Work done and power: formulas and examples

Work Done and Power

• When a force moves an object through a distance, WORK IS DONE on the object and ENERGY IS TRANSFERRED. In order to make something move, forces need to act upon it.
• E = F X d
• Work Done(J) = Force (N) X distance moved (m)
• e..g. Find the energy transferred when a tyre weighing 70N is lifted 1.2m in the air.
• work done = force X distance = 70 X 1.2 = 84J
• A force doing work can often cause a rise in temperature as energy is dissipated to the thermal energy of the moving object.
• e.g. efficiency = useful energy / total energy transferred by the device

Slide 2

• Power is how much work is done per second
• power = work done (J) / time taken (s)
• e.g. a motor does 4.8 kJ of work in 2 minutes. Find its power output.
• Convert the values to the correct units first (Joules in this case).
• Substitute the values into the power equation shown above.
• 4.8kJ = 4800 J and 2 minutes = 120 s
• P = E / t = 4800 /120 = 40W

Slide 3