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Chapter 7 Kinetic Energy and Work

Chapter 7 Kinetic Energy and Work. Energy What is energy? Energy - is a fundamental, basic notion in physics Energy is a scalar , describing state of an object or a system Description of a system in ‘energy language’ is equivalent to a description in ‘force language’

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Chapter 7 Kinetic Energy and Work

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  1. Chapter 7 Kinetic Energy and Work

  2. Energy • What is energy? • Energy - is a fundamental, basic notion in physics • Energy is a scalar, describing state of an object or a system • Description of a system in ‘energy language’ is equivalent to a description in ‘force language’ • Energy approach is more general and more effective than the force approach • Equations of motion of an object (system) can be derived from the energy equations

  3. Some calculus • In 1D case

  4. Some calculus • In 1D case • In 3D case, similar derivations yield • K – kinetic energy

  5. James Prescott Joule (1818 - 1889) • Kinetic energy • K = mv2/2 • SI unit: kg*m2/s2 = J (Joule) • Kinetic energy describes object’s ‘state of motion’ • Kinetic energy is a scalar

  6. Chapter 7 Problem 4

  7. Work–kinetic energy theorem • Wnet – work (net) • Work is a scalar • Work is equal to the change in kinetic energy, i.e. work is required to produce a change in kinetic energy • Work is done on the object by a force

  8. Work: graphical representation • 1D case: Graphically - work is the area under the curve F(x)

  9. Net work vs. net force • We can consider a system, with several forces acting on it • Each force acting on the system, considered separately, produces its own work • Since

  10. Work done by a constant force • If a force is constant • If the displacement and the constant force are not parallel

  11. Chapter 7 Problem 14

  12. Work done by a spring force • Hooke’s law in 1D • From the work–kinetic energy theorem

  13. Work done by the gravitational force • Gravity force is ~ constant near the surface of the Earth • If the displacement is vertically up • In this case the gravity force does a negative work (against the direction of motion)

  14. Lifting an object • We apply a force F to lift an object • Force F does a positive work Wa • The net work done • If in the initial and final states the object is at rest, then the net work done is zero, and the work done by the force F is

  15. Chapter 7 Problem 20

  16. James Watt (1736-1819) • Power • Average power • Instantaneous power – the rate of doing work • SI unit: J/s = kg*m2/s3 = W (Watt)

  17. Power of a constant force • In the case of a constant force

  18. Chapter 7 Problem 44

  19. Chapter 7 Problem 50

  20. Answers to the even-numbered problems Chapter 7: Problem 8 5.0 kJ

  21. Answers to the even-numbered problems Chapter 7: Problem 40 2.7 × 105 W

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