Physics 6.3

• There is work done
• Laws of thermodynamics that will be useful when understanding and calculating the work done (energy)

  • Law 1
  • Law 2
• Law 1


  Energy is never lost nor gained, it transfers by changing from one form into another form
• Law 2


  Energy spreads out and dissipates, taking on forms that isn't needed, wasting the energy. This means that no energy transfers is a 100% efficient
• Work done


  A measure of how much the energy transfer of energy was used by force to move an object a certain amount of distance
• Forms of energy


  • Light
  • Chemical
  • Kinetic
  • Thermal (heat)
  • Electrical
  • Sound
  • Nuclear
  • Elastic (strain)
  • Gravitational
• Work done has the same units as energy, Joules
• Work done examples


  • 1180N x 1.5m = 1770 J
  • 96,000J + 20m = 4800N
  • 21003700N=3m
• Gravitational potential energy (GPE)


  Potential energy stored due to the force of gravity acting upon an object above the Earth's surface
• Gravitational potential energy example

  • A book has a mass of 0.75 kg and is lifted 2 m in the air. The change in GPE is 0.75 kg x 10 m/s^2 x 2 m = 15 J
• Kinetic energy (KE)


  Energy possessed by an object due to its motion
• Kinetic energy example


  • A car travelling at 5 m/s has a kinetic energy of 1/2 x 1000 kg x (5 m/s)^2 = 12,500 J
• When the speed of an object doubles, its kinetic energy quadruples
• Efficiency


  Calculated by dividing the useful energy output by the total energy input
• Improving vehicle efficiency


  • Reduce air resistance by having a streamlined shape
  • Reduce rolling resistance by using the right tyres and tyre pressure
  • Reduce inertia by using lightweight materials
• Vehicle safety features


  • Seat belts
  • Airbags
  • Crumple zones
• Seat belts


  Stop people from tumbling and moving around inside a car in a collision, but are designed to stretch a bit to increase the time taken for the body's momentum to reach zero
• Airbags


  Increase the time taken for the head's momentum to reach zero, and act as a soft cushion to prevent cuts
• Crumple zones


  Areas of a vehicle designed to crush in a controlled way in a collision, increasing the time taken to change the momentum of the driver and passenger
• Safety features in a car reduce the force and severity of injuries in a crash
• Hooke's Law


  The extension of an elastic object is directly proportional to the force applied to it
• Spring constant


  The greater the spring constant, the stiffer the spring will be
• The area under the force-extension graph is the work done to stretch or compress the spring
• To calculate the total energy, use the equations for kinetic energy, gravitational potential energy, and work done on a spring, and add them together