P1 - Conservation and dissipation of energy

• Conservation of energy


  The principle that energy cannot be created or destroyed, only transferred or transformed
• Closed system


  • A system in which no energy transfers take place out of or into the energy stores of the system
• Changes to energy stores in a closed system

  1. Energy can be transferred between energy stores within the closed system

  2. The total energy of the system is always the same, before and after any such transfers
• Never use the terms 'movement energy' or 'motion energy' - only 'kinetic energy'
• Work


  The transfer of energy by a force causing an object to move in the direction of the force

  
• One joule of work is done when a force of one newton causes an object to move a distance of one metre in the direction of the force
• Experiments to calculate work done

  • Drag a small box a measured distance across a rough surface
  • Repeat the test above with two rubber bands wrapped around the box
• Work done


  Force applied x distance moved in the direction of the force
• To calculate the work done by a force when it causes displacement of an object, use the equation: work done, W = force applied, F x distance moved along the line of action of the force, s

  
• Very few people can manage to pull with such force. Don't even try it, though. The people who can do it are very, very strong and have trained specially for it
• Experiments to calculate the work done

  1. Measure force applied using a newton-meter

  2. Measure distance moved using a metre ruler
• Resolution


  Precision of measuring instruments
• Work done to overcome friction is mainly transferred to thermal energy stores by heating
• Friction examples


  • Rubbing hands together
  • Brake pads on a vehicle
  • Meteorites passing through the Earth's atmosphere

  
• Energy transferred


  Equal to the work done
• When an object moves, the energy transferred is equal to the work done
• Work done to overcome friction is transferred as energy to the thermal energy stores of the objects that rub together and to the surroundings
• Gravitational potential energy


  Energy in the gravitational energy store of an object
• When an object is moved upwards, the energy in its gravitational potential energy store increases. This increase is equal to the work done on it by the lifting force to overcome the gravitational force on the object

  
• When an object moves down, the energy in its gravitational potential energy store decreases. This decrease is equal to the work done by the gravitational force acting on it

  
• Work done when an object moves up or down

  Depends on: 1) how far it is moved vertically (its change of height), 2) its weight
• The change in the gravitational potential energy store of an object is AE = m g Ah

  
• Astronauts on the Moon can lift objects much more easily than they can on the Earth. This is because the gravitational field strength on the Moon's surface is only about a sixth of the gravitational field strength on the Earth's surface

  
• The change of gravitational potential energy store = mass x gravitational field strength x change of height

  
• Stepping up experiment


  1. Measure your mass in kilograms

  2. Step on and off a sturdy box or low platform

  3. Measure the height of the box

  4. Use the equation AE = m g Ah to calculate the energy transferred to your gravitational potential energy store

  
• Kinetic energy


  • The energy an object has because of its motion
  • Depends on its mass and speed

  
• Investigating kinetic energy stores

  1. Release a ball on a slope and measure the decrease in its gravitational potential energy store

  2. Time the ball over a measured distance after the slope to calculate its speed

  3. Use light gates to improve data collection

  
• The kinetic energy store of the ball increases if the speed increases
• Kinetic energy equation


  E = 1/2 m v^2

  
• Elastic potential energy


  Energy stored in a stretched or compressed elastic object
• Hooke's Law


  Force F needed to stretch a spring varies linearly with extension e: F = k e, where k is the spring constant
• Elastic potential energy equation

  E = 1/2 k e^2

  
• Kinetic energy


  Energy store of a moving object that depends on its mass and speed

  
• Elastic potential energy


  Energy stored in an elastic object when work is done on the object
• Calculating kinetic energy


  Kinetic energy = 1/2 x mass x speed^2

  
• Calculating kinetic energy


  • Vehicle of mass 500kg moving at 12 m/s

  ii) Football of mass 0.44kg moving at 20 m/s

  
• Calculating velocity with known kinetic energy


  Kinetic energy = 1/2 x mass x velocity^2

  Rearrange to find velocity

  
• Energy transfers in a catapult

  When stretched: Work done stores elastic potential energy

  When released: Elastic potential energy transfers to kinetic energy

  
• Calculating force that stops a vehicle

  Use kinetic energy equation to find force from known kinetic energy and stopping distance

  
• Calculating mass from known kinetic energy and speed

  Rearrange kinetic energy equation to find mass