In the first part of class on Friday, we established Cameron/Huff's Law, "The Fu and acceleration point in the same direction.
In the elevator activity, we drew velocity v. time graphs that made it look like the acceleration would be upwards during part #2 and downwards during part #4.
We then took a closer look at what the FBD and FAD would need to look like for part #2, and settled on either C or D because their FADS had Fu's that point upwards (A and B had FADs that showed balanced forces).
We can analyze the motion of the elevator based on the amount of UnBalanced Force the guy is experiencing. Given that C and D are the only options with UnBalanced Forces, and they both show an UnBalanced force of 55 lb directed up, they'll both result in the same acceleration for the guy. We need to use conversion factors to make this problem manageable. First, since the scale reads 157 pounds when he first steps on it, we can use the knowledge that 1 kg weighs 2.25 lbs to determine that his mass is around 71 kg. Then, we can use the understanding that 1 lb is roughly 4.5 Newtons in order to determine that the Fu of 55 lb would be 245 Newtons. Using a=Fu/m we can then find the acceleration by taking the 245 N divided by 71 kg of mass. The guy is accelerating at 3.45 m/s/s at that point in time. Use this process as you work through the problems on the worksheet. Good luck!