So now you want to speed up, slow down, or change direction.

First of all, you need to to figure out a way to do that. If you lived in on an ice world, where there were no friction, you would find it next to impossible to do so. The same way a car going around a corner on an icy road slides off into the trees, the same way a car on ice is incapable of stopping, the same way you spin your tires on ice or mud, if there were no friction, you would find that the only way to change your speed (even if it were zero) would be to bump into something. (kind of)

Changing speed or direction (velocity) requires FORCE.

We call the change in velocity, acceleration