Energy equations (work, efficiency, power)

• • Work done - energy transferred
• W = F x d
• Work = force x distance
• • Power - the rate at which energy is transferred
• Power = e / t
• Power = energy transferred / time
• • Efficiency - how much energy is useful 
• efficiency = useful energy / total energy
• A man pushes a box through a distance of 12m, and he exerts 14N of force on the box. Calculate the work done by the man. Ignore the frictional force.

  [2 marks]

  Using the work done equation:

  $W=$$f\times d$

  Substitute in the values:

  $W=$$14\times12$

  Calculate the answer:

  W = 168 J
• A man pushes a box 4m, and 100J of energy is transferred to the kinetic energy store of the box. Calculate the force exerted by the man on the box.

  Using the work done equation:

  $W=$$f\times d$

  Rearrange:

  $f =$$\frac{W}{d}$

  Substitute:

  $f =$$\frac{100}{4}$

  Calculate:

  f = 25 N
• The car uses 50000J whilst driving forwards for 10s. Calculate the power of the car’s engine

  [2 marks]

  P = e / t

  P = 50000 / 10

  P = 5000 W
• It takes 50kJ of work for a crane to lift a storage container and place it on top of another. The storage container is in the air for 50s, calculate the power of the crane.

  P = W / t

  P = 50000 / 50

  P = 1000 W
• A lightbulb is switched on for a period of time. During this time, the lightbulb uses up 100J of energy and 80J of useful light energy is produced by the lightbulb.

  Calculate the efficiency of the lightbulb.

  efficiency = useful energy / total energy

  efficiency = 80 / 100

  efficiency = 80%