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:
Wtotal = ΔK = ½ mvf2 -
½ mvi2
- 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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