Ever tried to push a door and seen how it opens easily when you push in the right direction? That's dot product in real life. In Class 12 Maths Chapter 10 Exercise 10.3, you will find out how the angle between the vectors determines the outcome a useful concept in physics, maths, and even basic motion.
Class 12 Maths chapter 10 Exercise 10.3 solutions of NCERT are simplified and easy to understand, with step-by-step explanations for every question. Practising these will help you grasp the dot product more clearly and give you confidence during exams. You can find all NCERT solution for Class 12 chapter 10 exercises combined from the link below for easy revision.
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Get simple and clear NCERT solutions for Class 12 Maths Chapter 10 Exercise 10.3 to understand dot product of vectors in an easy way.
Answer:
Given
$\left | \vec a \right |=\sqrt{3}$
$\left | \vec b \right |=2$
$\vec a . \vec b = \sqrt 6$
As we know
$\vec a . \vec b = \left | \vec a \right |\left | \vec b \right |cos\theta$
where $\theta$ is the angle between two vectors
So,
$cos\theta =\frac{\vec a.\vec b}{\left | \vec a \right |\left | \vec b \right |}=\frac{\sqrt{6}}{\sqrt{3}*2}=\frac{1}{\sqrt{2}}$
$\theta=\frac{\pi}{4}$
Hence the angle between the vectors is $\frac{\pi}{4}$ .
Answer:
Given two vectors
$\vec a=\hat i - 2 \hat j + 3 \hat k \: \:and \: \: \vec b=3 \hat i - 2 \hat j + \hat k$
Now As we know,
The angle between two vectors $\vec a$ and $\vec b$ is given by
$\theta=cos^{-1}\left ( \frac{\vec a.\vec b}{\left | \vec a \right |\left | \vec b \right |}\right )$
Hence the angle between $\vec a=\hat i - 2 \hat j + 3 \hat k \: \:and \: \: \vec b=3 \hat i - 2 \hat j + \hat k$
$\theta=cos^{-1}\left ( \frac{(\hat i-2\hat j+3\hat k).(3\hat i-2\hat j+\hat k)}{\left | \hat i-2\hat j+3\hat k \right |\left |3\hat i-2\hat j+\hat k \right |}\right )$
$\theta=cos^{-1}\left ( \frac{3+4+3}{\sqrt{1^2+(-2)^2+3^3}\sqrt{3^2+(-2)^2+1^2}} \right )$
$\theta=cos^{-1}\frac{10}{14}$
$\theta=cos^{-1}\frac{5}{7}$
Question 3: Find the projection of the vector $\hat i - \hat j$ on the vector $\hat i + \hat j$
Answer:
Let
$\vec a=\hat i - \hat j$
$\vec b=\hat i + \hat j$
Projection of vector $\vec a$ on $\vec b$
$\frac{\vec a.\vec b}{\left | \vec b \right |}=\frac{(\hat i-\hat j)(\hat i+\hat j)}{\left |\hat i+\hat j \right |}=\frac{1-1}{\sqrt{2}}=0$
Hence, Projection of vector $\vec a$ on $\vec b$ is 0.
Question 4: Find the projection of the vector $\hat i + 3 \hat j + 7 \hat k$ on the vector $7\hat i - \hat j + 8 \hat k$
Answer:
Let
$\vec a =\hat i + 3 \hat j + 7 \hat k$
$\vec b=7\hat i - \hat j + 8 \hat k$
The projection of $\vec a$ on $\vec b$ is
$\frac{\vec a.\vec b}{\left | \vec b \right |}=\frac{(\hat i+3\hat j+7\hat k)(7\hat i-\hat j+8\hat k)}{\left | 7\hat i-\hat j+8\hat k \right |}=\frac{7-3+56}{\sqrt{7^2+(-1)^2+8^2}}=\frac{60}{\sqrt{114}}$
Hence, projection of vector $\vec a$ on $\vec b$ is
$\frac{60}{\sqrt{114}}$
Answer:
Given
$\\\vec a=\frac{1}{7}( 2 \hat i + 3 \hat j + 6 \hat k ), \\\ \vec b =\frac{1}{7}( 3 \hat i- 6 \hat j + 2 \hat k ),\\\vec c = \frac{1}{7}( 6\hat i + 2 \hat j -3\hat k )$
Now magnitude of $\vec a,\vec b \:and\: \vec c$
$\left | \vec a \right |=\frac{1}{7} \sqrt{2^2+3^2+6^2}=\frac{\sqrt{49}}{7}=1$
$\left | \vec b \right |=\frac{1}{7} \sqrt{3^2+(-6)^2+2^2}=\frac{\sqrt{49}}{7}=1$
$\left | \vec c \right |=\frac{1}{7} \sqrt{6^2+2^2+(-3)^2}=\frac{\sqrt{49}}{7}=1$
Hence, they all are unit vectors.
