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JEE Advanced 2016
Paper-1 2016
Multiple correct answers - Select all that apply
Question
The position vector $\vec{r}$ of a particle of mass $m$ is given by the following equation $\vec{r}(t)=a t^3 \hat{i}+\beta t^2 \hat{j}$, where $\alpha=10 / 3 \mathrm{~ms}^{-3}, \beta=5 \mathrm{~ms}^{-2}$ and $\mathrm{m}=0.1 \mathrm{~kg}$. At $\mathrm{t}=1 \mathrm{~s}$, which of the following statement (s) is (are) true about the particle?
Select ALL correct options:
A
The velocity $\vec{v}$ is given by $\vec{v}=(10 \hat{i}+10 \hat{j}) \mathrm{ms}^{-1}$
B
The angular momentum $\vec{L}$ with respect to the origin is given by $\vec{L}=-(5 / 3) \hat{k} \mathrm{~N} \mathrm{~m} \mathrm{~s}$
C
The force $\vec{F}$ is given by $\vec{F}=(\hat{i}+2 \hat{j}) \mathrm{N}$
D
The torque $\vec{r}$ with respect to the origin is given by $\vec{r}=-(20 / 3) \hat{k} \mathrm{Nm}$
✓ Correct! Well done.
✗ Incorrect. Try again or view the solution.
⚠ Partially correct. Some answers are missing.
Solution
$ r(t)=\alpha t^3 \hat{i}+\beta t^2 \hat{j} $
Given, $\alpha=10 / 3 \mathrm{~m} / \mathrm{s}^3, \beta=5 \mathrm{~m} / \mathrm{s}^2, \mathrm{~m}=0.1 \mathrm{~kg}$; at $\mathrm{t}=1 \mathrm{~s}$
$ \begin{aligned} & \vec{v}=\alpha\left(3 t^2\right) \hat{i}+2 \beta t \hat{j} \\ & \vec{v}=(10 \hat{i}+10 \hat{j}) \\ & \frac{d \vec{v}}{d t}=\vec{a}=6 \alpha t \hat{i}+2 \beta \hat{j} \\ & \vec{F}=m \vec{a}=0.1[6 \alpha t \hat{i}+2 \beta \hat{j}] \end{aligned} $
$\begin{aligned} & =2 \hat{\mathrm{i}}+\hat{\mathrm{j}} \\ & \mathrm{m}(\overrightarrow{\mathrm{r}} \times \overrightarrow{\mathrm{v}}) \\ & \overrightarrow{\mathrm{i}}=-\frac{5}{3} \hat{\mathrm{k}} \mathrm{Nms} \\ & \vec{\tau}=\overrightarrow{\mathrm{r}} \times \overrightarrow{\mathrm{F}} \\ & =\left(\alpha t^3 \hat{\mathrm{i}}+\beta t^2 \hat{\mathrm{j}}\right) \times \mathrm{m}(6 \alpha t \hat{\mathrm{i}}+2 \beta t \hat{\mathrm{j}}) \\ & =\mathrm{m}\left(2 \alpha \beta t^3 \hat{\mathrm{k}}-6 \alpha \beta t^3 \hat{\mathrm{k}}\right) \\ & =\frac{-20}{3} \hat{\mathrm{k}} \mathrm{Nm}\end{aligned}$
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