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Impulse and Momentum question

2012 · Shift 1 · Q59
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Impulse and Momentum question

2012 · Shift 1 · Q59

JEE AdvancedPhysicsImpulse and MomentumNumerical+4 / −1
A lamina is made by removing a small disc of diameter 2R from a bigger disc of uniform mass density and radius 2R, as shown in the figure. The moment of inertia of this lamina about axes passing through O and P is IO and IP respectively. Both these axes are perpendicular to the plane of the lamina. The ratio IO/IP to the nearest integer is ‾\underline{\hspace{2cm}}​. IIT-JEE 2012 Paper 1 Offline Physics - Impulse & Momentum Question 9 English
Numerical answer
View written solutionFree

Correct answer: 1

  1. Interpret the figure and geometry

A bigger disc has radius 2R2R2R. A small disc of diameter 2R2R2R is removed, so its radius is RRR.

From the standard shown configuration, the small disc is cut out internally and tangent to the outer boundary, with centers on the same line. Hence:

  • center of big disc =O= O=O
  • center of small removed disc =P= P=P
  • distance between centers: OP=2R−R=ROP = 2R - R = ROP=2R−R=R

We need the moments of inertia of the remaining lamina about axes through OOO and PPP, perpendicular to the plane.


  1. Take areal mass density as σ\sigmaσ

Since the discs are uniform,

  • Mass of big disc: MB=σπ(2R)2=4σπR2M_B = \sigma \pi (2R)^2 = 4\sigma \pi R^2MB​=σπ(2R)2=4σπR2

  • Mass of removed small disc: Ms=σπR2M_s = \sigma \pi R^2Ms​=σπR2


  1. Moment of inertia about axis through OOO

For the remaining lamina, IO=IO(big disc)−IO(small removed disc)I_O = I_O(\text{big disc}) - I_O(\text{small removed disc})IO​=IO​(big disc)−IO​(small removed disc)

(a) Big disc about its own center OOO

IO(big)=12MB(2R)2I_O(\text{big}) = \frac12 M_B (2R)^2IO​(big)=21​MB​(2R)2 =12⋅4σπR2⋅4R2=8σπR4= \frac12 \cdot 4\sigma\pi R^2 \cdot 4R^2 = 8\sigma\pi R^4=21​⋅4σπR2⋅4R2=8σπR4

(b) Small disc about OOO

Its own central MOI is IP(small)=12MsR2=12σπR4I_{P}(\text{small}) = \frac12 M_s R^2 = \frac12 \sigma\pi R^4IP​(small)=21​Ms​R2=21​σπR4

Using parallel axis theorem to shift from PPP to OOO: IO(small)=IP(small)+Ms(OP)2I_O(\text{small}) = I_P(\text{small}) + M_s(OP)^2IO​(small)=IP​(small)+Ms​(OP)2 =12σπR4+σπR2⋅R2= \frac12 \sigma\pi R^4 + \sigma\pi R^2 \cdot R^2=21​σπR4+σπR2⋅R2 =32σπR4= \frac32 \sigma\pi R^4=23​σπR4

Therefore, IO=8σπR4−32σπR4=132σπR4I_O = 8\sigma\pi R^4 - \frac32\sigma\pi R^4 = \frac{13}{2}\sigma\pi R^4IO​=8σπR4−23​σπR4=213​σπR4


  1. Moment of inertia about axis through PPP

Similarly, IP=IP(big disc)−IP(small disc)I_P = I_P(\text{big disc}) - I_P(\text{small disc})IP​=IP​(big disc)−IP​(small disc)

(a) Big disc about PPP

First about OOO: IO(big)=8σπR4I_O(\text{big}) = 8\sigma\pi R^4IO​(big)=8σπR4

Shift to parallel axis through PPP: IP(big)=IO(big)+MB(OP)2I_P(\text{big}) = I_O(\text{big}) + M_B(OP)^2IP​(big)=IO​(big)+MB​(OP)2 =8σπR4+4σπR2⋅R2= 8\sigma\pi R^4 + 4\sigma\pi R^2\cdot R^2=8σπR4+4σπR2⋅R2 =12σπR4= 12\sigma\pi R^4=12σπR4

(b) Small disc about its own center PPP

IP(small)=12σπR4I_P(\text{small}) = \frac12 \sigma\pi R^4IP​(small)=21​σπR4

Hence, IP=12σπR4−12σπR4=232σπR4I_P = 12\sigma\pi R^4 - \frac12\sigma\pi R^4 = \frac{23}{2}\sigma\pi R^4IP​=12σπR4−21​σπR4=223​σπR4


  1. Compute the ratio

IOIP=132σπR4232σπR4=1323≈0.565\frac{I_O}{I_P} = \frac{\frac{13}{2}\sigma\pi R^4}{\frac{23}{2}\sigma\pi R^4} = \frac{13}{23} \approx 0.565IP​IO​​=223​σπR4213​σπR4​=2313​≈0.565

The question asks for the ratio IO/IPI_O/I_PIO​/IP​ to the nearest integer: 1323≈1?\frac{13}{23} \approx 1 \text{?}2313​≈1? Actually, 0.5650.5650.565 rounds to 111.

So the required nearest integer is 1\boxed{1}1​


  1. Comparison with stored answer

Stored correct answer: 333

My derived answer is 111, so I do not agree with the stored answer.

The stored answer may correspond to a different interpretation, such as asking for IP/IOI_P/I_OIP​/IO​ or a different geometry. For the standard tangent-inside configuration with big radius 2R2R2R and removed disc radius RRR, we get: IOIP=1323\frac{I_O}{I_P} = \frac{13}{23}IP​IO​​=2313​ whose nearest integer is 111.

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