Now,
$\vec a.\vec b=\frac{1}{7}(2\hat i+3\hat j+6\hat k)\frac{1}{7}(3\hat i-6\hat j+2\hat k)=\frac{1}{49}(6-18+12)=0$
$\vec b.\vec c=\frac{1}{7}(3\hat i-6\hat j+2\hat k)\frac{1}{7}(6\hat i+2\hat j-3\hat k)=\frac{1}{49}(18-12-6)=0$
$\vec c.\vec a=\frac{1}{7}(6\hat i+2\hat j-3\hat k)\frac{1}{7}(2\hat i+3\hat j-6\hat k)=\frac{1}{49}(12+6-18)=0$
Hence all three are mutually perpendicular to each other.
Answer:
Given in the question
$( \vec a + \vec b ). ( \vec a - \vec b )=8$
$\left | \vec a \right |^2-\left | \vec b \right |^2=8$
Since $|\vec a |\: \:= 8 \: \:|\vec b |$
$\left | \vec {8b} \right |^2-\left | \vec b \right |^2=8$
$\left | \vec {63b} \right |^2=8$
$\left | \vec {b} \right |^2=\frac{8}{63}$
$\left | \vec {b} \right |=\sqrt{\frac{8}{63}}$
So, answer of the question is
$\left | \vec {a} \right |=8\left | \vec {b} \right |=8\sqrt{\frac{8}{63}}$
Question 7: Evaluate the product $( 3\vec a - 5 \vec b ). ( 2 \vec a + 7 \vec b )$ .
Answer:
To evaluate the product $( 3\vec a - 5 \vec b ). ( 2 \vec a + 7 \vec b )$
$( 3\vec a - 5 \vec b ). ( 2 \vec a + 7 \vec b )=6\vec a.\vec a+21\vec a.\vec b-10\vec b.\vec a-35\vec b.\vec b$
$=6\vec a.^2+11\vec a.\vec b-35\vec b^2$
$=6\left | \vec a \right |^2+11\vec a.\vec b-35\left | \vec b \right |^2$
Answer:
Given two vectors $\vec a \: \: and \: \: \vec b$
$\left | \vec a \right |=\left | \vec b\right |$
$\vec a.\vec b=\frac{1}{2}$
Now Angle between $\vec a \: \: and \: \: \vec b$
$\theta=60^0$
Now As we know that
$\vec a.\vec b=\left | \vec a \right |\left | \vec b \right |cos\theta$
$\frac{1}{2}=\left | \vec a \right |\left | \vec a \right |cos60^0$
$\left | a \right |^2=1$
Hence, the magnitude of two vectors $\vec a \: \: and \: \: \vec b$
$\left | a \right |=\left | b \right |=1$
Question 9: Find $|\vec x |$ , if for a unit vector $\vec a , ( \vec x -\vec a ) . ( \vec x + \vec a ) = 12$
Answer:
Given in the question that
$( \vec x -\vec a ) . ( \vec x + \vec a ) = 12$
And we need to find $\left | \vec x \right |$
$\left | \vec x \right |^2-\left | \vec a \right |^2 = 12$
$\left | \vec x \right |^2-1 = 12$
$\left | \vec x \right |^2 = 13$
$\left | \vec x \right | = \sqrt{13}$
So the value of $\left | \vec x \right |$ is $\sqrt{13}$
Answer:
Given in the question is
$\vec a = 2 \hat i + 2 \hat j + 3 \hat k , \vec b = - \hat i + 2 \hat j + \hat k \: \: and \: \: \vec c = 3 \hat i + \hat j$
and $\vec a + \lambda \vec b$ is perpendicular to $\vec c$
and we need to find the value of $\lambda$ ,
so the value of $\vec a + \lambda \vec b$ -
$\vec a + \lambda \vec b=2\hat i +2\hat j +3\hat k+\lambda (-\hat i+2\hat j+\hat k)$
$\vec a + \lambda \vec b=(2-\lambda)\hat i +(2+2\lambda)\hat j +(3+\lambda)\hat k$
As $\vec a + \lambda \vec b$ is perpendicular to $\vec c$
