A water wheel with diameter 3 m is used to measure the approximate speed of the water in a river. If the angular velocity of the wheel is 15 rpm, what is the speed of the river, in kilometres per hour?
linear velocity = circumferential distance*number of revolutions per minute so:
linear velocity = 15*pi*3 metres/minute = (15*pi*3/1000)*60 km/hour
You can crunch the numbers.