Work, Power & Energy Resources

What it means to "do work"

• Work is done whenever a force moves the object it is acting on through a distance in the direction of that force
• If the force does not move the object, no work has been done. Pushing on a stationary wall for ten minutes transfers no energy to the wall, no matter how tired your arms feel
• "Doing work" is just the mechanical pathway in different language. The amount of work done equals the amount of energy transferred between stores:

energy transferred = work done

• Both quantities are measured in joules (J); 1 J = 1 N m (one newton acting through one metre)

Sign of the energy transfer

• If a force acts in the same direction as the motion, the object gains energy (usually into its kinetic store, sometimes into gravitational potential if it is being lifted)
• If a force acts in the opposite direction to the motion, the object loses energy; in everyday cases this is friction or drag dissipating energy as heat into the surroundings
• Lifting a 5 kg sack of flour onto a kitchen shelf does work against gravity and fills the sack's gravitational potential store; sliding a box across a carpet does work against friction and dumps that energy into the thermal stores of the carpet and the box

The work-done equation

W = F × d

• where:
  • W = work done (J or N m)
  • F = force acting on the object (N)
  • d = distance the object moves in the direction of the force (m)
• Use SI units throughout (newtons and metres); the result then comes out in joules