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)
• Unit for gravitational field strength


  N/kg
• 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 against frictional forces

  • Rise in temperature of the object
  • Kinetic energy converted to heat
• Air resistance slowing down a projectile

  • The object does work against the air resistance
  • Kinetic energy converted to heat, slowing down the object
• Force applied


  Extension of an elastic object
• Limit of proportionality


  The point at which the extension is no longer directly proportional to the force applied
• 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

  1. Force = Spring Constant x Extension

  2. Force (N), Spring Constant (N/m), Extension (m)
• Elastic potential energy


  The type of energy stored in a spring when it is stretched
• Compression


  Can replace extension in the equation for spring force
• Moment


  Alternative name for the turning effect of a force
• Moment of force


  Equation: Moment = Force x Distance

  Units: 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
• Parts of a lever system


  • Load
  • Effort
  • Pivot
• How a lever can be used as a force multiplier

  • If the distance between the effort and pivot is greater than the distance between the pivot and load, the force applied on the load is greater than the effort force. This is since the moment on both the effort and load must be the same.
• Example of when a lever may be used to multiply a force

  • A wrench, which has a long handle so that the force applied by the user is multiplied
• What determines the moment of a gear wheel

  The size of the wheel
• A larger gear being driven by a smaller gear will rotate more slowly but will also produce a larger moment
• Fluid


  A liquid or a gas
• In any fluid, the forces due to pressure act on a given surface at right angles (normal to) the surface
• Calculating pressure


  Pressure = Force/Area