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Question

A circular running track has six lanes, each $1$ $\text{m}$ wide. How far ahead (in metres) should the runner in the outermost lane start from, so as to cover the same distance in one lap as the runner in the innermost lane?

The correct answer is

$10 \pi$

The problem asks for the starting distance difference required for a runner in the outermost lane of a circular track to cover the same distance as a runner in the innermost lane.

Calculating Lane Radius Difference

There are 6 lanes, each 1 meter wide. The difference in the radius of the running path between the outermost lane and the innermost lane is determined by the number of lanes and their width.

  • Number of lanes = 6
  • Width of each lane = $1 \text{ m}$
  • The difference in radius corresponds to the width of the lanes between the innermost and outermost lane. This is $(6 - 1)$ lanes.
  • Radius difference $= (6 - 1) \times 1 \text{ m} = 5 \text{ m}$

Let $r_{in}$ be the radius of the path for the runner in the innermost lane. The radius for the runner in the outermost lane, $r_{out}$, is $r_{out} = r_{in} + 5 \text{ m}$.

Determining Circumference Difference

The distance covered in one lap is the circumference of the circular path. The formula for the circumference ($C$) of a circle is $C = 2 \pi r$.

  • Circumference for the innermost lane: $C_{in} = 2 \pi r_{in}$
  • Circumference for the outermost lane: $C_{out} = 2 \pi r_{out} = 2 \pi (r_{in} + 5 \text{ m})$
  • The difference in distance covered per lap is $C_{out} - C_{in}$.
  • Difference $= 2 \pi (r_{in} + 5) - 2 \pi r_{in}$
  • Difference $= 2 \pi r_{in} + 10 \pi - 2 \pi r_{in}$
  • Difference $= 10 \pi \text{ m}$

Finding the Starting Offset

The runner in the outermost lane covers $10 \pi$ meters more distance than the runner in the innermost lane for each lap completed around the same starting point. To cover the *same* distance, the runner in the outermost lane must start ahead. The distance they need to start ahead is exactly the difference in the distance they cover in a lap.

Therefore, the runner in the outermost lane should start $10 \pi$ meters ahead.

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Important Questions from Mensuration 2D (Notes)

  1. A 2 cm wide wooden strip is to be fixed on a photo of 40 cm x 30 cm size all along its four sides. What is the minimum length of the wooden strip required?
  2. $110$ मी. $\times 60$ मी. घास-आच्छादित आयताकार प्लॉट के अंदर चारों ओर $2$ मी. चौड़ा बजरी का रास्ता बनाना है। $₹2$ प्रति वर्ग मी. की दर से बजरी बिछाने का लागत ज्ञात कीजिए।
  3. The perimeter of a square is $596$ m. Its area (in m$^2$) is:
  4. A hollow spherical shell is made of a metal of density 4 g/cm$^3$. Its internal and external radius are 15 cm and 18 cm, respectively. What is the weight (in kg) of the shell?
    (Use $\pi = \frac{22}{7}$ and Density = $\frac{\text{Mass}}{\text{Volume}}$)
  5. Water flows out through a circular pipe whose internal diameter is 2 cm, at the rate of 4 metres per second into a cylindrical tank, the radius of whose base is 80 cm. By how much will the level of water rise in 16 minutes?
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