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Multiplication of Vectors
Multiplication of vector by a scalar
Multiplication of a vector by a vector
Dot product or scalar product
Some examples of dot product
Characteristics of dot product
Cross product or vector product
Characteristics of cross product
Related Resources
The vectors cannot be added algebraically, like scalar quantities. The angle between the two vectors plays an important role in addition of vectors. Similarly, angle between two vectors plays an important role when the two vectors are multiplied together.
Consider two quantities having magnitudes ‘3’ and ‘2’ respectively. If the quantities are scalars then then the result of their multiplication cannot be anything other than 6. We are at liberty to express these product as 3.2 = 6 or 32 = 6. If the two quantities are vector quantities, the result of their multiplication can have any value lying in between -6 and +6. The result depends upon the angle between them. Moreover we are not free to put a dot (.) or a cross () in between.
The dot product of two vectors a and b is defined as the product of their magnitudes and the cosine of the smaller angle between the two.
It is written by putting a dot (.) between two vectors. The result of this product does not possess any direction. So, it is a scalar quantity. Hence it is also called a scalar product. = || || cosθ = ab cosθ
where a and b are the magnitudes of the respective vectors and θ is the angle between them. The final product is a scalar quantity. If two vectors are mutually perpendicular then θ = 900 and cos 90 = 0, Hence, their dot product is zero.
Work (W):- Work is the dot product of the force () and displacement (). Work is scalar quantity.
W = = Fs cosθ
Power (P):- Power is the dot product of force () nad velocity (). Power is also a scalar quantity.
Dot product of two vectors obeys the following characteristics. (i) Commutative:-
Dot product between two vectors is commutative in nature.
If and are two vectors,
then,
It means that the order of vectors in the product can be changed without affecting the result.
(ii) Distributive:-
Dot product of vector with the sum of a number of other vectors is equal to the sum of the dot products of the vector taken with other vectors separately.
Mathematically, it can be expressed as
(iii) Dot product in terms of rectangular component:-
Here,
(iv) Perpendicular vectors:-
For two perpendicular vectors and , θ = 90º.
= || || cosθ = ab cos 90º = 0
Thus, the product of two non-zero vectors, which are perpendicular to each other, is always zero. This statement is known as condition of perpendicularity.
(v) Collinear vectors:-
(a) Parallel vectors:-
In this case, θ = 0º,
So, = || || cosθ = ab cos 0º = ab
(b) Anti-parallel vectors:-
In this case, θ = 180º,
= || || cosθ = ab cos 180º = -ab
Therefore, the dot product of collinear vectors is equal to the product of their magnitudes. It is positive if they are parallel and negative if they are anti-parallel. The statement is called condition of co-linearity.
(vi) Equal vectors:-
Vectors are equal if they possess same magnitude and direction, i.e.,
θ = 0º
So dot product of two equal vectors is given by
= |a||a| cos0º = A^{2}
Dot product two equal vectors is equal to the square of the magnitude of the either.
Thus,
Problem 1:-
Find the angle between the vectors A and B where
= 2i + 3j + 3k, = i + 2j - 3k
Solution : We know that,
= |A||B| cosθ where |A| = √(22 +32 + 32) = √22
|B| =√(12 + 22+ 32) = √14
Hence cosθ = ()/(|A||B|)
=((2i +3j +3k )(i +2j -3k ))/(√22×√14) =(2+6-9)/(2√77)=(-1)/(2√77)
Cross product of two vectors and is defined as a single vector whose magnitude is equal to the product of their individual magnitudes and the sine of the smaller angle between them and is directed along tyhe normal to the plane containing and .
x = = ab sinθ
Here is the unit vector in a direction perpendicular to the plane containing and .
Cross product of two vectors being a vector quantity is also known as vector product. To specify the sense of the vector c, refer to the figure given below.
Imagine rotating a right hand screw whose axis is perpendicular to the plane formed by vectors a and b so as to turn it from vectors a to b trough the angle θ between them. Then the direction of advancement of the screw gives the direction of the vector product vectors and .
Examples:-
Angular momentum ():- Angular momentum () which is the cross product of position vector () and momentum () is also a vector quantity.
= x
(i) Not commutative:-
Cross product of two vectors is not commutative in nature.
Cross product of a vector with the sum of a number of vectors is equal to the sum of cross product of the vector taken with other vectors separately.
So,
(iii) Collinear vectors:-
In this case, θ = 0º, sinθ = 0,
So, = a null vector
In this case, θ =180º, sinθ = 0
Thus, again, = a null vector
Therefore, cross product of two linear vectors is always a null vector (zero magnitude). The statement is known as condition of co-linearity.
(iv) Equal vectors:-
For equal vectors,
θ = 0º, sinθ = 0,
(v) Perpendicular vectors:-
For perpendicular vectors,
θ = 90º, sinθ = 1,
So, = ab
The magnitude of the cross product of two perpendicular vectors is equal to the product of their individual magnitudes.
(vi) Orthogonal unit vectors:-
In case of orthogonal unit vectors,
Problem 2:-
Obtain a unit vector perpendicular to the two vectors = 2i + 3j + 3k, = i – 2j + 3k
Solution:-
We know that = AB sin θ n n = ()/ABsinθ We have = 17i-2j-7k A = √29 B = √14 and θ = cos-1 8/(√14 √29) (Use concept of dot product to find θ). From the above values we can find n Solving we get,
n = (17i-2j-7k)/(√29 √14 sinθ) where θ = cos-1 8/(√14 √29)
The result of dot product of two vectors is a scalar quantity.
Dot product of the perpendicular vectors is always zero.
Dot product of two similar vectors is equal to the square of the magnitude of either of them.
Change in the order of vectors does not produce a change in the result of dot product.
Cross product of two vectors is a vector quantity.
Change in the order of vectors, in a cross product, results in the introduction of a negative sign.
Direction of cross product is always perpendicular to the plane containing two vectors.
While applying right thumb rule, for cross product, it must be ensured that both the vectors are directed away or directed towards the point of intersection.
Cross product of two linear vectors is always a null vector
Area of the parallelogram formed by the two vectors is equal to the magnitude of their cross-product.
Question 1:-
If x = , then:
(a) is perpendicular to only.
(b) is parallel to .
(c) is perpendicular to both and .
(d) is parallel to both and .
Question 2:-
Which of the following is correct?
(a)
(b)
(c)
(d)
Question 3:-
The angle between the two vectors and will be:
(a) zero (b) 45^{0 }(c) 90^{0 }(d) 180^{0}
Question 4:-
If || = || = || then angle between A and B will be:
(a) 90^{0 } (b) 120^{0} (c) 0^{0 }(d) 60^{0}
Question 5:-
A boat which has a speed of 5 km/h in still water crosses a river of width 1 km along the shortest possible path in 15 minutes. The velocity of the river was in km/h is
(a) 1 (b) 4 (c) √ 21 (d) 3
c
d
b
You might like to refer Scalar and Vectors.
For getting an idea of the type of questions asked, refer the Previous Year Question Papers.
Click here to refer the most Useful Books of Physics.
To read more, Buy study materials of Units & Dimensions comprising study notes, revision notes, video lectures, previous year solved questions etc. Also browse for more study materials on Physics here.
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