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Average velocity | Motion in a plane



1. Introduction

  • In previous chapter we have learned about the motion of any particle along a straight line
  • Straight line motion or rectilinear motion is motion in one dimension.Now in this chapter ,we will consider both motion in two dimension and three dimension.
  • In two dimensional motion path of the particle is constrained to lie in a fixed plane.Example of such motion motion are projectile shot from a gun ,motion of moon around the earth,circular motion and many more.
  • To solve problems of motion in a plane,we need to generalize kinematic language of previous chapter to a more general using vector notations in two and three dimensions.

Difference between Scalar and Vector

Physical quantities may be divided into two categories
(1) Scalars are physical quantities that only have magnitude for example mass, length, time, temperature ,the distance between two points, mass of an object, and the time at which a certain event happened. The rules for combining scalars are the rules of ordinary algebra. Scalars can be added, subtracted, multiplied and divided just as the ordinary numbers
(2)Vectors are physical quantities having both magnitude and direction for example velocity, force, electric field, torque etc. It can be represented by an arrow in space.It obeys the triangle law of addition or equivalently the parallelogram law of addition.

What is vector

  • A quantity that has magnitude as well as direction is called vector.From a geometric point of view, a vector can be defined as a line segment having a specific direction and a specific length
  • It is denoted  by the letter bold letter a  or it can be denoted as a
  • Magnitude of a vector a is denoted by |a| or a.It is a positive quantity
  • It obeys the triangle law of addition or equivalently the parallelogram law of addition
  • Example velocity, force, electric field, torque,acceleration etc

Type Of Vectors

(i) Zero or Null Vector :A vector whose initial and terminal points are coincident is called zero or null vector
(ii) Unit Vector :A vector whose magnitude is unity is called a unit vector which is denoted by n^
(iii) Free Vectors: If the initial point of a vector is not specified, then it is said to be a free vector.
(iv) Negative of a Vector A vector having the same magnitude as that of a given vector a and the direction opposite to that of a is called the negative of a and it is denoted by —a.
(v) Like and Unlike Vectors Vectors are said to be like when they have the same direction and unlike when they have opposite direction.
(vi) Collinear or Parallel Vectors Vectors having the same or parallel supports are called collinear vectors.
(vii) Coinitial Vectors Vectors having same initial point are called coinitial vectors.
(viii) Coplanar Vectors A system of vectors is said to be coplanar, if their supports are parallel to the same plane. Otherwise they are called non-coplanar vectors
(iX) Equal vectors: Two vectors a and b are said to be equal written as a = b, if they have  same magnitude and same direction regardless of the there initial point

Addition Of vector:

Let a and b be any two vectors. From the terminal point of a, vector b is drawn. Then, the vector from the initial point  of a to the terminal point B of b is called the sum of vectors a and b and is denoted by a + b. This is called the triangle law of addition of vectors
c= a + b
Motion in a plane

Properties of Vector addition
(1) Vector addition is commutative i.e. A+B=B+A
(2) Vector addition is associative i.e. A+B+C=(A+B)+C=A+(B+C)=(A+C)+B
(3)  A +0= A
4) A+ (-A)=0

Subtraction of vectors

Let a and b be any two vectors.
Now a-b = a+ (-b)
Motion in a plane
So we will first reverse the direction of vector b and then follow the vector addition process
From the terminal point of a, vector -b is drawn. Then, the vector from the initial point  of a to the terminal point B of -b is called the sum of vectors a and -b and is denoted by a - b.

Another method to find substraction of vectors would be
Let draw vector a and vector b from the same initial point. And then draw the line from end point of vector b to vector a.This will  give a-b
Motion in a plane
 

Scalar multiplication of vectors

When we multiply any vector A with any scalar quantity 'n' then it's direction remains unchanged and magnitude gets multiplied by 'n'. Thus, n(A) = nA
Scalar multiplication of vectors is distributive i.e.,
n(A + B) = nA +nB

Important Properties
(i) |k a| = |k| |a|
(ii) k O = O
(iii) m (-a) = – ma = – (m a)
(iv) (-m) (-a) = m a
(v) m (n a) = mn a = n(m a)
(vi) (m + n)a = m a+ n a
(vii) m (a+b) = m a + m b
 
