Chapter 7. Work, energy and power

Chapter 7. Work, energy and power

1. Doing work, transferring energy

   • Factors for doing work
     • The size of the force F - bigger the force, greater work
     • The distance travelled x - Further distance, more work
   • Work done = force x distance moved in direction of force
     • W=Fx
     • Work in Nm (Newton metres) or J (Joules)
     • Work Done = Energy transferred
     • If F at angle theta to direction of motion then W = FxCos(theta)
   • Change in GPE = Weight x Change in height
     • Ep = mgh
   • Faster an object, greater kinetic energy
     • Kinetic energy = 0.5 x mass x speed^2
       • Ek = 0.5mv^2
   • If no energy is lost during energy transfer then Change in GPE = Change in KE
     • mgh = 0.5mv^2

2. Energy transfers and conservation

   • Energy efficiency = (Useful output energy / Total input energy) x 100%
   • Sankey diagram can represent energy transfer
   • Conservation of energy - The total energy at the beginning = Total energy at the end in a closed system

3. Power

   • Power is the rate at which work is done
     • P = W / t
     • Power measured in Watts (W) (Joules per second)