Physics Circle home Student Login

Home›Chapters›Rotational Motion PYQs›Angular Momentum & its Conservation›Question 21

JEE Main 2025 (22 Jan, Shift 1) · Angular Momentum & its Conservation

Angular Momentum & its Conservation question from JEE Main 2025 (22 Jan, Shift 1)

Q21JEE Main 2025 · 22 JAN · SHIFT 1Difficult

Angular Momentum & its Conservation

The position vectors of two 1 kg particles, (A) and (B), are given by

r⃗A=(α1t2i^+α2tj^+α3tk^) m\vec{r}_A = \left(\alpha_1 t^2\hat{i} + \alpha_2 t\hat{j} + \alpha_3 t\hat{k}\right)\text{ m} andr⃗B=(β1ti^+β2t2j^+β3tk^) m\vec{r}_B = \left(\beta_1 t\hat{i} + \beta_2 t^2\hat{j} + \beta_3 t\hat{k}\right)\text{ m}, respectively;

(α1=1 m/s2,α2=3n m/s,α3=2 m/s,β1=2 m/s,β2=−1 m/s2,β3=4p m/s)\left(\alpha_1 = 1\text{ m/s}^2, \alpha_2 = 3n\text{ m/s}, \alpha_3 = 2\text{ m/s}, \beta_1 = 2\text{ m/s}, \beta_2 = -1\text{ m/s}^2, \beta_3 = 4p\text{ m/s}\right), where t is time, n and pp are constants. At Figure and velocities V⃗A\vec{V}_A and V⃗B\vec{V}_B of the particles are orthogonal to each other. At t=1 st = 1\text{ s}, the magnitude of angular momentum of particle (A) with respect to the position of particle (B) is L kg m2 s−1\sqrt{L}\text{ kg m}^{2}\text{ s}^{-1}. The value of L is _________.

Numerical answer type. Work it out, then open the answer.

Show answer and solution

Answer: 90

  1. V⃗A=2ti^+3nj^+2k^\vec{V}_A = 2t\hat{i} + 3n\hat{j} + 2\hat{k}, V⃗B=2i^−2tj^+4pk^\vec{V}_B = 2\hat{i} - 2t\hat{j} + 4p\hat{k}; at t=1t = 1 s: V⃗A=(2,3n,2)\vec{V}_A = (2, 3n, 2), V⃗B=(2,−2,4p)\vec{V}_B = (2, -2, 4p).
  2. Equal magnitudes: 9n2=16p29n^2 = 16p^2. Orthogonal: 4−6n+8p=04 - 6n + 8p = 0. Only 3n=−4p3n = -4p works, giving p=−14p = -\frac{1}{4}, n=13n = \frac{1}{3}.
  3. r⃗A=(1,1,2)\vec{r}_A = (1, 1, 2), r⃗B=(2,−1,−1)\vec{r}_B = (2, -1, -1), so r⃗A/B=(−1,2,3)\vec{r}_{A/B} = (-1, 2, 3) and p⃗A=(2,1,2)\vec{p}_A = (2, 1, 2).
  4. L⃗=r⃗A/B×p⃗A=(1,8,−5)\vec{L} = \vec{r}_{A/B}\times\vec{p}_A = (1, 8, -5), ∣L⃗∣2=1+64+25=90|\vec{L}|^2 = 1 + 64 + 25 = 90
  5. Answer: L=90L = 90
Report a mistake

More questions on Angular Momentum & its Conservation