Work, Energy and Power

• Work is done when the point of application of a force is moved in the direction of the force
• work done = force x distance moved by the force in the direction of the force. W = Fd. Work is measure in joules
• One joule is the name for a newton metre; it is the work done when the point of application of a force of one newton is moved through one metre
• The work done by the pulling force in the forward direction is the product of the horizontal component of the force and the horizontal displacement Δ𝑥, work done = Fcosθ x Δ 𝑥 
• In most cases, the forces acting on moving objects are not constant, and it is usual to express the work done in terms of the average force: ΔW = F(average force) x Δ s
• As work is done in both directions it follows that the total work done is the sum of these values and must be a vector quantity
• Work and energy are closely linked. Energy is transferred to an object when work is done on it. When energy is transferred from the object to another system we say that the object does work
• Energy is the ability to do work
• Potential energy is the ability of an object to do work by virtue of its position or state
• Gravitational potential energy is the energy an object possesses by virtue of its position in a gravitational field
• ΔGPE = mgΔh
• Elastic potential energy is the ability of an object to do work by virtue of a change in its shape. The EPE of a deformed object can be represented in terms of the work done to change its shape. For a spring or wire being stretched using an average force, F(average force), so that it extends Δx ΔEPE = F(average force) x Δ𝑥. The EPE of stretched bands and wires is usually referred to as the Elastic Strain Energy
• Kinetic energy is the ability of an object to do work by virtue of its motion
• The KE of an object can be calculated using the formula: KE = 1/2 x mass x velocity^2
• If a mass changes speed from u to v for KE then the change in KE is calculated through the formula: ΔKE = 1/2mv^2 - 1/2mu^2
• Chemical energy is the ability to do work using chemical reactions
• Nuclear energy is the ability to do work by changes in the constitution of nuclei
• Electrostatic potential energy is the energy of a charged particle by virtue of its position in an electric field
• Radiant energy is the energy transferred by electromagnetic waves
• The principle of conservation of energy states that energy cannot be created or destroyed, it can only be transferred from one form to another
• Efficiency is the ability to transfer the input energy to the useful energy in a system
• Efficiency = (Useful energy or power output/energy or power input) x 100%
• power = work done (energy transferred)/time taken
• Power is measured in watts (W)
• One watt is a rate of conversion of energy of one joule per second. 1W = 1J/s
• Power = Force x velocity