Paragraph: A frame of reference that is accelerated with respect to an inertial frame of reference is called…

Paragraph: A frame of reference that is accelerated with respect to an inertial frame of reference is called a non-inertial frame of reference. A coordinate system fixed on a circular disc rotating about a fixed axis with a constant angular velocity $\omega$ is an example of a non-inertial frame of reference. The relationship between the force $\vec{F}_{\text {rot }}$ experienced by a particle of mass $m$ moving on the rotating disc and the force $\vec{F}_{\text {in }}$ experienced by the particle in an inertial frame of reference is $\vec{F}_{\mathrm{rot}}=\vec{F}_{\mathrm{in}}+2 m\left(\vec{v}_{\mathrm{rot}} \times \vec{\omega}\right)+m(\vec{\omega} \times \vec{r}) \times \vec{\omega}$, where $\vec{v}_{\text {rot }}$ is the velocity of the particle in the rotating frame of reference and $\vec{r}$ is the position vector of the particle with respect to the centre of the disc. Now consider a smooth slot along a diameter of a disc of radius $R$ rotating counter-clockwise with a constant angular speed $\omega$ about its vertical axis through its center. We assign a coordinate system with the origin at the center of the disc, the $x$-axis along the slot, the $y$-axis perpendicular to the slot and the $z$-axis along the rotation axis $(\vec{\omega}=\omega \hat{k}) .$ A small block of mass $m$ is gently placed in the slot at $\vec{r}=(R / 2) \hat{i}$ at $t=0$ and is constrained to move only along the slot. Question: The distance $r$ of the block at time $t$ is

1. $\frac{R}{4} \left(e^{2\omega t}+e^{-2\omega t}\right)$
2. $\frac{R}{2}\cos 2\omega t$
3. $\frac{R}{2}\cos \omega t$
4. $\frac{R}{4} \left(e^{\omega t}+e^{-\omega t}\right)$

Solution

Asked in: JEE Advanced 2016 (Paper 2)

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