Physics
Vandebilt Catholic High School
W. Dupre

Energy – Notes

  1. Definition - An object posses energy if it has the ability to cause changes in its surroundings.

  2. Forms
    3.    
    1. Mechanical Energy (E)- the energy in material objects
      2.        
      1. Kinetic Energy (K) - energy of motion
        2.           
        1. Kinetic Energy is described by the formula: K = 1/2mv2.
        2. Therefore K a m ; if m doubles then K also doubles. For example if an objects mass doubles from 2kg to 4 kg;its K would also double from lets say 3 J to 6 J.
        3. Also K a v2 ; if v doubles then K would quadruple. For example if an objects velocity increased from 2 m/s to 4 m/s; the relative K increased would be from let say 3 J to 12 J.
      2. Potential Energy (U) - enrergy of position
        3.           
        1. When work is done Potential energy is transfered to an object.
        2. The amount of potential energy depends on the position, shape, or form of the object.
        3. Potential Energy can be gravitational (UG). It is acquired by doing work against the force of gravity.
        4. UG = mgh; where m is mass, g is acceleration due to gravity, and h is the height above the frame of reference.
        5. The amount of potential energy an object gains or looses is not affected by the path the object travels, but only by its change in height from the frame of referenece. For example a person will loose just as much potential energy sliding down a slide as he would jumping straight down from the top of the slide.
        6. Potential Energy can be elastic or spring potential (Us). It is acquired by doing work on a spring or eleastic substance.
        7. Us = 1/2 kx2; where k is the spring constant in (N/m) and x is the displacement of the spring.
      3. Work (W) - the role of work is to transfer energy from one place or object to another.
    2. Heat - a by product of friction that causes movement at the molecular level
    3. Electricity
    4. Magnetism
    5. Light
    6. Sound
    7. Nuclear
    8. Chemical
    9. Mass

  3. Units
    4.    
    1. Like work the standard mks unit for energy is the joule (J).
    2. Other units for energy include the erg (1 erg = 10-7 J), the calorie (1 cal = 4.184 J), British Thermal Units (1 B.T.U. = 1055 J), kilowatt hours (kWh), etc.

  4. The Work - Energy Theorem
    5.    
    1. This tells us that the work done on a mass will always equal the mass' change in energy.
    2. Therefeore the works done tells us either how much change in kinetic energy an objects undergoes or how much change in potential energy an object has.
    3. SW = DK ; or SW = DU
    4. If work is positive then the change in kinetic energy is positive, which means the object's speed is increasing. If work is negative then the change in kinetic energy is negative, which means the object's speed is decreasing.

  5. Law of Conservation of Energy
    6.    
    1. The total energy in a closed, isolated system is always conserved. it may change from one form to another but the total amount of energy is always constant.
      2.       
      1. A closed system means that no objects may enter or leave the system.
      2. An isolated system means that no external force is acting on the system.
    2. Therefore the initial energy a system has will be equal to the final energy the system has; and any gain in one form of enegy must be accounted for by an equal loss in another form of energy.
    3. In a formula think of the law as follows:
      Ei = E f
      -DU = DK
      mghf - mghi = 1/2mvf2 - 1/2mvi2