$(\vec a + \lambda \vec b).\vec c=0$
$((2-\lambda)\hat i +(2+2\lambda)\hat j +(3+\lambda)\hat k)(3\hat i+\hat j)=0$
$3(2-\lambda)+2+2\lambda=0$
$6-3\lambda+2+2\lambda=0$
$\lambda=8$
the value of $\lambda=8$ ,
Answer:
Given in the question that -
$\vec a \: \: \: and \: \: \vec b$ are two non-zero vectors
According to the question
$\left ( |\vec a | \vec b + |\vec b | \vec a\right )\left (|\vec a | \vec b - |\vec b | \vec a \right )$
$=|\vec a |^2 |\vec b|^2 - |\vec b |^2 |\vec a|^2+|\vec b||\vec a|\vec a.\vec b-|\vec a||\vec b|\vec b.\vec a=0$
Hence $|\vec a | \vec b + |\vec b | \vec a$ is perpendicular to $|\vec a | \vec b - |\vec b | \vec a$ .
Question 12: If $\vec a . \vec a = 0 \: \: and \: \: \vec a . \vec b = 0$ , then what can be concluded about the vector $\vec b$ ?
Answer:
Given in the question
$\\\vec a . \vec a = 0 \\|\vec a|^2=0$
$\\|\vec a|=0$
Therefore $\vec a$ is a zero vector. Hence any vector $\vec b$ will satisfy $\vec a . \vec b = 0$
Answer:
Given in the question
$\vec a , \vec b , \vec c$ are unit vectors $\Rightarrow |\vec a|=|\vec b|=|\vec c|=1$
and $\vec a + \vec b + \vec c = \vec 0$
and we need to find the value of $\vec a . \vec b + \vec b. \vec c + \vec c . \vec a$
$(\vec a + \vec b + \vec c)^2 = \vec 0$
$\vec a^2 + \vec b^2 + \vec c ^2+2(\vec a . \vec b + \vec b. \vec c + \vec c . \vec a)=0$
$|\vec a|^2 + |\vec b|^2 + |\vec c |^2+2(\vec a . \vec b + \vec b. \vec c + \vec c . \vec a)=0$
$1+1+1+2(\vec a . \vec b + \vec b. \vec c + \vec c . \vec a)=0$
$\vec a . \vec b + \vec b. \vec c + \vec c . \vec a=\frac{-3}{2}$
Answer- the value of $\vec a . \vec b + \vec b. \vec c + \vec c . \vec a$ is $\frac{-3}{2}$
Answer:
Let
$\vec a=\hat i-2\hat j +3\hat k$
$\vec b=5\hat i+4\hat j +1\hat k$
we see that
$\vec a.\vec b=(\hat i-2\hat j +3\hat k)(5\hat i+4\hat j +1\hat k)=5-8+3=0$
we now observe that
$|\vec a|=\sqrt{1^2+(-2)^2+3^2}=\sqrt{14}$
$|\vec b|=\sqrt{5^2+4^2+1^2}=\sqrt{42}$
Hence here converse of the given statement is not true.
Answer:
Given points,
A=(1, 2, 3),
B=(–1, 0, 0),
C=(0, 1, 2),
As need to find Angle between $\overline{BA}\: \: and\: \: \overline{BC} ]$
$\vec {BA}=(1-(-1))\hat i+(2-0)\hat j+(3-0)\hat k=2\hat i+2\hat j+3\hat k$
$\vec {BC}=(0-(-1))\hat i+(1-0)\hat j+(2-0)\hat k=\hat i+\hat j+2\hat k$
Hence angle between them ;
$\theta=cos^{-1}(\frac{\vec {BA}.\vec {BC}}{\left | \vec {BA} \right |\left | \vec {BC} \right |})$
$\theta=cos^{-1}\frac{2+2+6}{\sqrt{17}\sqrt{6}}$
$\theta=cos^{-1}\frac{10}{\sqrt{102}}$
Answer - Angle between the vectors $\overline{BA}\: \: and\: \: \overline{BC}$ is $\theta=cos^{-1}\frac{10}{\sqrt{102}}$
Question 16: Show that the points A(1, 2, 7), B(2, 6, 3) and C(3, 10, –1) are collinear.
