acceleration

Anything proportional to zero will be zero, so there must be some initial speed that is added non-proportionally to start it moving. The simplest way would be something like this: However, the actor will quickly accelerate off to the edge of the window. Placing a cap on the speed might be helpful. Also, using location values that are smaller that one pixel would aid in a slower speed-up as well as allowing a smaller proportionality. Unfortunately, this becomes not so simple.