Final answer:
To make her test fair, Sue must keep the initial velocity of the car the same, ensuring only friction from the surfaces affects the stopping distance.
Step-by-step explanation:
When Sue rolls a car down a ramp and investigates how far the car travels along different surfaces before coming to a stop due to friction, to make her test fair, she must keep the initial velocity of the car the same. By doing this, she ensures that the only variable affecting the distance the car travels is the friction of the different surfaces, not any differing initial force applied. This allows for a proper comparison of the effect of surface texture on stopping distance alone.