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Question

What is the area of the region swept by the minute hand $6\text{ cm}$ long, of a wall clock, in an interval of 5 minutes?

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper PDF (31-Jan-2021) (Shift 1)
The correct answer is
$9.43\text{ cm}^2$

Calculating Sector Area Swept by Clock Hand

The problem asks for the area swept by the minute hand of a wall clock over a specific time interval. The minute hand's length acts as the radius of the sector it sweeps.

Key Information Extraction

  • Length of the minute hand (Radius, $r$): $6\text{ cm}$
  • Time interval: 5 minutes

Determining the Angle Swept

A clock face represents a full circle ($360^\circ$). The minute hand completes a full circle ($360^\circ$) in 60 minutes.

  • Angle swept per minute = $\frac{360^\circ}{60} = 6^\circ$ per minute.
  • Angle swept in 5 minutes = $5 \times 6^\circ = 30^\circ$.

To use the sector area formula, convert the angle to radians:

  • Angle in radians, $\theta$ = $30^\circ \times \frac{\pi}{180^\circ} = \frac{\pi}{6}$ radians.

Calculating the Area of the Sector

The area of a sector is given by the formula $A = \frac{1}{2} r^2 \theta$, where $r$ is the radius and $\theta$ is the angle in radians.

  • Substitute the known values: $r = 6\text{ cm}$ and $\theta = \frac{\pi}{6}$.
  • Area, $A$ = $\frac{1}{2} \times (6\text{ cm})^2 \times \frac{\pi}{6}$
  • Area, $A$ = $\frac{1}{2} \times 36\text{ cm}^2 \times \frac{\pi}{6}$
  • Area, $A$ = $18\text{ cm}^2 \times \frac{\pi}{6}$
  • Area, $A$ = $3\pi\text{ cm}^2$.

Numerical Calculation and Final Answer

Using the approximate value of $\pi \approx 3.14159$:

  • Area, $A$ $\approx 3 \times 3.14159 \text{ cm}^2$
  • Area, $A$ $\approx 9.42477 \text{ cm}^2$.

Rounding to two decimal places gives $9.42\text{ cm}^2$. Comparing this to the options, $9.43\text{ cm}^2$ is the closest value.

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Similar Questions

  1. A circle, made of wire, of diameter 49 cm is formed into a rectangle whose sides are in the ratio 7 : 4. Find the area of the rectangle. [Take $\pi = \frac{22}{7}$]
  2. The area of a rectangle with a length of 121 m is the same as that of a square with a side of 44 m. Find the width of the rectangle.
  3. Find the cost of carpeting a 15-m-long and 11-m-wide room with a 75-cm-wide carpet, if the price of the carpet is ₹13 per meter (not considering the cost of labour).
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Important Questions from Mensuration

  1. A solid cube is painted yellow, blue and black such that opposite faces are of same colour. The cube is then cut into 36 cubes of two different sizes such that 32 cubes are small and the other four cubes are Big. None of the faces of the bigger cubes is painted blue. How many cubes have only one face painted?

  2. A and B are two heavy steel blocks. If B is placed on the top of A, the weight increases by 60%. How much weight will reduce with respect to the total weight of A and B, if B is removed from the top of A?

  3. A gardener increased the area of his rectangular garden by increasing its length by 40% and decreasing its width by 20%. The area of the new garden

  4. A village having a population of 4000 requires 150 liters of water per head per day. It has a tank measuring 20 m x 15 m x 6 m. The water of this tank will last for

  5. The centroid of an equilateral triangle ABC is G. If AB is 6 cms, the length of AG is

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