Resultant torque on the waterwheel at Acorn Bank

You'll notice that when you drop coins onto the wheel above, they fall off centre. This is so that they create a resultant torque around the centre. When the coin hits the wheel, the wheel applies a force to decelerate the coin, by Newton's Second Law. In turn, the coin applies an equal and opposite reaction force on the wheel, by Newton's Third Law. This reaction force is off centre, so there is a force x perpendicular distance from the middle - a TORQUE. In rotational terms, Newton's Second Law is written as resultant torque = moment of inertia x angular acceleration. So the wheel starts to turn.

And there was more than one way of having water flow onto the wheel to create a resultant torque. At Acorn Bank, the water came from above but on one wheel it fell down the back of the wheel and on the other wheel down the front. They would spin in opposite directions.