5 - forces

• Vector quantity


  Magnitude + associated direction (represented by an arrow)
• Forces


  • Contact forces
  • Non-contact forces
• Contact forces


  Objects are physically touching (friction, air resistance, tension + normal contact force)
• Non-contact forces


  Objects are physically separated (gravitational force, electrostatic force + magnetic force)
• Weight


  Force acting on an object due to gravity (due to the gravitational field around the Earth)
• Weight of an object

  Depends on the gravitational field strength at the point where the object is
• Weight formula


  weight = mass × gravitational field strength (W =mg)
• Units


  weight - newtons (N)

  mass - kilograms (kg)

  gravitational field strength - newtons per kilogram (N/kg)
• Weight and mass of an object

  Directly proportional
• Measure weight


  Calibrated spring balance (a newton meter)
• Resultant force


  A number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together
• Resolving a force


  A single force can be resolved into two components (forces have to have the same effect as a single force) acting at right angles to each other
• Work done


  When a force causes an object to move through a distance (causes a displacement)
• Work done formula


  work done = force × distance (moved along the line of action of the force)

  W =Fs
• Units


  work done - joules (J)

  force - newtons (N)

  distance - metres (m)
• 1 joule of work is done when a force of 1 newton causes a displacement of 1 metre
• Work done against the frictional forces

  Causes a rise in the temperature
• Hooke's Law


  Extension of an elastic object is directly proportional to the force applied (limit of proportionality is not exceeded)
• Hooke's Law formula


  force = spring constant × extension

  F =k e
• Elastic potential energy


  Work done on spring = elastic potential energy stored (provided spring is not inelastically deformed)
• Elastic potential energy formula

  Elastic potential energy = 0.5 × spring constant × extension^2
• Units


  force - newtons (N)

  spring constant - newtons per metre (N/m)

  extension - metres (m)
• Moment of a force

  The turning effect of a force
• Moment formula


  Moment of a force= force × distance (M = F d)
• Units


  moment - newton-metres (Nm)

  force - newtons (N)

  distance - is the perpendicular distance from the pivot to the line of action of the force (metres, m)
• Balanced


  Total clockwise moment about a pivot = the total anticlockwise moment about that pivot
• Lever and gear system

  Used to transmit the rotational effects of forces
• Fluid


  Either a liquid or a gas
• Pressure formula


  Pressure = force normal to a surface / area of that surface (p=F/A)
• Units


  pressure - pascals (Pa)

  force - newtons (N)

  area - metres (m2)
• Pressure in liquids formula

  Pressure = height of the column × density of the liquid × gravitational field strength (p = h ρ g)
• Units


  pressure - pascals (Pa)

  height of the column - metres (m)

  density - kilograms per metre squared (kg/m3)

  Gravitational field strength - newtons per kilogram (N/kg)
• Submerged object


  Experiences a greater pressure on the bottom surface than on the top surface (resultant force upwards - upthrust)
• Atmospheric pressure


  Air molecules colliding with a surface, it gets less dense with increasing altitude
• Atmospheric pressure


  Decreases with an increase in height
• Distance


  How far an object moves (scalar quantity - does not involve direction)
• Displacement


  Includes both the distance (measured in a straight line from start to finish) and the direction (vector quantity)
• Speed


  Does not involve direction (scalar quantity)
• Speed of a moving object is rarely constant (constantly changing)</b>
• Typical speed values: walking ̴ 1.5 m/s, running ̴ 3 m/s, cycling ̴ 6 m/s, sound in air 330 m/s