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Multiplying a Vector by a Scalar If vector A is multiplied by a positive scalar quantity m, then the product mA is a vector that has the same direction as A and magnitude mA. If vector A is multiplied by a nega- tive scalar quantity # m, then the product # m A is directed opposite A. For example, the vector 5 A is five times as long as A and points in the same direction as A; the vector is one-third the length of A and points in the direction opposite A. 3.4 Components of a Vector and Unit Vectors The graphical method of adding vectors is not recommended whenever high accuracy components of the vector. Any vector can be com- pletely described by its components. Consider a vector A lying in the xy plane and making an arbitrary angle ! with the positive x axis, as shown in Figure 3.13a. This vector can be expressed as the sum of two A x and A y . From Figure 3.13b, we see that the three vectors form a right triangle and that A " A x % A y . We shall often refer to the “components of a vector A,” written A x and A y (without the boldface notation). The component A x represents the projection of A along the x axis, and the component A y represents the projection of A along the y axis. These components can be positive or negative. The component A x is positive if A x points in the positive x direction and is negative if A x points in the nega- tive x direction. The same is true for the component A y . From Figure 3.13 and the definition of sine and cosine, we see that cos ! " A x /A and that sin ! " A y /A. Hence, the components of A are (3.8) (3.9) These components form two sides of a right triangle with a hypotenuse of length A. A are related to its components through the expressions (3.10) (3.11) Note that the signs of the components A x and A y depend on the angle !. For example, if ! " 120°, then A x is negative and A y is positive. If ! " 225°, then both A x and A y are negative. Figure 3.14 summarizes the signs of the components when A lies in the various quadrants. When solving problems, you can specify a vector A either with its components A x and A y or with its magnitude and direction A and !. ! " tan # 1 " A y A x # A " √ A x
2 % A y
2 A y " A sin ! A x " A cos ! # 1 3 A SECTION 3.4 • Components of a Vector and Unit Vectors 65 y x A O A y A x θ (a) y x A O A x θ (b) A y Figure 3.13 (a) A vector A lying in the xy plane can be represented by its component vectors A x and A y . (b) The y component vector A y can be moved to the right so that it adds to A x . The vector sum of the component vectors is A. These three vectors form a right triangle. ▲ PITFALL PREVENTION 3.2 Component Vectors versus Components The vectors A x and A y are the component vectors of A. These x and A y , which we shall always refer to as the components Quick Quiz 3.5 Choose the correct response to make the sentence true: A component of a vector is (a) always, (b) never, or (c) sometimes larger than the magni- y x A x
positive A y
positive A x
positive A y
negative A x
negative A y
positive A x
negative A y
negative Figure 3.14 The signs of the com- ponents of a vector A depend on the quadrant in which the vector is located. Components of the vector A Suppose you are working a physics problem that requires resolving a vector into its components. In many applications it is convenient to express the components in a co- |