Welcome to the Physics forum
GameStudio (like many other physics sims) does not simulate rolling friction. The ball rolling down the slope is treated as though it was sliding instead of rolling. You can get the expected acceleration by setting all friction related parameters to their lower limits. This is not recommended for stability reasons though:
Code:
phent_setfriction(my,0);
phent_setdamping(my,0,0)
phent_setelasticity(my,40,1000);
For more realistic motion you should reduce damping to a low value like 5%. Then utilize events to find the normal of the plane your player is on and give it some extra push downward if it's not level.
