 # Work and Energy Work done by a constant force Work-Energy Theorem and KE Gravitational Potential Energy Conservative Forces vs Non- conservative Forces.

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Work and Energy Work done by a constant force Work-Energy Theorem and KE Gravitational Potential Energy Conservative Forces vs Non- conservative Forces Conservation of Mechanical Energy Power

 Work is a scalar product of force and displacement with a unit of N*m or Joules (J).  A joule is the work done by applying a force of one newton through a displacement of one meter.  For work to be done on an object, the force must have a component in the same direction as the displacement.  Ex: pg 187 #1, pg 188 # 1, 3

 Energy is the ability to do work.  When work is done, energy is transferred.  Unit for energy is the same as for work, Joule.  *** The amount of work done on a system is exactly equal to the change in energy of the system.  Kinetic Energy is the energy an object has b/c of its motion.  Work = ∆ KE = ½ mv f 2 – ½ mv i 2  Ex: pg 189 #13, 15

 Potential Energy is energy a system has because of its position or configuration.  An example of a rubber band, you store energy in the rubber band as elastic potential energy as you stretch it.  Another example is when you lift an object up against gravity.  PE = g * m * h

 A force is conservative if the work done by the force does not depend on the path taken between any two points.  Examples of conservative forces are the gravitational force and the spring force as they conserve energy during a round trip.  Nonconservative forces are those that the work done depends on the path taken. Friction is an example as a longer path will dissipate more heat energy.  Work done by NCF generally cannot be recovered as usable energy.  Ex: pg 190 #29, 33 pg 191 #51, 53

 When work is done the energy of the system changes form but the TOTAL AMOUNT of energy stays the same. Total energy is conserved.  **Law of Conservation of Energy  The sum of the kinetic and potential energies of a system is called the total mechanical energy of the system.  During freefall examples we can follow: GPE top + KE top = GPE bottom + Ke bottom EX: pg 190 #36, 41

 Work can be done slowly or quickly.  Power is the rate at which work is done.  Power = work / time  The units for power are J/s or Watts (W).  A 75- watt light bulb is using 75 joules of energy each second.  Ex: pg 192 # 63, 69

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