1. The problem statement, all variables and given/known data It's webbased, so I just took a screenshot: http://img27.yfrog.com/img27/2237/pyquestion.jpg [Broken] 2. Relevant equations F = ma 3. The attempt at a solution Well, the way I figured it was that we could determine the force by calculating the average velocity between two time periods and subtracting to see how much it changed. Doing this I came up with that the acceleration is -75 cm/s. Converting to m/s and plugging in to F = ma (along with the 4 kg) I got an answer that is apparently wrong. I tried some other ways as well to no avail. Help!