FACULTY OF SCIENCE, AUTUMN 2006

PHYS 124 LEC A1 : Particles and Waves (Instructor: Marc de Montigny)

Walker, Physics, Chapter 7: Work and Kinetic Energy

• Section 7-1: Work Done by a Constant Force
  • Force in the direction of displacement:
    • Definition: p. 180, Eq. 7-1 : W = Fd
    • SI Unit: 1 joule [J] = 1 newton-meter [N⋅m]; p. 180, Eq. 7-2
    • Unlike Force, Work is a scalar quantity
    • See p. 180 Figure 7-1
  • Force at an Angle to the Displacement:
    • Definition: p. 182, Eq. 7-3 : W = (F cosθ)d = Fd cosθ
    • See p. 182 Figure 7-2
  • Negative Work and Total Work:
    • See bottom of p. 184:
      • W + if F has a component in the direction of displacement (-90°<θ<90°)
      • W = 0 if F is perpendicular to displacement (θ=±90°)
      • W - if F has a component opposite to the direction of displacement (90°<θ<270°)
    • See p. 184 Figure 7-4
  • Read p. 185, Example 7-3.
• Section 7-2: Kinetic Energy and the Work-Energy Theorem
  • Definition of Kinetic Energy: p. 187, Eq. 7-6 : K=½mv²
  • SI Units: 1 J = 1 kg⋅m²/s²
  • Work-Energy Theorem: p. 188, Eq. 7-7: W_total = ΔK = ½ mv_f² - ½ mv_i²
  • We will discuss p. 189, Example 7-6.
• Section 7-3 (Brief): Work Done by a Variable Force
  • Given a graph of F as a function of x, then the work done by F to move an object from position x₁ to position x₂ is given by the surface area under the curve.
  • See p. 191, Figure 7-7a and Figure 7-8
  • Work needed to stretch a spring a distance x, p. 192, Figure 7-10
  • Work to stretch or compress a spring a distance x from equilibrium: p. 192, Eq. 7-8: W = ½ kx²
• Section 7-4: Power
  • Definition of average power: p. 195, Eq. 7-10: P = W/t
  • SI Unit: 1 watt [W] = 1 J/s
  • See p. 195, Table 7-3, for typical values of power.
  • Other form in p. 196, Eq. 7-13: P = W/t = (Fd)/t = F(d/t) = Fv
  • We will discuss p. 202, Prob. 49.

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