The present study aimed at investigating children's and adolescents' understanding of constant and accelerated motions. The main objectives were (1) to investigate whether different task formats would affect the performance and (2) to track developmental changes in this domain. Five to 16 year olds (N = 157) predicted the distances of a moving vehicle on the basis of its movement durations on both a horizontal and an inclined plane. The task formats involved: (1) nonverbal action tasks, (2) number-based missing-value word problems, and (3) verbal judgments. The majority of participants of all age groups based their reactions in the first two task types on the assumption of a linear relationship between time and distance--which is correct for motions with constant speed but incorrect for accelerated motions. However, in the verbal judgments that tapped conceptual understanding, children from the age of 8 years onwards correctly assumed that an object rolling down an inclined plane would accelerate. The role of the task format in evoking erroneous beliefs and strategies is discussed.