# Plane Polar coordinates for simple pendulum in moving lift(elevator) [closed]

Can any one help me with the following.

A simple pendulum is suspended from the ceiling of a lift. It is moving upward with acceleration $a$. The string of the effective length of the pendulum makes angle $\alpha$ with the vertical. Choose the point of suspension as origin, $x$-axis along the horizontal and $y$-axis vertically downwards. Express $x$ and $y$ in terms of plane polar coordinates at time $t$ after the lift takes off

Now I understand it will be

$x = l \sin\alpha$ and $y = l \cos\alpha$ where $l$ is the length of the pendulum string

but it seems correct answer is $y = l \cos\alpha - \frac{1}{2}at^2$

can any one please explain why and how the other factor is included in the equation and why it is not affecting the $x$ and affecting only $y$

## closed as off-topic by Brandon Enright, Jim, John Rennie, jinawee, Valter MorettiApr 17 '14 at 18:27

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