Chapter 6 - Work, Energy and Power


Describe the energy changes that take place when a ball is being thrown upwards by a hand. (Neglecting air resistance)

  • As the ball just leaves the hand, it possesses maximum amount of KE. Its GPE is zero due to the zero height from the hand.
  • As it moves upwards, it loses speed. The KE is converted to GPE as the ball looses speed and gain height.
  • At the highest point from the ground, the stone is momentarily at rest. The KE is zero while the GPE reaches a maximum. All the KE have been converted to GPE. The GPE at the top equals the KE as the ball just left the hand (at the bottom).
  • As the object moves downwards, its speed increases. GPE is converted back to KE.
  • Just before hitting the hand, the KE reaches maximum and GPE is zero.
  • According the Principal of Conservation of Energy, the sum of KE and GPE is always constant at any point in time.



Explain why in practical, the actual useful work done by a motor is less than the calculated value.

  • Some mechanical energy is converted to thermal energy as it is used to overcome frictional force.
  • Some energy is also converted to sound energy.
  • The thermal energy and sound energy is lost to the surroundings.
  • Therefore, the actual useful work done is always less than the calculated value.