Say I have a sphere (planet). If I have object on the surface, I want to limit movement so that it can only travel in directions along or above the surface.
Imagine a coin on a table. If I were to push the coin from up above at an angle, the table would cancel out any downward vector component and the coin would move in 2 dimensions across the table. This is what I would like to achieve.
In the case of a planet (sphere) then I can calculate the cancelling vector to some extent (this is just the line from the centre of the sphere to the point on the surface).
Now I struggle - how do I calculate the magnitude of the opposing vector such that it is sufficient to stop the object falling through the surface but not too big that it starts to launch the object away from the surface?