AQA GCSE Physics contents

Unit 5 · Forces

Scalars, Vectors & Forces

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Scalar and vector quantities

  • A scalar quantity has magnitude only: size, with a unit
  • A vector quantity has magnitude and a direction
  • A vector is drawn as an arrow: the length represents the magnitude and the direction of the arrow the direction
Scalars Vectors
speed, distance, mass, time, energy, temperature, density velocity, displacement, force, weight, acceleration, momentum

Contact and non-contact forces

  • A force is a push or pull that acts on an object due to its interaction with another object. Force is a vector
  • Contact forces: the objects are physically touching. Friction, air resistance, tension, normal contact force
  • Non-contact forces: the objects are physically separated. Gravitational, electrostatic, magnetic
  • Forces come in pairs: when a book rests on a table, the book pushes down on the table and the table pushes up on the book with a normal contact force of the same size. Each force acts on a different object

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forces — a book on a tablebookcentre of massweight Wnormal contact force NThe book is not moving, so the resultant force on itmust be ZERO.So the two forces are EQUAL in size and OPPOSITEin direction — the arrows are drawn the same length.Weight is a NON-CONTACT force: the pull of gravity,acting from the centre of mass. W = m × g g = 9.8 N/kg on EarthThe normal contact force is a CONTACT force: thetable pushing back, at right angles to the surface.Contact forces: friction, air resistance, tension, normal contact force.Non-contact forces: gravity, magnetism, electrostatic force.Force is a VECTOR — it has size AND direction, which is why it is drawn as an arrow.
A book on a table with two force arrows: the weight of the book acting down from its centre, and the normal contact force from the table acting up, equal in length