3.3

• Work done

  The amount of energy transferred when an external force causes an object to move over a certain distance.

  If the force is parallel to the objects direction of displacement W = Fx can be used.

  W = work done (J) F = average force applied (N) x = displacement (m)

  If a force acts in the direction that an object is moving it will gain energy

  If the force acts in the opposite direction to the movement the object will lose energy

  When pushing a block work is done against friction to give the box kinetic energy to move
• The joule

  Unit of work

  Si unit is kgm^2s^-2

  One joule is the energy transferred to an object when a force of 1N acts on that object parallel to its motion through a distance of 1m
• Work done equation

  when direction of motion is not parallel to the direction of the force

  If the force is at an angle work done is calculated with W = Fxcosθ for horizontal motion.

  For vertical motion the formula would be W = Fxsinθ and the component needed will be the one that is parallel to the displacement
• Conservation of energy

  Energy cannot be created or destroyed it can only be transferred from one form to another

  Total amount of energy in a closed system remains constant
• Energy dissipation

  When energy is transferred from one form to another not all energy will end up in the useful form so some energy is wasted

  Any energy not transferred to useful energy stores is wasted as it is lost to the surroundings
• Energy conservation on a trampoline

  Work done against resistive forces like friction

  Loss in kinetic energy = Gain in gravitational potential energy + work done against friction

  Elastic potential energy is converted to kinetic energy when your on the trampoline about to jump

  Kinetic energy is converted to gravitational potential energy after you jump
• Energy and work done

  Transfer of energy = work done

  Type of transfer of energy changes based on the scenario
• Kinetic energy

  Energy an object has due to its motion

  When an object is falling it is gaining kinetic energy because it is gaining speed

  A objects kinetic energy is constant unless speed changes

  KE = 1/2mv^2

  KE - Kinetic energy (J)

  m - Mass (kg)

  v - Velocity (ms^-1)
• Deriving Kinetic energy (real flashcard)

  right
• Gravitational potential energy

  Energy stored in a mass due to its position in a gravitational field

  If a mass is lifted up it gains GPE

  If a mass falls it loses GPE

  ΔGPE = mgΔh

  ΔGPE - Change in gravitation potential energy (J)

  m - Mass (kg)

  g - Gravitational field strength (9.81 Nkg^-1)

  Δh - Change in height (m)
• Deriving GPE equation (real flashcard)

  Right
• Exchange between GPE and KE

  Loss in GPE = Gain in KE

  Happens in objects like:

  Swinging pendulums

  Objects in free fall
• Power

  Power of a machine is the rate at which it transfers energy

  Power is the work done per unit time

  SI unit for power is the watt (W)

  P = E/t or P = W/t

  P - Power(W)

  E - Energy (J)

  t - Time (s)

  W - Work done (J)
• The watt

  Unit for power

  1W = 1Js^-1

  SI unit for energy is kgm^2s^-3

  One watt is a transfer of energy of 1J in 1s
• Power and force

  P = Fv

  Equation is only relevant when a constant force moves a body at constant velocity

  Power is needed to produce acceleration

  The force must be applied in the same direction as the velocity
• Deriving P = Fv

  Right
• Efficiency

  Efficiency of a system measures how successfully energy is transferred in a system

  Efficiency is the ratio of the useful power output from a system to its total power output

  If the system has a high efficiency more of the energy transfer is useful

  If the system has a low efficiency less of the energy transfer is useful so is wasted
• Efficiency equation

  Efficiency = (useful power output/total power input)x100%

  Power = energy/time

  So:

  Efficiency = (useful energy output/total energy output)x100%