NCERT Solutions
Class 11 Physics
Motion in a Plane

Q. 3.19
î and ĵ are unit vectors along x- and y- axis respectively. What is the magnitude and direction of the vectors î + ĵ and î - ĵ? What are the components of a vector A= 2 î + 3 ĵ along the directions of (î + ĵ) and (î - ĵ)? [You may use graphical method]
- Magnitude of (î + ĵ) = | î + ĵ|
=√ (1)2 + (1)2 = √2
Let the vector (î + ĵ) make an angle θ with the direction of î, then
cos θ = ( (î + ĵ). Î)/ (| î + ĵ|| î|
= (1/√2) = cos 450 or θ = √2
Magnitude of (î - ĵ) = | î - ĵ|
=√ (1)2 + (-1)2 = √2
Similarly, if vector (î - ĵ) makes an angle θ with the direction of î, then
cos θ = ((î - ĵ). Î)/ (| î - ĵ|. Ĵ)
= (1/√2) (1)
= (1/√2)
cos 450 or θ =450
Here θ = -450 with x-axis.
- Given :- A = 2 î + 3 ĵ
Ax î + Ay ĵ = 2 î + 3 ĵ
On Comparing: - Ax = 2 and Ay = 3
| A| =√ (2)2 + (3)2
=√13
Let Ax make an angle θ with the x-axis, as shown in the following figure.
Therefore, tan θ = (Ax/Ay)
θ = tan-1(3/2)
=tan-1(1.5) = 56.310
Angle between the vectors (2 î + 3 ĵ) and (î + ĵ),
θ = (56.310 – 450) = 11.310
Component of vector A, along the direction of P, making and angle θ
= (A cos θ) P
= (A cos11.31) (î + ĵ)/ (√2)
= √13 x (0.9806) (î + ĵ) / (√2)
=2.5 (î + ĵ)
= (25/10) x (√2)
= (5/ (√2))
Let θ be the angle between the vector :( 2 î + 3 ĵ) and (î - ĵ),
θ = (450 +56.310) = 101.310
Component of vector A, along the direction of Q, making and angle θ.
= (A cos θ) Q
= (A cos θ) (î – ĵ)/ (√2)
= (√13) cos (901.310) (î – ĵ)/ (√2)
= (√13)/ (√2) sin (11.300) (î – ĵ)
=-2.550 x 0.196(î – ĵ)
=-0.5(î – ĵ)
=-(5/10) x (√2)
=-(1/ (√2))