Simple harmonic motion question about amplitude

by TheWhizzz   Last Updated September 11, 2019 18:20 PM - source

I am stuck on a question which describes that there is a particle $P$ of mass 0.6kg (which is not so relevant) attached to an elastic spring of natural length 2.5m. The other end of the spring is attached to a fixed point $A$ on the smooth horizontal plane on which $P$ lies. The particle is held at point $B$ where AB=4m and released from rest. The motion of the particle can be described using the equation $f''(x)=-\frac{50}{3}x$ where $x$m is the displacement of $P $ from $A$. The question asks for the amplitude and period of the subsequent motion. How can the simple harmonic motion be taking place about centre $A$ and not the point 2.5m away from $A$ of the natural length of the spring especially since the amplitude is given as 1.5m in the answers?

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Updated July 27, 2020 16:20 PM