Vectors - University of x y z i j k These are special unit vectors directed along the x, y, and z...

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Transcript of Vectors - University of x y z i j k These are special unit vectors directed along the x, y, and z...

  • 28/06/13 Vectors (© F.Robilliard) 1

    x

    y

    s

    θ" 4

    3

    2.8

  • 28/06/13 Vectors (© F.Robilliard) 2

    Introduction:

    To make precise descriptions of the physical world we define measurable physical quantities that represent particular aspects of that world. These quantities must be realizable experimentally, and are specified as numeric multiples of a standard amount of

    the quantity, called a unit.

    A model is then constructed, that precisely describes the interrelationships, that exist between the various quantities

    associated with a given physical system.

    In these notes, we will identify two different types of quantity used in Physics, and develop methods for their use.

  • 28/06/13 Vectors (© F.Robilliard) 3

    Scalar & Vector Quantities:

    We find two types of quantity in Physics:

    Scalars: have a magnitude only

    Vectors: have a magnitude and act in a direction in space

  • 28/06/13 Vectors (© F.Robilliard) 4

    Examples: Scalars: mass m time t

    volume V energy E density ρ

    temperature T

    Vectors: displacement r

    velocity v acceleration a

    force f momentum p

    electric field vector E area A

    Is area a scalar, or a vector?

  • 28/06/13 Vectors (© F.Robilliard) 5

    Representation of Scalars:

    Scalars can be fully represented by a single number, representing their magnitude in terms of a unit–

    Example: mass = 5 kg.

  • 28/06/13 Vectors (© F.Robilliard) 6

    Representation of Vectors:

    The direction of the arrow corresponds to the direction of the vector in space, relative to stated reference axes.

    For Vectors, magnitude and direction must both be represented.

    This can be done using an arrow, drawn on a graph page.

    The length of the arrow is proportional to the magnitude of the vector, according to a stated scale.

    Note: Because our vectors exist in 3-dimensional space, they require three numbers for their representation.

    Note: the location of the arrow doesn t matter – only its length, and angle of inclination.

  • 28/06/13 Vectors (© F.Robilliard) 7

    Example: Represent a force of 3.0 N acting on a body in the +x direction

    3.0 N

    x

    y

    x

    y Scale:

    1 cm = 1 N

    2.8 N

    x

    y

    x

    y Scale:

    1 cm = 1 N

    Represent a force of 2.8 N acting on a body at 45 deg to the +x direction

  • 28/06/13 Vectors (© F.Robilliard) 8

    Notation: The fact that a symbol represents a vector quantity is

    indicated in a number of alternative ways:

    The magnitude of a vector can be explicitly represented by the use of modulus bars, or by not using the above notational

    devices

    (Magnitude of vector v) = |v| = v Not bold

    v :gleUndersquig v :Underarrow :Font Bold

    ~

    v

  • 28/06/13 Vectors (© F.Robilliard) 9

    Addition of Vectors:

    If two, or more, forces act on the same body, the motion of the body will be determined by the total overall effective

    force (also called the resultant force), acting on the body. To find this we add the individual forces.

    The sum, s, of two vectors a and b, is represented by s = a + b

    Notation:

    In many physical situations, we need to know the total effect of several simultaneous individual vectors. For example -

    If each of two charges, produces an electric field force at a point in space, the total field force at that point will be the sum of the two individual field force components.

  • 28/06/13 Vectors (© F.Robilliard) 10

    Rule for Graphical Addition of Vectors:

    Arrange the arrows representing the individual vectors, on a graph page so that the head of one touches the tail of the next.

    Draw a new arrow from the tail of the first vector, to the head of the last vector.

    This new arrow represents the sum of the individual vectors.

    Note: the sum will be the same, no matter in what order the arrows are placed.

  • 28/06/13 Vectors (© F.Robilliard) 11

    Example: Add 2 Vectors Two forces act

    on a body

    4 N

    3 N

    4

    3 s

    Total effective (resultant) force acting on the body is 5 N acting at 36.9 deg to the direction of the 4 N force

    θ"

    Scale: 1 cm = 1 N

    Using a ruler: s has a magnitude of 5

    ( |s| = 5 units)

    Using a protractor: s has a direction of 36.9 deg

    above the 4 N force ( θ = 36.9 deg )

    s = (4 + 3) N

    5 N

    36.90

  • 28/06/13 Vectors (© F.Robilliard) 12

    Example: Add 3 Vectors

    Using a ruler: s has a magnitude of 5.4

    ( |s| = 5.4 units)

