Physics Notes Class 11 CHAPTER 4
MOTION IN A
PLANE part 1
Those physical
quantities which require magnitude as well as direction for their complete
representation and follows vector laws are called vectors.
Vector can be
divided into two types
1. Polar Vectors
These are those
vectors which have a starting point or a point of application as a
displacement, force etc.
2. Axial Vectors
These are those
vectors which represent rotational effect and act along the axis of rotation in
accordance with right hand screw rule as angular velocity, torque, angular
momentum etc.
Scalars
Those physical
quantities which require only magnitude but no direction for their complete
representation, are called scalars.
Distance, speed,
work, mass, density, etc are the examples of scalars. Scalars can be added,
subtracted, multiplied or divided by simple algebraic laws.
Tensors
Tensors are
those physical quantities which have different values in different directions
at the same point.
Moment of
inertia, radius of gyration, modulus of elasticity, pressure, stress,
conductivity, resistivity, refractive index, wave velocity and density, etc are
the examples of tensors. Magnitude of tensor is not unique.
Different Types of Vectors
(i)
Equal Vectors Two vectors of equal
magnitude, in same direction are called equal vectors.
(ii)
Negative Vectors Two vectors of equal
magnitude but in opposite directions are called negative vectors.
(iii) Zero Vector or Null Vector A vector whose magnitude is zero is known as a zero or null vector.
Its direction is not defined. It is denoted by 0.
Velocity of a
stationary object, acceleration of an object moving with uniform velocity and
resultant of two equal and opposite vectors are the examples of null vector.
(iv) Unit Vector A vector having unit magnitude is called a unit vector.
A unit vector in
the direction of vector A is given
by
 = A / A
A unit vector is
unitless and dimensionless vector and represents direction only.
(v)
Orthogonal Unit Vectors The unit vectors
along the direction of orthogonal axis, i.e., X – axis, Y – axis and Z – axis
are called orthogonal unit vectors. They are represented by
(vi)
Co-initial Vectors Vectors having a
common initial point, are called co-initial vectors.
(vii)
Collinear Vectors Vectors having equal
or unequal magnitudes but acting along the same or Ab parallel lines are called
collinear vectors.
(viii)
Coplanar Vectors Vectors acting in the
same plane are called coplanar vectors.
(ix)
Localised Vector A vector whose initial
point is fixed, is called a localised vector.
(x)
Non-localised or Free Vector A vector
whose initial point is not fixed is called a nonlocalised or a free vector.
(xi)
Position Vector A vector representing
the straight line distance and the direction of any point or object with
respect to the origin, is called position vector.
Addition of Vectors 1. Triangle Law of Vectors
If two vectors acting at a point are
represented in magnitude and direction by the two sides of a triangle taken in
one order, then their resultant is represented by the third side of the
triangle taken in the opposite order.
If two vectors A and B acting at a point
are inclined at an angle θ, then their resultant
R = √A2 + B2 + 2AB
cos θ
If the resultant
vector R subtends an angle β with vector A, then tan β = B sin θ / A + B cos θ
2. Parallelogram Law of Vectors
If two vectors
acting at a point are represented in magnitude and direction by the two
adjacent sides of a parallelogram draw from a point, then their resultant is
represented in magnitude and direction by the diagonal of the parallelogram
draw from the same point.
Resultant of vectors A and B is given by
√A2 +
B2 + 2AB cos θ
If the resultant
vector R subtends an angle β with vector A, then tan β = B sin θ / A + B cos θ
Polygon Law of Vectors
It states that
if number of vectors acting on a particle at a time are represented in
magnitude and – direction by the various sides of an open polygon taken in same
order, their resultant vector E is represented in magnitude and direction by
the closing side of polygon taken in opposite order. In fact, polygon law of
vectors is the outcome of triangle law of vectors.
