Question Bank • QB_EM_ADV_046 MCQ • 1 Mark

Section 1: Engineering Mathematics

Vector Calculus

Subtopic: Vector differential operators – del, gradient

Question Statement:

The unit outward normal vector to the level surface $x^2 + y^2 - z = 1$ at the point $(1, 1, 1)$ is:

Options:

  • (A) $\frac{2\hat{i} + 2\hat{j} - \hat{k}}{3}$
  • (B) $\frac{\hat{i} + \hat{j} - \hat{k}}{\sqrt{3}}$
  • (C) $\frac{2\hat{i} + 2\hat{j} + \hat{k}}{3}$
  • (D) $\frac{2\hat{i} - 2\hat{j} - \hat{k}}{3}$

Answer Key:

A

Step-by-Step Solution & Explanation

Let the surface be defined as $F(x, y, z) = x^2 + y^2 - z - 1 = 0$. The normal vector is given by the gradient: $\nabla F = 2x \hat{i} + 2y \hat{j} - \hat{k}$ At the point $(1, 1, 1)$: $\nabla F = 2(1)\hat{i} + 2(1)\hat{j} - \hat{k} = 2\hat{i} + 2\hat{j} - \hat{k}$ Magnitude of the normal vector: $|\nabla F| = \sqrt{2^2 + 2^2 + (-1)^2} = \sqrt{4 + 4 + 1} = \sqrt{9} = 3$ Unit normal vector: $\hat{n} = \frac{\nabla F}{|\nabla F|} = \frac{2\hat{i} + 2\hat{j} - \hat{k}}{3}$
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