Work, energy and power

• Work done is the amount of energy transferred from one form to another when a force causes a movement.
• Work done = force x distance
• Work is the energy that’s been changed from one form to another - it’s not necessarily the total energy.
• The force (F) will be a fixed value in most calculations. Either because it’s constant or because it’s the average force.
• One joule (J) is the work done when a force of 1 newton (N) moves an object 1 metre
• If the direction of the movement is different from the direction of the force:

  • Work done = Force x distance x cos θ
• Power is the amount of energy transferred per unit time.
• Watt is the rate of energy transfer equal to 1 joule per second.
• Power = Force x velocity

  If force and motion are in different directions,

  P = Fv cos θ
• Transferred energies are kinetic, thermal, sound, electrical.
• Stored energies are elastic, gravitational potential, chemical, nuclear.
• Kinetic energy = 1/2 mv^2 - 1/2 mu^2
• Principle of conservation of energy = Energy cannot be created or destroyed.
• Efficiency = useful energy output / total energy input (x100%)
• Loss in kinetic energy = Gain in gravitational potential energy + work done against friction (and vice versa)
• If there is no friction:

  • mgh = 1/2 mv^2
• If the object is moving & gaining speed:

  • Power (of engine) = (resistive) Force x velocity + kinetic energy gain by second
• If the object is moving & gaining height:

  • Power (of engine) = (resistive) Force x velocity + gravitational potential energy gain per second