When a car is driving on a flat road, how many distinct action/reaction pairs of forces exist between the car and the road surface according to Newton’s Third Law?

• A. One pair: the normal force and the $weight$ of the car
• B. Three pairs: normal force, friction force, and air resistance, each with their reaction forces
• C. Four pairs: normal force, friction force, air resistance, and engine force, each with their reaction forces
• D. Two pairs: the normal force and friction force, each with their respective reaction forces

Answer: (D)

Verified step by step guidance

1. Recall Newton's Third Law, which states that for every action force, there is an equal and opposite reaction force acting on a different object.
2. Identify the forces acting between the car and the road surface specifically. These include the normal force (the road pushing up on the car) and the friction force (the road pushing horizontally on the car to prevent slipping).
3. Recognize that the weight of the car acts between the car and the Earth, not directly between the car and the road surface, so it does not form an action/reaction pair with a force from the road.
4. Understand that air resistance and engine force involve interactions with the air and the car's engine mechanism, not directly with the road surface, so they do not count as action/reaction pairs between the car and the road.
5. Conclude that there are two distinct action/reaction pairs of forces between the car and the road surface: one pair for the normal force and its reaction, and one pair for the friction force and its reaction.