Final answer:
Friction is the force that causes particles to slow down in forward motion, acting in opposition to the direction of motion. It can occur as a result of contact between surfaces, like a car tire on the road, or as air resistance during free fall.
Step-by-step explanation:
The force that causes particles to slow down in their forward motion is known as friction. Friction is an external force that acts in opposition to the direction of motion. This resistance to motion slows things down. In the context of the examples provided, when a car sits in a dip or on a hill, pushing it encounters friction between the car tires and the road surface, resulting in a force that resists the push. When objects fall through the air, air resistance is a form of friction that acts opposite to the direction of motion, affecting objects differently based on their surface area. Moreover, in high-energy conditions within particles, the strong force interacts, and due to friction at a microscopic level, particles can be slowed or their movement can become more random.