R = A + B + C +
D + E
OE = OA + AB +
BC + CD + DE
Properties of Vector Addition
(i)
Vector addition is commutative,
i.e., A + B = B + A
(ii) Vector addition is associative, i.e.,
A +(B + C)= B + (C + A)= C + (A + B)
(iii) Vector
addition is distributive, i.e., m (A +
B) = m A + m B
Rotation of a Vector
(i)
If a vector is rotated through
an angle 0, which is not an integral multiple of 2 π, the vector changes.
(ii)
If the frame of reference is
rotated or translated, the given vector does not change. The components of the
vector may, however, change.
Resolution of a Vector into Rectangular Components
If any vector A subtends an angle θ with
x-axis, then its
Horizontal component Ax = A cos
θ
Vertical
component Ay = A sin θ
Magnitude of vector A = √Ax2
+ Ay2
tan θ = Ay
/ Ax
Direction Cosines of a Vector
If any vector A
subtend angles α, β and γ with x – axis, y – axis and z – axis respectively and
its components along these axes are Ax, Ay and Az,
then cos α= Ax / A, cos β = Ay / A, cos γ = Az
/ A and cos2 α + cos2 β + cos2 γ = 1
Subtraction of Vectors
Subtraction of a
vector B from a vector A is defined as the addition of vector -B (negative of
vector B) to vector A
Thus, A
– B = A + (-B) Multiplication of a
Vector 1. By a Real Number
When a vector A
is multiplied by a real number n, then its magnitude becomes n times but
direction and unit remains unchanged.
2. By a Scalar
When a vector A
is multiplied by a scalar S, then its magnitude becomes S times, and unit is
the product of units of A and S but direction remains same as that of vector A.
Scalar or Dot Product of Two Vectors
The scalar product of two vectors is equal
to the product of their magnitudes and the cosine of the smaller angle between
them. It is denoted by . (dot).
A * B = AB cos θ
The scalar or
dot product of two vectors is a scalar.
Properties of Scalar Product
(i)
Scalar product is commutative,
i.e., A * B= B * A
(ii)
Scalar product is distributive,
i.e., A * (B + C) = A * B + A * C
(iii) Scalar product of two perpendicular vectors is zero.
A * B = AB cos 90° = O
(iv)
Scalar product of two parallel
vectors is equal to the product of their magnitudes, i.e., A * B = AB cos 0° = AB
(v) Scalar product of a vector with itself is equal to the square of its
magnitude, i.e.,
A *
A = AA cos 0° = A2
(vi)
Scalar product of orthogonal
unit vectors
and
(vii) Scalar product in cartesian coordinates
= AxBx
+ AyBy + AzBz
Vector or Cross Product of Two Vectors
The vector
product of two vectors is equal to the product of their magnitudes and the sine
of the smaller angle between them. It is denoted by * (cross).
A * B = AB sin θ
n
The direction of
unit vector n can be obtained from right hand thumb rule.
If fingers of
right hand are curled from A to B through smaller angle between them, then
thumb will represent the direction of vector (A * B).
The vector or
cross product of two vectors is also a vector.
Properties of Vector Product
(i) Vector product is not commutative,
i.e.,
A * B ≠ B * A [ (A * B) = — (B * A)]
(ii) Vector
product is distributive, i.e.,
A * (B + C) = A * B + A * C
(iii) Vector
product of two parallel vectors is zero, i.e.,
A * B = AB sin O° = 0
(iv) Vector product
of any vector with itself is zero.
A * A = AA sin O° = 0
(v)
Vector product of orthogonal
unit vectors
(vi) Vector product in cartesian coordinates
Direction of Vector Cross Product
When C = A * B,
the direction of C is at right angles to the plane containing the vectors A and
B. The direction is determined by the right hand screw rule and right hand
thumb rule.
(i)
Right Hand Screw Rule Rotate a right
handed screw from first vector (A) towards second vector (B). The direction in
which the right handed screw moves gives the direction of vector (C).
(ii)
Right Hand Thumb Rule Curl the fingers
of your right hand from A to B. Then, the direction of the erect thumb will
point in the direction of A * B.
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