Final answer:
Without additional context, it's unclear if the truck is speeding up on a horizontal surface. A position vs. time graph with a straight line indicates constant velocity, not acceleration. For velocity vs. time graphs, a positive slope represents acceleration, which is crucial in understanding vehicle dynamics.
Step-by-step explanation:
The question "Is the truck speeding up on a horizontal surface?" cannot be definitively answered with a simple 'True' or 'False' without additional context or information such as a velocity vs. time graph or data on the truck's acceleration. Nonetheless, if we are to determine whether an object is speeding up based on a position vs. time graph, a straight line would indicate a constant velocity, not acceleration. Therefore, if this was the basis of the question, the answer would be False (regarding the 18th statement in the reference), since an object that is speeding up would have a position vs. time graph that is curved, reflecting a change in velocity over time.
Regarding the acceleration of a vehicle on a velocity vs. time graph, the correct interpretation is that a constant positive slope indicates a constant acceleration, while a varying slope indicates changing acceleration. The 21st statement seems to concern the direction of acceleration vectors; the True statement there would be that the vector for negative acceleration points in the opposite direction to that of positive acceleration.