Juku found a hemispherical rock sticking up from the ground. Measuring its circumference with a tape he got a result of a=\SI2,4m. Next he took a match box and started to slowly move it down from top of the hemisphere, until the box finally slid off the rock. Measuring along the hemisphere with a tape, Juku found that the distance from the tip of the rock to the point where the box slid off was b=\SI20cm. What was the coefficient of friction between the rock and the match box?