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SECTION 7.2 • Work Done by a Constant Force 185 application. That is, if ! # 90°, then W # 0 because cos 90° # 0. For example, in r. The sign of the work also depends on the direction of F relative to "r. The work done by the applied force is positive when the projection of F onto "r is in the same direction as the displacement. For example, when an object is lifted, the work done F onto "r is in the direction opposite the displacement, W is negative. For exam- ple, as an object is lifted, the work done by the gravitational force on the object is W (Eq. 7.1) automatically takes care of the sign. If an applied force F is in the same direction as the displacement "r, then ! # 0 and cos 0 # 1. In this case, Equation 7.1 gives Work is a scalar quantity, and its units are force multiplied by length. Therefore, the SI unit of work is the newton ! meter (N · m). This combination of units is used so frequently that it has been given a name of its own: the joule ( J). An important consideration for a system approach to problems is to note that work is an energy transfer. If W is the work done on a system and W is positive, energy is W # F
" r Quick Quiz 7.1 The gravitational force exerted by the Sun on the Earth holds the Earth in an orbit around the Sun. Let us assume that the orbit is perfectly cir- Quick Quiz 7.2 Figure 7.4 shows four situations in which a force is applied to an object. In all four cases, the force has the same magnitude, and the displacement of Figure 7.4 (Quick Quiz 7.2) F (c) (d) (b) F (a) F F ▲ PITFALL PREVENTION 7.4 Cause of the Displacement We can calculate the work done F θ n ∆ r mg Figure 7.3 When an object is dis- placed on a frictionless, horizontal surface, the normal force n and the gravitational force mg do no work on the object. In the situation shown here, F is the only force do- ing work on the object. |