Here is a problem from our freshman physics homework--it seemed pretty simple and self-explanatory: (refering to a diagram) 22.5 newtons of force must be used to pull the 4 kg block across the rough surface at a constant velocity of 6.7 m/sec to the right. If the string pulling the block rightward were to break, what would be... (a) the acceleration of the block: Now, isn't the formula for acceleration a=F/m? Wouldn't a = 22.5N/4kg = 5.625 m/s^2??? It seems simple enough, yet it is marked wrong. If I made a silly mistake, I would like to know why, because it seems really easy and I don't know where I went wrong. THEN, after that, it prompts me to find the time it would take for the block to stop. Not quite sure how to figure that out... any suggestions?