams

Friction always acts at the point where the objects are in contact, and in the opposite direction to the direction of motion

Free-body diagrams

• Free body diagrams are useful for modelling the forces that are acting on an object

• Each force is represented as a vector arrow, where each arrow:

  • Is scaled to the magnitude of the force it represents

  • Points in the direction that the force acts

  • Is labelled with the name of the force it represents

• Free body diagrams can be used:

  • To identify which forces act in which plane

  • To resolve the net force in a particular direction

• The net force in a particular direction can be calculated by:

  • Using the labelled angles and magnitudes

  • Resolving each force into horizontal and vertical components