ofxBullet how to add momentum to moving objects

I am looking at the MeshExample which can place new spheres on to a mesh floor.
The sphere rolls down hills and if it has enough momentum it can roll up hills also.
But what if I want the sphere to act like a car that listens to gravity but also has added momentum via it’s “engine” ?

I see that mouse picking can move objects but probably ignores it’s already existing motion.

Thanks fro any advice.