The distance around a circle ... often referred to as the circle's "circumference" ... is
C = (π) · (diameter)
But the diameter is (2 · radius), so the circumference is also
C = (2·π) · (radius) .
The 43kg person in the question is going around a circle with radius = 1.5 m, so the distance all around the circle is
C = (2·π) · (radius) = (2·π) · (1.5 m) = 9.42 meters .
Now, Speed = (distance covered) / (time to cover the distance), so his speed is
S = (9.42 meters) / (24 seconds)
S = 0.39 m/s .
Notice that we don't need to know his mass in order to calculate his speed. In the calculations, we never actually needed to use the "43kg". The answer would be the same no matter what his mass is.
What I AM curious about is why it's taking him 24 seconds to move a distance of only about 31 feet ? ? Normal walking speed is about 3 mph, but our man is only moving a little slower than 0.9 mph !
