Lesson 21.2

• Law of Conservation of Mechanical Energy

  The total mechanical energy in a system remains the same during any process
• Joule's mechanical equivalent of heat set up

  1. Used a moving wheel to stir water inside an insulated box to produce heat

  2. Showed that mechanical energy can be converted to heat energy
• Galileo's frictionless ramp experiment showed that a ball will reach the same height as its initial position regardless of the length of the ramp on the other side
• Law of Conservation of Energy

  Energy can neither be created nor destroyed, it is only transformed from one form to another, but the total amount of energy in the system remains the same
• The law of conservation of mechanical energy states that the total mechanical energy in a system remains the same during any process
• A book placed on top of a table does not conserve mechanical energy because there is friction between the book and the table, which causes the mechanical energy to be transformed into other forms of energy such as heat
• Julius Robert Mayer

  • a German scientist was highly interested in one experiment that he thinks is beneficial for him and his family
  • He needs to bring up water from a river upstream
  • To do this, he designed an Archimedean screw to pump up water to a higher level
• Upon trying a lot of designs, Mayer realized that there was no way to arrange a machine to do work without anyone doing it. He thought that work is needed to lift the water up and energy must be spent to do this
• James Prescott Joule

  • was able to provide an experimental evidence for the equivalence of work and heat
  • he came up with an apparatus that converts mechanical energy to produce heat
  • To produce heat, a moving wheel is used to stir water inside an insulated
• In Joules experiment, every time the wheel is turned, heat is generated because the water present in the insulated box increases temperature. Hence, mechanical energy can be converted to heat energy
• The total mechanical energy is constant for a closed system
• Mathematically, the law of conservation of mechanical energy can be expressed as:

  • MEbefore = MEafter
  • PEbefore + KEbefore = PEafter + KEafter
  • mghi + 1/2mvi2 = mghf + 1/2mvf2