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Chapter 6

Chapter 6. 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 Done by a Constant Force.

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Chapter 6

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  1. Chapter 6 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

  2. Work Done by a Constant Force • 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

  3. Work-Energy Theorem and KE • 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 mvf2 – ½ mvi2 • Ex: pg 189 #13, 15

  4. Gravitational Potential Energy • 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

  5. Conservative vs Non-conservative Forces • 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

  6. Conservation of Mechanical Energy • 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: GPEtop + KE top = GPEbottom + Kebottom EX: pg 190 #36, 41

  7. Power • 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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