A force of 78 grams (weight) acts at the point (2, 3, 5), the direction ratios of its line of action being 2, 2, 1. The magnitude of its moment about the line joining the origin to the point (12, 3, 4) is

Answer (Detailed Solution Below)View answer ▾

Answer: Option B — 136

Detailed Solution

The unit vector along (2, 2, 1) is $\frac{1}{3} (2, 2, 1)$ , so $\hat{F} = 26 (2, 2, 1) = (52, 52, 26)$ .

Moment about the origin: $\overset{r}{^} \times \hat{F} = (2, 3, 5) \times (52, 52, 26) = (- 182, 208, - 52)$ .

Moment about the line = component along $\overset{n}{^} = \frac{( 12 , 3 , 4 )}{13}$ : $\frac{- 2184 + 624 - 208}{13} = \frac{- 1768}{13} = - 136$ , so the magnitude is 136.

✓ Check

∣ (12, 3, 4) ∣ = 144 + 9 + 16 = 13

and

1768 = 13 \times 136

⚠ Common trap

(D) 0 would need the line of action to meet or be parallel to the axis; here

\overset{r}{^} \times \hat{F}

has a non-zero component along it.

Quick tip

Moment about a line through O $= \frac{∣ ( r \times F ) \cdot L ∣}{∣ L ∣}$

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Vectors — Preparation

This question tests your understanding of Vectors, a key topic in the Quantitative Aptitude section. The detailed solution above uses a step-by-step approach to show the reasoning, and where available, a shortcut that helps you solve similar questions in under 90 seconds during the actual exam.

A vector is a quantity with both magnitude and direction, such as displacement or force. This topic covers types of vectors (zero, unit, collinear, coplanar and position vectors), vector addition, multiplication by a scalar, the section formula, and the components of a vector along the i, j and k axes. It also covers the scalar (dot) product and its use in finding the angle between two vectors and the projection of one vector on another, the vector (cross) product and its use in finding areas of parallelograms and triangles, and the scalar triple product for the volume of a parallelepiped and the condition for coplanarity. Remember that two vectors are perpendicular when their dot product is zero, parallel when their cross product is the zero vector, and three vectors are coplanar when their scalar triple product is zero. Practise the vectors questions below, each with a step-by-step solution.

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A force of 78 grams (weight) acts at the point (2, 3, 5), the direction ratios of its line of action being 2, 2, 1. The magnitude of its moment about the line joining the origin to the point (12, 3, 4) is

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