Momentum: How can a rolling wheeled vehicle turn 180 degrees without stopping? "In Newtonian mechanics momentum is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. An object will stay still or keep moving at the same speed and in a straight line, unless it is acted upon by an external force."
How then, is it possible for a rolling wheeled vehicle to turn 180 degrees (reversing the direction of its momentum/kinetic energy) without stopping?
An example: when driving my car, to bring my forward kinetic energy to zero I apply my brakes and convert it to heat energy. To move in the opposite direction I then need to set the gearbox in reverse and apply extra energy. Yet if I switch off the engine (adding no new kinetic energy) and turn the wheel, then I can achieve the same effect (with small losses due to wheel friction) without dispersing or exerting any energy. How is this possible?
 A: 
How can a rolling wheeled vehicle turn 180 degrees without stopping ?

The answer is in the last sentence of the passage you quote:

An object will stay still or keep moving at the same speed and in a straight line, unless it is acted upon by an external force

The external force that acts on a coasting car to change its direction is the sideways friction between the car's tyres and the road. If the road were perfectly smooth then you could not steer the car. Similarly, if the car's wheels could not turn at an angle to its direction of motion, then it could not be steered.
There are vehicles without steerable wheels, but they all use various mechanisms for   creating a difference in friction from one side of the vehicle to the other. A sledge is steered by the rider shifting their weight from one side to the other. A tank is steered by making its tracks move at different speeds.
A: Take note of the last part of Newton's law quoted above that states unless it is acted upon by an external force. If there are no externals forces, an object will remain at rest or in a state of uniform motion with constant velocity.
The examples you provide include the existence of external forces. Namely, the frictional forces provided by the road on the tyres.
A: Think of the semicircular motion of a body which is moving at constant speed.
That body must have a force on it to cause the centripetal acceleration which in the case of a vehicle is provided by the force of friction on the tyres due to the road.
