The crank of a slider-crank mechanism rotates counter-clockwise (CCW) with a constant angular velocity $\omega$, as shown. Assume the length of the crank to be $r$.
Using exact analysis, the acceleration of the slider in the $y$-direction, at the instant shown, where the crank is parallel to $x$-axis, is given by
1. $- \omega ^2r$
2. $2 \omega ^2r$
3. $\omega ^2r$
4. $-2 \omega ^2r$