Answer:
Given in the question
A=(1, 2, 7), B=(2, 6, 3) and C(3, 10, –1)
To show that the points A(1, 2, 7), B(2, 6, 3) and C(3, 10, –1) are collinear
$\vec {AB}=(2-1)\hat i+(6-2)\hat j+(3-7)\hat k$
$\vec {AB}=\hat i+4\hat j-4\hat k$
$\vec {BC}=(3-2)\hat i+(10-6)\hat j+(-1-3)\hat k$
$\vec {BC}=\hat i+4\hat j-4\hat k$
$\vec {AC}=(3-1)\hat i+(10-2)\hat j+(-1-7)\hat k$
$\vec {AC}=2\hat i+8\hat j-8\hat k$
$|\vec {AB}|=\sqrt{1^2+4^2+(-4)^2}=\sqrt{33}$
$|\vec {BC}|=\sqrt{1^2+4^2+(-4)^2}=\sqrt{33}$
$|\vec {AC}|=\sqrt{2^2+8^2+(-8)^2}=2\sqrt{33}$
As we see that
$|\vec {AC}|=|\vec {AB}|+|\vec {BC}|$
Hence point A, B , and C are colinear.
Answer:
Given the position vector of A, B , and C are
$2 \hat i - \hat j + \hat k , \hat i - 3 \hat j - 5 \hat k \: \: and \: \: 3 \hat i - 4 \hat j - 4 \hat k$
To show that the vectors $2 \hat i - \hat j + \hat k , \hat i - 3 \hat j - 5 \hat k \: \: and \: \: 3 \hat i - 4 \hat j - 4 \hat k$ form the vertices of a right angled triangle
$\vec {AB}=(1-2)\hat i + (-3-(-1))\hat j+(-5-1)\hat k=-1\hat i -2\hat j-6\hat k$
$\vec {BC}=(3-1)\hat i + (-4-(-3))\hat j+(-4-(-5))\hat k=-2\hat i -\hat j+\hat k$
$\vec {AC}=(3-2)\hat i + (-4-(-1))\hat j+(-4-(1))\hat k=\hat i -3\hat j-5\hat k$
$|\vec {AB}|=\sqrt{(-1)^2+(-2)^2+(-6)^2}=\sqrt{41}$
$|\vec {BC}|=\sqrt{(-2)^2+(-1)^2+(1)^2}=\sqrt{6}$
$|\vec {AC}|=\sqrt{(1)^2+(-3)^2+(-5)^2}=\sqrt{35}$
Here we see that
$|\vec {AC}|^2+|\vec {BC}|^2=|\vec {AB}|^2$
Hence A,B, and C are the vertices of a right angle triangle.
$\\A ) \lambda = 1 \\\\ B ) \lambda = -1 \\\\ C ) a = |\lambda | \\\\ D ) a = 1 / |\lambda |$
Answer:
Given $\vec a$ is a nonzero vector of magnitude ‘a’ and $\lambda$ a nonzero scalar
$\lambda \vec a$ is a unit vector when
$|\lambda \vec a|=1$
$|\lambda|| \vec a|=1$
$| \vec a|=\frac{1}{|\lambda|}$
Hence the correct option is D.
Product of Two Vectors
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Frequently Asked Questions (FAQs)
The main three topics are the addition of vectors, the dot product of vectors and the cross product of vectors.
The work done is the dot product of force and displacement. The dot product of two vectors is a scalar (real number).
The dot product of force and velocity gives power.
i.k=0 as the angle between them is 90 degrees
Yes, the dot product of two vectors can be either positive, negative or zero based on the angle between them.
There are a total of 5 exercises including miscellaneous.
Yes. For two perpendicular vectors, the dot product is zero.
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