FORCES

• Scalar quantity


  • A quantity that only has a magnitude
  • A quantity that isn't direction dependent
• Vector quantity

  A quantity that has both a magnitude and an associated direction
• How a vector quantity can be drawn

  • As an arrow
  • The length of the arrow represents the magnitude
  • The arrow points in the associated direction
• Force


  A push or pull acting on an object due to an interaction with another object
• Categories that all forces can be split into

  • Contact forces (objects touching)
  • Non-contact forces (objects separated)
• Contact forces


  • Friction
  • Air resistance
  • Tension
• Force is a vector quantity
• Vector quantities

  • Velocity
  • Displacement
  • Force
• Weight


  The force that acts on an object due to gravity and the object's mass
• What quantities weight depends on

  • The object's mass
  • The gravitational field strength at the given position in the field
• Unit for weight

  Newton (N)
• Centre of mass


  The single point where an object's weight can be considered to act through
• Equipment to measure weight

  A calibrated spring-balance or newton-meter
• Resultant force


  The single force that is equivalent to all the other forces acting on a given object
• Work done by a force


  The force causes an object to be displaced through a distance
• Calculating work done


  1. Work done = Force x Distance

  2. Work done (Joules)

  3. Force (Newtons)

  4. Distance (metres)
• Distance used when calculating work done

  The distance that is moved along the line of action of the force
• 1 joule of work is done when a force of 1 Newton causes a displacement of 1 metre
• Relationship between Joules and Newton-metres

  1 Nm = 1 J
• Work done by a 10N force over 5m


  • 10 x 5 = 50 Nm
  • 50 J
• When work is done against frictional forces:
• Air resistance slowing down a projectile


  • The object does work against the air resistance
  • Kinetic energy is converted in to heat, slowing down the object
• Work done by a force


  The force causes an object to be displaced through a distance
• Calculating work done


  1. Work done = Force x Distance

  2. Work done (Joules)

  3. Force (Newtons)

  4. Distance (metres)
• Distance used when calculating work done

  The distance that is moved along the line of action of the force
• 1 joule of work is done when a force of 1 Newton causes a displacement of 1 metre
• Relationship between Joules and Newton-metres

  1 Nm = 1 J
• Work done by a 10N force over 5m


  • 10 x 5 = 50 Nm
  • 50 J
• When work is done against frictional forces, a rise in temperature of the object occurs and kinetic energy is converted to heat
• Air resistance slowing down a projectile


  The object does work against the air resistance, converting kinetic energy into heat, slowing down the object
• Relationship between the force applied and the extension of an elastic object


  The extension is directly proportional to the force applied, provided that the limit of proportionality is not exceeded
• Inelastic (plastic) deformation

  • A deformation which results in the object being permanently stretched
  • The object doesn't return to its original shape when the force is removed
• Equation relating force, spring constant and extension

  Force = Spring Constant x Extension
• Units

  • Force (N), Spring Constant (N/m), Extension (m)
• Energy stored in a stretched spring

  Elastic potential energy
• Extension in the equation for spring force


  Can be replaced with compression
• Moment


  Alternative name for the turning effect of a force
• Calculating the moment of a force

  1. Moment of force = Force x Distance

  2. Moment (Nm), Force (N), Distance (m)
• Distance measurement used when calculating a moment

  The perpendicular distance from the pivot to the line of action of the force
• If an object is in equilibrium, the clockwise moments are equal to the anticlockwise moments