Rotational Motion

JEE Main level, numerical answer, medium

199 of 280

Question 199 of 280

The position vectors of two 1 kg particles, (A) and (B), are given by
r→A=(α1t2i^+α2tj^+α3tk^)m\overrightarrow{\mathrm{r}}_{\mathrm{A}}=\left(\alpha_1 \mathrm{t}^2 \hat{i}+\alpha_2 \mathrm{t} \hat{j}+\alpha_3 \mathrm{t} \hat{k}\right) \mathrm{m} and r→B=(β1ti^+β2t2j^+β3tk^)m\overrightarrow{\mathrm{r}}_{\mathrm{B}}=\left(\beta_1 \mathrm{t} \hat{i}+\beta_2 \mathrm{t}^2 \hat{j}+\beta_3 \mathrm{t} \hat{k}\right) \mathrm{m}, respectively; (α1=1 m/s2,α2=3nm/s,α3=2 m/s,β1=2 m/s,β2=−1 m/s2,β3=4pm/s)\left(\alpha_1=1 \mathrm{~m} / \mathrm{s}^2, \alpha_2=3 \mathrm{n} \mathrm{m} / \mathrm{s}, \alpha_3=2 \mathrm{~m} / \mathrm{s}, \beta_1=2 \mathrm{~m} / \mathrm{s}, \beta_2=-1 \mathrm{~m} / \mathrm{s}^2, \beta_3=4 \mathrm{pm} / \mathrm{s}\right), where t is time, n and p are constants. At t=1 s,∣VA→∣=∣V⃗B∣t=1 \mathrm{~s},\left|\overrightarrow{V_A}\right|=\left|\vec{V}_B\right| and velocities V⃗A\vec{V}_A and V⃗B\vec{V}_B of the particles are orthogonal to each other. At t=1 st=1 \mathrm{~s}, the magnitude of angular momentum of particle (A) with respect to the position of particle (B) is Lkgm2 s−1\sqrt{\mathrm{L}} \mathrm{kgm}^2 \mathrm{~s}^{-1}. The value of L is _______ .

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Answer: 90

At t=1t=1
rAB=−1i^+(3n+1)j^+(2−4p)k^r_{A B}=-1 \hat{i}+(3 n+1) \hat{j}+(2-4 p) \hat{k}

At
t=1t=1
$\begin{aligned}

& v_A=2 \hat{i}+3 n \hat{j}+2 \hat{k} \\

& v_B=2 \hat{i}-2 \hat{j}+4 p \hat{k} \\

& \vec{v}_A-\vec{v}_B=0, \quad 4-6 n+8 p=0

\end{aligned}$

∣vA∣=∣vB∣(3n)2+4=4+16p23n=−4p4+16p=0p=−14,n=13rAB=−i^+2j^+3k^vA=2i^+j^+2k^∴L=m∣r⃗AB×v⃗A∣=90\begin{gathered}\left|v_A\right|=\left|v_B\right| \quad(3 n)^2+4=4+16 p^2 \\ 3 n=-4 p \\ 4+16 p=0 \\ p=-\frac{1}{4}, n=\frac{1}{3} \\ r_{A B}=-\hat{i}+2 \hat{j}+3 \hat{k} \\ v_A=2 \hat{i}+\hat{j}+2 \hat{k} \\ \therefore L=m\left|\vec{r}_{A B} \times \vec{v}_A\right|=90\end{gathered}

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