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Concepts Of Physics MCQ Edition [Volume 1]
Exam
Concepts Of Physics MCQ Edition [Volume 1]
Subject
Physics
Chapter
Rotational Mechanics

Concepts Of Physics MCQ Edition [Volume 1] · Physics · Rotational Mechanics

Particles having masses of 1 g , 2 g , 3 g , ..., 100 g are placed...

Particles having masses of 1 g1 \text{ g}, 2 g2 \text{ g}, 3 g3 \text{ g}, ..., 100 g100 \text{ g} are placed at the marks 1 cm1 \text{ cm}, 2 cm2 \text{ cm}, 3 cm3 \text{ cm}, ..., 100 cm100 \text{ cm} respectively on a metre scale. Determine the moment of inertia of this system of particles about the perpendicular bisector of the metre scale.

Options

  1. A0.43 kg m20.43 \text{ kg m}^2
  2. B4.3 kg m24.3 \text{ kg m}^2
  3. C0.83 kg m20.83 \text{ kg m}^2
  4. D2.55 kg m22.55 \text{ kg m}^2

Correct answer

A. 0.43 kg m20.43 \text{ kg m}^2

Step-by-step solution

The perpendicular bisector of a metre scale passes through its midpoint at 50 cm50 \text{ cm}.

The moment of inertia of the system about this axis is given by:

I=∑i=1100miri2I = \sum_{i=1}^{100} m_i r_i^2

where mi=i gm_i = i \text{ g} and the distance from the axis is ri=∣i−50∣ cmr_i = |i - 50| \text{ cm}.

I=∑i=1100i(i−50)2I = \sum_{i=1}^{100} i (i - 50)^2

Let j=i−50j = i - 50. As ii varies from 11 to 100100, jj varies from −49-49 to 5050.

I=∑j=−4950(j+50)j2=∑j=−4950j3+50∑j=−4950j2I = \sum_{j=-49}^{50} (j + 50) j^2 = \sum_{j=-49}^{50} j^3 + 50 \sum_{j=-49}^{50} j^2

Since j3j^3 is an odd power, the sum of symmetric terms is zero, giving ∑j=−4949j3=0\sum_{j=-49}^{49} j^3 = 0.

∑j=−4950j3=503=125000\sum_{j=-49}^{50} j^3 = 50^3 = 125000

For the second term, using the sum of squares formula ∑j=1nj2=n(n+1)(2n+1)6\sum_{j=1}^{n} j^2 = \dfrac{n(n+1)(2n+1)}{6}:

∑j=−4950j2=502+2∑j=149j2\sum_{j=-49}^{50} j^2 = 50^2 + 2 \sum_{j=1}^{49} j^2

∑j=149j2=49×50×996=40425\sum_{j=1}^{49} j^2 = \dfrac{49 \times 50 \times 99}{6} = 40425

∑j=−4950j2=2500+2(40425)=83350\sum_{j=-49}^{50} j^2 = 2500 + 2(40425) = 83350

Substituting these values back into the equation for II:

I=125000+50(83350)=125000+4167500=4292500 g cm2I = 125000 + 50(83350) = 125000 + 4167500 = 4292500 \text{ g cm}^2

Converting the units to kg m2\text{kg m}^2:

I=4292500×10−3 kg×(10−2 m)2I = 4292500 \times 10^{-3} \text{ kg} \times (10^{-2} \text{ m})^2

I=4292500×10−7 kg m2=0.42925 kg m2I = 4292500 \times 10^{-7} \text{ kg m}^2 = 0.42925 \text{ kg m}^2

Rounding to two decimal places, we get 0.43 kg m20.43 \text{ kg m}^2.

Answer: 0.43 kg m20.43 \text{ kg m}^2
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