Final answer:
It is more difficult to stop a vehicle going downhill primarily due to gravity, as it adds to the vehicle's forward momentum, thereby increasing its velocity and making it harder to decelerate.
Step-by-step explanation:
It is more difficult to stop a vehicle going downhill due to gravity. This is because when a vehicle is moving downhill, the downhill component of gravity is adding to the forward momentum of the vehicle, making it harder to stop. The force of gravity working in the same direction as the vehicle's movement increases its velocity and hence its momentum. In contrast, friction is the force that opposes the motion, and it works to slow the vehicle down. If the opposing force due to friction is less than the force due to gravity, the vehicle will continue to accelerate downhill.
The concept of inertia – the tendency of an object to remain at rest or, if moving, to continue in motion – is also relevant to the discussion, but it refers to the resistance to changes in an object's state of motion rather than the force that is causing the motion itself.