Just before letting go, a hammer thrower swings the ball round a circle of radius $1.8\ \text{m}$ at $25\ \text{m/s}$, as the figure shows. What is the ball's acceleration at that moment?

Fig. 1: View from above: a hammer thrower at the centre swings the ball on a wire 1.8 m long, at 25 m/s along the circle.