Important Note :Addition and subtraction of scalars make sense only for quantities with same units. However, you can multiply and divide scalars of different units


Components of the vector

Let a and b be any two non-zero vectors in a plane with different directions and let A be another vector in the same plane A can be expressed as a sum of two vectors ? one obtained by multiplying a by a real number and the other obtained by multiplying b by another real number. To see this, let O and P be the tail and head of the vector A. Then, through O, draw a straight line parallel to a, and through P, a straight line parallel to b. Let them intersect at Q. Then, we have
A = OP = OQ + QP
But since OQ is parallel to a, and QP is parallel to b, we can write :
OQ = ? a, and QP = ? b ( where ? and ? are real numbers.)
Therefore, A = ? a + ? b
Motion in a plane
We say that A has been resolved into two component vectors ? a and ? b along a and b respectively.
Similarly We can  represent any vector in rectangular components form. Let us assume an xyz coordinate plane and unit vector i,j andk are defined across x,y,z respectively
Then we can represent any vector in the components forms like
r= xi+yj+ zk
Important take aways
  • x,y and z are scalar components of vector r
  • xi,yj,zk are called the vector components
  • x,y,z are termed as rectagular components
  • Length of vector or magnitude of the vector is defined as
  • Motion in a plane
  • x,y,z are called the direction ratio of vector r
  • In case it is given l,m,n are direction cosines of a vector then
li+mj+nk=(cosθx) i +(cosθy) j+(cosθz) k    is the unit vector in the direction of the vector and θx,θy,θz    are the angles which the vector makes with x,y and z axis
Addition,substraction and multiplication,equality in component form can be expressed
In component form addition of two vectors is
C=A+B
C = (Ax+ Bx)i + (Ay+ By)j + (Az+ Bz)k
Where, A = (Ax, Ay, Az) and B = (Bx, By, Bz)
Thus in component form resultant vector C becomes,
Cx = Ax+ Bx : Cy = Ay+ By : Cz = Az+ Bz
In component form  substraction of two vectors is
D=A -B
D = (Ax- Bx)i + (Ay- By)j + (Az- Bz)k
where, A = (Ax, Ay, Az) and B = (Bx, By, Bz)
Thus in component form resultant vector D becomes,
Dx = Ax - Bx : Dy = Ay- By : Dz = Az- Bz

Equality of vector in components form
A=B
Axi +Ayj+Azk= Bxi+Byj+Bzk
Ax=Bx
Ay=By
Az=Bz
Multiplication of scalar by vector in components form
A=kB
=K(Bxi+Byj+Bzk)
=(kBx)i + (kBy)j+ (kBz)k
Position vector
In the coordinate system, the line joining the origin O to the point P in the system is called the position vector of Point P
OP= xi+yj+zk
 
Vector joining two points in the Coordinate system
Let Point P and Q are there
Postion Vector of Point P
OP= x1 i+y1 j+z1k
Postion Vector of Point Q
OQ= x2 i+y2 j+z2k
Then Vector PQ is defined as
PQ=OQ-OP
=(x2 -x1) i +(y2 -y1) j +(z2 -z1) k
 

2.Average velocity

  • Consider a particle moving along a curved path in x-y plane shown below in the figue
  • Suppose at any time,particle is at the point P and after some time 't' is at point Q where points P and Q represents the position of particle at two different points.


    Displacement vector in xy coordinate system

  • Position of particle at point P is described by the Position vector r from origin O to P given by
    r=xi+yj
    where x and y are components of r along x and y axis
  • As particle moves from P to Q,its displacement would be would be Δr which is equal to the difference in position vectors r and r'.Thus
    Δr = r'-r = (x'i+y'j)-(xi+yj) = (x'-x)i+(y'-y)j = Δxi+Δyj                                          (1)
    where Δx=(x'-x) and Δy=(y'-y)
  • If Δt is the time interval during which the particle moves from point P to Q along the curved path then average velocity(vavg) of particle is the ratio of displacement and corresponding time interval

    average velocity equation in two-dimension
    since vavgr/Δt , the direction of average velocity is same as that of Δr
  • Magnitude of Δr is always the straight line distance from P to Q regardless of any shape of actual path taken by the particle.
  • Hence average velocity of particle from point P to Q in time interval Δt would be same for any path taken by the particle.





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