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SECTION 3.3 • Some Properties of Vectors 61 Another example of a vector quantity is displacement, as you know from Chapter 2. Suppose a particle moves from some point ! to some point " along a straight path, as , with the tip of the arrow pointing away from the starting point. The direction of the arrowhead represents the direction of the displacement, and the length of the arrow In this text, we use a boldface letter, such as A, to represent a vector quantity. An- other notation is useful when boldface notation is difficult, such as when writing on pa- : A. The magnitude of the vector A is written either A or !A!. The magnitude of a vector has physical units, such as meters for displacement or meters per second for velocity. The A vector quantity is completely specified by a number and appropriate units plus a ! " Figure 3.4 As a particle moves from ! to " along an arbitrary path represented by the broken line, its displacement is a vector quantity shown by the arrow drawn from ! to ". O y x Figure 3.5 These four vectors are equal because they have equal lengths and point in the same direction. Quick Quiz 3.1 Which of the following are vector quantities and which are scalar quantities? (a) your age (b) acceleration (c) velocity (d) speed (e) mass ▲ PITFALL PREVENTION 3.1 Vector Addition versus Scalar Addition Keep in mind that A % B " C is B A R = A + B Active Figure 3.6 When vector B is added to vector A, the resultant R is the vector that runs from the tail of A to the tip of B. 3.3 Some Properties of Vectors Equality of Two Vectors For many purposes, two vectors A and B may be defined to be equal if they have the same magnitude and point in the same direction. That is, A " B only if A " B and if A and B point in the same direction along parallel lines. For example, all the vectors in Figure 3.5 are equal even though they have different starting points. This property al- Adding Vectors The rules for adding vectors are conveniently described by graphical methods. To add B to vector A, first draw vector A on graph paper, with its magnitude repre- sented by a convenient length scale, and then draw vector B to the same scale with its tail starting from the tip of A, as shown in Figure 3.6. The resultant vector R " A % B is the vector drawn from the tail of A to the tip of B. For example, if you walked 3.0 m toward the east and then 4.0 m toward the north, as shown in Figure 3.7, you would find yourself 5.0 m from where you started, mea- A geometric construction can also be used to add more than two vectors. This is shown in Figure 3.8 for the case of four vectors. The resultant vector R " A % B % C % D is the vector that completes the polygon. In other words, R is the vector drawn When two vectors are added, the sum is independent of the order of the addition. (This fact may seem trivial, but as you will see in Chapter 11, the order is important Go to the Active Figures link at http://www.pse6.com. |