In the speed-time graph provided, the resultant force on the car is zero when the car is moving at a constant speed because there is no acceleration (Newton's First Law of Motion).

Looking at the graph:

• Section A: The car is accelerating, so the resultant force is not zero.
• Section B: The speed is constant, which means no acceleration and thus the resultant force is zero.
• Section C: The speed is constant again, so the resultant force is zero.
• Section D: The car is decelerating, so there is a negative acceleration and the resultant force is not zero.

Therefore, the resultant force on the car is zero during sections B and C of the journey.

Would you like more details or have any other questions? Here are some related questions:

1. What does the area under the speed-time graph represent?
2. How does the slope of the speed-time graph relate to acceleration?
3. What would the graph look like if the car came to a stop?
4. How does the concept of inertia relate to a flat section on a speed-time graph?
5. Can you explain how friction affects the speed-time graph?

Tip: Remember, on a speed-time graph, a flat horizontal line indicates constant speed, where no net force is acting on the object.