Work, Energy and Power

Notes

The following topics are covered in this chapter.

• (a) Definition of Work
  • Define the work done on an object by a force as the product of the displacement and the component of the force parallel to the displacement
  • Solve problems using: W  F.s or W  Fx or W  Fx cos is allowed 
  • Know that work is a scalar quantity and is measured in joules (J)
• (b) Mechanical Energy
  • Define gravitational potential energy as the energy an object possesses due to its position relative to a reference point
  • Calculate the gravitational potential energy of an object using p E  mgh
  • Define kinetic energy as the energy an object has as a result of the object's motion
  • Calculate the kinetic energy of an object using 1 2 K 2 E  mv
  • Define mechanical energy as the sum of gravitational potential and kinetic energy at a point
  • Use the equation: M p K E  E  E
  • State the law of conservation of energy as the total energy in a system cannot be created nor destroyed; only transferred from one form to another
  • State the principle of conservation of mechanical energy: In the absence of air resistance or any external forces, the mechanical energy of an object is constant
  • Apply the principle of conservation of mechanical energy and solve problems using:     p K i p K f E  E  E  E
• (c) Work – Energy Theorem
  • State that the work done by a net force on an object is equal to the change in the kinetic energy of the object – the workenergy theorem
  • Apply the work-energy theorem to objects on horizontal and inclined planes (frictionless and rough)
  • Kinetic energy of a system is increased when net F is in the same direction as s or x
  • Kinetic energy of a system is decreased when net F is in the opposite direction to s or x
• (d) Conservation of Energy with External Forces and/or Resistive Forces Present
  • Solve conservation of energy problems (with and without external forces and/or resistive forces present) by applying the law of conservation of energy
• (e) Power
  • Define power as the rate at which work is done or the rate at which energy is transferred
  • State that the unit of power is the watt (W). One watt is defined as the power when one joule of work is done in one second. (1 W = 1 J.s-1)
  • Calculate the power involved when work is done using P W t 
  • If a force causes an object to move at a constant velocity, calculate the power using P  Fv
  • Define efficiency as the ratio of output power to input power
  • Calculate percentage efficiency using out 100 in efficiency power power  