I want to have a body move at a constant velocity, but react to collisions.
My example game is a train, I want it to move at 1 meter a second. If an obstacle appears on the tracks, I want it to react by smashing through it (which will normally slow it down). But when the obstacle is cleared, I want the train to accelerate back to 1 m/s and continue on.
It seems like the applyLinearImpulse() and applyLinearForce() methods are the place to start, but those are one-shot methods.