    Using a protractor: s has a direction

    of 68.2 deg to the +x-axis

    ( θ = 68.2 deg )

    4 ) + ( 3 ) + ( 2.8 S = ( )

    x

    y

    s

    θ" 4

    3

    2.8

  • 28/06/13 Vectors (© F.Robilliard) 13

    Vector Subtraction:

    Negative Vector: To get the negative of a vector, reverse its direction

    - ( ) = ( 3 3 )

    Subtraction: To subtract a vector, add the negative vector

    ( ) – ( 3 ) = ( 4 4 ) + ( 3 )

  • 28/06/13 Vectors (© F.Robilliard) 14

    Magnitude of d: | d | = 5 units

    Direction of d: θ = 36.9 deg

    Example: Difference Vector:

    x

    y

    4

    3 d

    θ"

    ( ) ( ) ( ) ( ) ( )3434d vectordifference ~

    ↓+=↑−=≡ →→

    Subtract a vector, of magnitude 3 units in the +y-direction, from a vector of magnitude 4 units in the +x-direction.

  • 28/06/13 Vectors (© F.Robilliard) 15

    Resolving Vectors: This is the inverse of addition. Any single vector, v, can be

    replaced by two vectors, a and b, which are perpendicular to each other, and which add together to equal v.

    v = a + b a b

    a = v cos θ b = v sin θ

    a and b are the components, or resolved parts, of v along the x & y axes, respectively.

    v b

    a a

    v b

    x

    y

    θ"

  • 28/06/13 Vectors (© F.Robilliard) 16

    Unit Vectors:

    The unit vector pointing in the direction of a vector v, is represented by ) ( v̂ hat" v" pronounced

    ( )

    |v|

    v v̂ ~

    vector)of (magnitude

    (vector) runit vecto

    =

    These are vectors whose magnitude is one unit.

    v̂ v

    3 ∧

    3

  • 28/06/13 Vectors (© F.Robilliard) 17

    i, j and k unit vectors:

    x

    y

    z

    i j

    k

    These are special unit vectors directed along the x, y, and z axes

    Any vector can be expressed in terms of i, j and k unit vectors.

    This representation gives us a powerful method for working with vectors.

  • 28/06/13 Vectors (© F.Robilliard) 18

    Representing Vectors in i, j & k:

    v = 4 i + 3 j

    v = OP = 4 i + 3 j + 2 k

    d = 4 i - 3 j

    x

    y

    4i

    3j v

    θ"

    i j

    x

    y

    4i -3j d

    θ" i

    j

    x

    y

    z

    i

    j k

    4

    3

    2

    v

    P

    O

  • 28/06/13 Vectors (© F.Robilliard) 19

    General i, j & k form:

    v = a i + b j = ( v cos θ ) i + ( v sin θ ) j

    Resolve a vector v into its components along the x and y axes

    a = v cos θ

    b = v sin θ a

    v b

    x

    y

    θ"

    This allows any vector to be expressed in i, j, k form.

  • 28/06/13 Vectors (© F.Robilliard) 20

    Adding Vectors using i, j &k:

    i j

    a b

    s

    a = 2 i + 3 j b = 3 i - 4 j

    sum = s = a + b = (2+3) i + (3-4) j = 5 i - j

    Graphically: Using i, j, k:

    Both methods are equivalent, but i, j, k is preferred.

    s = 5 i - j

  • 28/06/13 Vectors (© F.Robilliard) 21

    Polar & Cartesian forms:

    θ" x

    y

    v

    vx = v cos θ

    vy = v sin θ"

    vx

    vy

    Polar: magnitude and direction

    (v, θ)"

    Cartesian: x & y components

    v = vx i + vy j

    There are two forms in which a vector can be expressed.

    Cartesian to Polar: Polar to Cartesian: v2 = vx2 + vy2 (Pythagoras)

    tan θ = vy/vx

  • 28/06/13 Vectors (© F.Robilliard) 22

    Example: Polar to Cartesian:

    A force F of 8 N acts at an angle of 30 deg to the x axis.

    Express F in Cartesian form.

    x

    y F = 8 N

    θ = 30o

    34

    30cos8

    cos FFx

    =

    =

    = o

    θ

    4 30sin8

    sin FFy

    =

    =

    = o

    θ

    jijiF 4 )3(4 ) sinθ (F ) cosθ (F +=+=

  • 28/06/13 Vectors