Chapter 13

# Example 6 find the scalar projection and vector

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Unformatted text preview: LE 6 Find the scalar projection and vector projection of b a 1, 1, 2 onto 2, 3, 1 . SOLUTION Since a s compa b 2 2 32 12 ab a s14, the scalar projection of b onto a is 21 31 s14 12 3 s14 The vector projection is this scalar projection times the unit vector in the direction of a: proja b F P Q D ( W S ¨ 3 a 14 393 , , 7 14 14 One use of projections occurs in physics in calculating work. In Section 6.4 we deﬁned the work done by a constant force F in moving an object through a distance d as W Fd, but this applies only when the force is directed along the line of motion of the object. l Suppose, however, that the constant force is a vector F PR pointing in some other direction as in Figure 6. If the force moves the object from P to Q, then the displacement l vector is D PQ. The work done by this force is deﬁned to be the product of the component of the force along D and the distance moved: R FIGURE 6 3 a s14 a F cos ) D But then, from Theorem 3, we have 12 W F D cos FD 5E-13(pp 848-857) ❙❙❙❙ 848 1/18/06 11:18 AM Page 848 CHAPTER 13 VECTORS AND THE GEOMETRY OF SPACE Thus, the work done by a constant force F is the dot product F D, where D is the displacement vector. EXAMPLE 7 A crate is hauled 8 m up a ramp under a constant force of 200 N applied at an angle of 25 to the ramp. Find the work done. F 25° SOLUTION If F and D are the force and displacement vectors, as pictured in Figure 7, then the work done is D W FIGURE 7 FD F D cos 25 200 8 cos 25 1450 N m 1450 J EXAMPLE 8 A force is given by a vector F 3 i 4 j 5 k and moves a particle from the point P 2, 1, 0 to the point Q 4, 6, 2 . Find the work done. l SOLUTION The displacement vector is D PQ 2, 5, 2 , so by Equation 12, the work done is W FD 3, 4, 5 6 10 20 2, 5, 2 36 If the unit of length is meters and the magnitude of the force is measured in newtons, then the work done is 36 joules. |||| 13.3 Exercises 1. Which of the following expressions are meaningful? Which are meaningless? Explain. (a) a b c (c) a b c (e) a b c 11–12 |||| If u is a...
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