Lecture II Vectors and Vector Algebra

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Lecture II. Vectors and Vector Algebra. A set S of rays is called a direction if it satisfies the following laws: (1) Any two rays in S have the same direction. (2) Every ...
Lecture II Vectors and Vector Algebra

A set S of rays is called a direction if it satisfies the following laws: (1) Any two rays in S have the same direction. (2) Every ray that has the same direction as some member of S is in S. � consists of a non-negative real number, called the magnitude of A vector A � by |A � |. the vector, and a direction. We denote the magnitude of A � such that |A � | = 1 is called a unit vector. A vector A We will now describe four basic algebraic operations with vectors in E3 : 1 Multiplication by a scalar � by a vector, and let c by a real number. Multpying A � by the scalar Let A � The magnitude of the result is given c, we obtain a vector denoted by cA. � |. The direction of cA � is the same as the direction of A � if � | = |c||A by |cA � if c < 0. c ≥ 0, and opposite to the direction of A 2 Addition of vectors � and B � is denoted simply by A � + B. � We can The sum of two vectors A � such that its start point is the define this sum geometrically. Translate B � Then A � +B � will be the vector having the same start-point end-point of A. � and the same end-point as B. � as A 3 Scalar product (Dot product) � and B � is a scalar quantity denoted The scalar product of two vectors A � . B. � .B � Its value is A � = |A|| � B � |cosθ, where θ is the angle made by A � by A � if we translate B � such that it has the same start-point as A. � We and B � on A. � � |cosθ is the length of the projection of B observe that |B

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� B

� A � �+B A

Figure 1: Vector addition � is equal to the square root of the dot product of A The magnitude of A with itself: � = |A| Hence

� A � |A|

� = √A

� A� .A

� � � .B A

is a unit vector with the same direction as A.

4 Vector product (Cross product) � and B � is a vector denoted by A � × B. � The cross product of two vectors A The magnitude of the cross product is given by: �×B � | = |A � | |B � |sinθ. |A � and B � have the same start-point P and end-points Q1 and Q2 , Let A � and B. � The direction of A �×B � is respectively. Let M be the plane of A normal to M in the manner established by the right-hand rule: if a right hand is placed at P and the fingers are curling from P Q1 to T Q2 through � × B. � the angle smaller than π, then the thumb indicates the direction of A The following are some properties of these basic vector operations. � = aC � + bC � 1. (a + b)C � = a(bC) � 2. (ab)C � + C) � = aB � + aC � 3. a(B

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� + B � =B � +A � 4. A � + C) � =A � +A � � . (B � .B �.C 5. A � ×A � �×B � = −B 6. A

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