Topic 8 - Forces Doing work

• Energy is never created or destroyed, it is only transferred e.g electric energy changes into kinetic in a motor
• when a system changes energy is transferred from one form to another
• a closed system is is a system which experiences no external forces, e.g a circuit or a skydiver
• in a closed system the total energy remains the same at all times
• ∆GPE = mg∆h

  change in gravitational potential energy(J, joule) = mass(kg, kilogram) x gravitational field strength(N/kg, newton per kilogram) x change in vertical height(m, metre)
• KE = 1/2mv²

  Kinetic energy (joule, J)= ½ ×mass (kilogram, kg) × (speed)^2 (metre/second)
• work done

  when an external force transfers energy to a system, e.g a human doing work to push a ball up a hill
• electric

  chemical potential energy (stored in batteries) forms electric changes
• heating

  electric transfers to thermal energy, as work is done against the heating coils causing it to heat up
• E = Fd

  Work done (joule, J) = force (newton, N) × distance moved in the direction of the force (metre, m)
• energy is rarely 100% transferred into the desired form

  some energy is dissipated so it is stored in less useful ways
• mechanical processes are wasteful when they get hot(usually due to friction)

  energy is dissipated as heat, causing a rise in temperature

  so energy is lost to the surroundings
• power - the rate at which energy is transferred
• P = E/t

  Power (watt, W) = energy transferred or work done (joule, J) ÷ time taken (second, s), where 1 Watt is equal to 1 Joule per second.
• efficiency - measure of how much energy is not lost as waste
• efficiency = useful energy output / total energy input x 100
• closed system - a system that experiences no net change in its total energy when energy transfers occur within it
• dissipation of energy - energy that is transferred to, and spread out in the surroundings. usually in the form of heat as a result of a system undergoing a rise in temp
• efficiency - the ratio of useful output energy to the total energy input. it can never exceed 1 , or 100%, due to the conservation of energy.
• gravitational potential energy - the store of energy that all raised matter has. it is directly proportional to the mass of the object, the distance that it is raised and the gravitational field strength as that point
• kinetic energy - the store of energy that all moving matter has. it is directly proportional to the objects mass and to the square of its velocity
• power - the rate at which energy is transferred, or the rate at which work is done.
• resistive force - when a force opposes an objects motion, the moving object will do work against the resistive force
• watt - the unit of power, it is equal to 1 joule per second
• work done - work is done on an object when a force causes it to move through a distance. It is equal to the product of the distance travelled and the magnitude of the force in the direction of motion