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Vectors

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Notes de leçon

Vectors and notation

  • A vector has both magnitude (size) and direction. Written as a column \begin{pmatrix} a \\ b \end{pmatrix}, or \vec{AB}, or in bold \mathbf{a}.
  • \vec{AB} goes from A to B. Its negative reverses direction: \vec{BA} = -\vec{AB}.
  • The zero vector has magnitude 0 and no defined direction. Statements about parallel directions here assume nonzero vectors.

Vector magnitude from components

Vector magnitude from componentsa = (3, 0)b = (0, 4)a + b = (3, 4)ABC

Operations with vectors

  • Add vectors by adding components: \begin{pmatrix} 2 \\ 3 \end{pmatrix} + \begin{pmatrix} 1 \\ -4 \end{pmatrix} = \begin{pmatrix} 3 \\ -1 \end{pmatrix}.
  • Multiply by a scalar to scale: 3\begin{pmatrix} 2 \\ -1 \end{pmatrix} = \begin{pmatrix} 6 \\ -3 \end{pmatrix}.
  • The magnitude of \begin{pmatrix} a \\ b \end{pmatrix} is \√{a2 + b2}. For \begin{pmatrix} 3 \\ 4 \end{pmatrix} it is 5.

Parallel vectors

  • Two nonzero vectors are parallel if one is a nonzero scalar multiple of the other. Positive multiples point the same way; negative multiples point in opposite directions.

Geometry with vectors

  • Use the triangle rule: \vec{AC} = \vec{AB} + \vec{BC}. Route to a point by any path along known vectors.
  • The midpoint M of AB satisfies \vec{AM} = \frac{1}{2}\vec{AB}.
  • Vectors prove that points are collinear (on a straight line): if \vec{AB} is a multiple of \vec{AC} then A, B and C are collinear.

Adding vectors head to tail

Adding vectors head to taila = (3, 1)b = (1, 3)a + b = (4, 4)ABC

Diapos

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Questions d'entraînement

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  1. 1.What two properties does a vector have?

    Easy
    • Asize only
    • Bmagnitude and direction
    • Cdirection only
    • Dspeed and time
  2. 2.What is 2\begin{pmatrix} 3 \\ -1 \end{pmatrix}?

    Easy
    • A\begin{pmatrix} 6 \\ -2 \end{pmatrix}
    • B\begin{pmatrix} 5 \\ 1 \end{pmatrix}
    • C\begin{pmatrix} 6 \\ -1 \end{pmatrix}
    • D\begin{pmatrix} 3 \\ -2 \end{pmatrix}
  3. 3.What is \begin{pmatrix} 4 \\ 1 \end{pmatrix} + \begin{pmatrix} 2 \\ 5 \end{pmatrix}?

    Easy
    • A\begin{pmatrix} 6 \\ 6 \end{pmatrix}
    • B\begin{pmatrix} 8 \\ 5 \end{pmatrix}
    • C\begin{pmatrix} 2 \\ -4 \end{pmatrix}
    • D\begin{pmatrix} 6 \\ 4 \end{pmatrix}
  4. 4.What is \begin{pmatrix} 5 \\ 2 \end{pmatrix} - \begin{pmatrix} 1 \\ 4 \end{pmatrix}?

    Easy
    • A\begin{pmatrix} 4 \\ -2 \end{pmatrix}
    • B\begin{pmatrix} 6 \\ 6 \end{pmatrix}
    • C\begin{pmatrix} 4 \\ 2 \end{pmatrix}
    • D\begin{pmatrix} -4 \\ 2 \end{pmatrix}
  5. 5.What is -\begin{pmatrix} 2 \\ -3 \end{pmatrix}?

    Easy
    • A\begin{pmatrix} -2 \\ 3 \end{pmatrix}
    • B\begin{pmatrix} 2 \\ 3 \end{pmatrix}
    • C\begin{pmatrix} -2 \\ -3 \end{pmatrix}
    • D\begin{pmatrix} 3 \\ -2 \end{pmatrix}
  6. 6.If \vec{AB} = \mathbf{a}, what is \vec{BA}?

    Easy
    • A\mathbf{a}
    • B-\mathbf{a}
    • C2\mathbf{a}
    • D0
  7. 7.A has coordinates (1, 2) and B (4, 6). What is \vec{AB}?

    Easy
    • A\begin{pmatrix} 3 \\ 4 \end{pmatrix}
    • B\begin{pmatrix} 5 \\ 8 \end{pmatrix}
    • C\begin{pmatrix} -3 \\ -4 \end{pmatrix}
    • D\begin{pmatrix} 4 \\ 3 \end{pmatrix}
  8. 8.What is the magnitude of \begin{pmatrix} 6 \\ 8 \end{pmatrix}?

    Easy
    • A10
    • B14
    • C48
    • D100

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