Leaning on the playground, by changing your center of gravity, make the ball rolling along the surface!
If you move the surface from its horizontal position thenyou actually put the ball on a slope. Resultant of the forces affecting the ball moves it downwards along the slope, the faster, the steeper the slope. Of course, ultimate cause is: gravity.
You need continuous switching of the center of your gravity while surfing, too. Most of the winter sports need a slope, too. And balancing skills may be quite usseful, too.
Avalanches start because of slopes, too. This happens when the force affecting downwards the slope is stronger than the cohesive and rubbing force that keeps the mass of snow together.
