Mechanical energy and machines

• Mechanical energy


  Can either be potential or kinetic or both
• Potential Energy (P.E)


  The energy possessed by a body by virtue of its position/state/configuration. Potential energy also means the stored up energy of a body in particular position/state/configuration
• Forms of potential energy

  • Elastic potential energy (energy stored in a stretched or compressed elastic string/spring)
  • Gravitational potential energy (energy stored in a body by virtue of its position in relation to the centre of the earth)
• Work done = force x distance in direction of force
• Gravitational potential energy


  Equal to the work done in moving an object from a reference point, through a certain height to its present position. The force that is overcome is the weight, w on the object and the distance is the height h through which the object is moved
• Machine


  A tool or device with which work is done easily. An effort applied at one point is used to overcome a load at another point
• Mechanical Advantage (M.A)


  The ratio of the load to the effort. The effort is the applied force or input force while the load is the resistance to overcome or the output force
• Velocity Ratio (VR)


  The ratio of the distance moved by effort to the distance moved by load
• Efficiency


  The ratio of the useful work done (work output) by the machine to the work put into (work input) the machine, expressed as a percentage
• A perfect or ideal machine is 100% efficient, and the velocity ratio equals the mechanical advantage
• In a practical situation, the efficiency of a machine is usually less than 100% due to frictional forces
• Lever


  A rigid bar supported at the fulcrum or pivot, about which it can rotate. An effort (E) applied at one point of the lever overcomes a resistance or lifts a load (L) at another point
• Types of Levers


  • First order (first class) levers
  • Second order (second class) levers
  • Third order (third class) levers
• First order levers


  • The fulcrum (pivot point) is between the load and the effort
  • Mechanical Advantage and Velocity Ratio is usually greater than 1 but could be less than or equal to 1
• Second order levers


  • The load is between the fulcrum and the effort
  • Mechanical Advantage and Velocity Ratio are always greater than 1
• Third order levers


  • The effort is between the load and the fulcrum
• Newton's Second Law states that force equals rate of change of momentum