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2012-12-15 12:39:46

Physics: Work and Energy
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  1. When does an object has energy
    if it can produce a change in itself or in its surroundings.
  2. Work
    • is the product of the net force exerted on an object and the distance the object moves in the direction of the force.
    •  W = Fd   N·m = Joule (J)
    • If force is at an angle: W = Fd cos θ   (cos 90o = 0)
    • Work is the transfer of energy by mechanical means. Energy is the ability to change an object.
  3. Kinetic energy (KE):
    energy an object has because of its motion.
  4. Potential energy (PE):
    energy an object has because of position, shape or form.
  5. The net work done on an object is equal to its change in KE.
    F = ma   W = Fx     W = max
  6. Gravitational PE
    • depends on the weight of the body and its separation from Earth.
    • PE = mgh
  7. How does PE depend on height?
    • (Recall this equation:  vf2 = vi2 + 2gx and multiply each term by ½ m) 
    • E = ½ mvf2 = ½ mvi2 + mgh   
    • If h = 0, then E = ½ mv2    
    • If v = 0, then E = mgh
    • Total E = KE + PE
    • Note: need a reference level to calculate PE 
  8. Elastic potential energy
     is energy available for use when a deformed elastic object returns to its original configuration.
  9. The sum of KE + PE = mechanical energy.
  10. the Law of Conservation of Energy
    • According to the Law of Conservation of Energy, the total energy of a closed, isolated system is constant.
    • (If something is conserved, we mean it remains constant.)
    • Energy can change form but the total energy does not change.
    • Note: mechanical energy is not conserved in the presence of friction.
  11. Power
    • is the rate of doing work or the rate at which energy is transferred.
    • P = W/t        J/s = Watt (W)   P = Fd/t = Fv