C.a.R. > Applications > Area > Planimeter
This is a simulation of planimeter in C.a.R. These devices measure areas. A wheel partly rolls, partly slides when the point at the top is moved around the area. The rolling fraction of the movement measures the total area, once the area is completely surrounded once. If you look at the red mark on simulated wheel, you can see it turn.
Move the endpoint around the circle. You can reset the display with the slider.
The movement of the wheel is summed up using the d() and sum() functions of C.a.R. d() measures changes of objects or expressions, and sum() adds these changes.