I'm stuck on this textbook question about the minimum energy required to roll a cuboid through $90^\circ$ from an upright position to a horizontal position (see image). I believe the correct answer is A, 200 J.
I really don't know which approach to use here. Even my physics teacher is stumped. I don't think I can use difference in potential energy here since I'm not lifting the box, just rolling it, so the weight cannot be 4000N in any case. I'm wondering why the value of the answer is so small. Is the principle of moments applicable here? Or is it something to do with the change in centre of gravity? Can someone give me some hints to get to the correct answer?