(b) Here, $r = 4m,\ T = 2s,\ a = ?$ $a = r\omega^{2} = r\left( \frac{2\pi}{T} \right)^{2} = \frac{4\pi^{2}r}{T^{2}} = 4\pi^{2} \times \frac{4}{2^{2}} = 4\pi^{2}ms^{- 2}$
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Ph.D. in Physics with 19+ years of teaching experience.