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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 (8-Mar-2021) (Shift 2)
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.

Important Questions from Mensuration

  1. The areas of three adjacent faces of a cuboidal tank are 3 m 2, 12 m 2 and 16 m 2. the capacity of the tank, in litres, is:

  2. Volume of a cuboid is 4800 cm 3. If the height of this cuboid is 20 cm, then what will be the area of the base of cuboid ?

  3. Two similar cubes have heights of 8 cm and 12 cm, respectively. If the capacity of the smaller cube is 80 cm 3, what is the capacity of the bigger cube (in cm 3)?

  4. Three circles of radius 7 cm are kept touching each other. The string is tightly tied around these three circles. What is the length of the string?

  5. Three circles of radius 6 cm are kept touching each other. The string is tightly tied around these three circles. What is the length of the string?

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