Definition: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Discussion The minus sign indicates that acceleration is to the left. This sign is reasonable because the train initially has a positive velocity in this problem, and a negative acceleration would oppose the motion. Again, acceleration is in the same direction as the change in velocity, which is negative here. This acceleration can be called a deceleration because it has a direction opposite to the velocity. Answer: acceleration
Output:
Deceleration is the opposite of what?