The question has been the bane of a lot of people's textbook grind.
I hope you don't mind answering some more questions regarding this.
Q1: In Part (a) of this question, the acceleration of the lift was found, without the two people inside (without their weights being considered). But in the next part of the question, it was used that the acceleration of Albert was
1.5ms−2. Surely, this cannot be correct, as we didn't we even consider the weights of the two people inside the lift for part (a). The way I'm thinking about it is in part (a) the acceleration of the lift was found
WITHOUT two people inside which in my opinion would not equal the acceleration of the lift
WITH the two people inside.
Please consider this. Suppose we have a ball of mass 0.2kg that is attached to the upper end of a verticle light rod. The thrust in the rod when it raises the ball is
2.3N. Find the acceleration. So, the way I am going to solve this problem is comparable to how the above problem was solved. We have our thrust force pushing the ball up with a force of
2.3N and we have our force due to gravity of
0.2×9.8N.
2.3N=0.2aa=0.2kg2.3N However, clearly I have forgotten the force of thrust and so this incorrect. But why is this not analogous to what was done in the above question???