|If force causes only acceleration and not velocity, does a machine (i.e. an engine) that causes a constant velocity in an adjacent object not exert a force?
If that adjacent object is free of any other forces, then no, the machine does not exert a force on it! This is a wonderful question, because it points toward many of the issues concerning energy and work. The bottom line is this: if some object is truly free moving (no other forces on it), it will move along at constant velocity without anything having to push on it. For example, if your car were truly free moving (no friction or air resistance), then it would coast forever on a level surface and the engine wouldn't have to do anything. You could even put the car in neutral and turn off the engine. The only reason that you need an engine to keep pushing the car forward is because friction and air resistance push the car backwards.