A stone on a string 0.80 m long is whirled round a horizontal circle, 150 times each minute…

Step 1. Frequency and period. $f = \dfrac{150}{60\ \text{s}} = 2.5\ \text{Hz}$, so $T = \dfrac{1}{2.5} = 0.40\ \text{s}$.

Step 2. Angular velocity. $\omega = 2\pi \times 2.5\ \text{Hz} = 15.7\ \text{rad/s}$.

Step 3. Speed. $v = 15.7\ \text{rad/s} \times 0.80\ \text{m} = 12.6\ \text{m/s}$.

Step 4. Acceleration. $a = \omega^2 r = (15.7\ \text{rad/s})^2 \times 0.80\ \text{m} = 197\ \text{m/s}^2$, towards the centre. That is about $20$ times $g$.