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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.

enter image description here

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?

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closed as off-topic by Brian Moths, John Rennie, Martin, Danu, ACuriousMind May 13 '15 at 18:38

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Homework-like questions should ask about a specific physics concept and show some effort to work through the problem. We want our questions to be useful to the broader community, and to future users. See our meta site for more guidance on how to edit your question to make it better" – Brian Moths, John Rennie, Martin, Danu, ACuriousMind
If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Hi Lim and welcome to the Physics SE! Please note that this is not a homework help site. Please see this Meta post on asking homework questions and this Meta post for "check my work" problems. $\endgroup$ – John Rennie May 13 '15 at 5:57
  • $\begingroup$ It should shift to the edge of the cube until it lies out of the base area and topples. $\endgroup$ – Lim LS May 13 '15 at 6:05
  • $\begingroup$ It decreases by 0.35m. However,when I calculated the change in potential energy I get C as the answer $\endgroup$ – Lim LS May 13 '15 at 6:27
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    $\begingroup$ The title of this question contains almost no information. "Need help" is obvious because this is a website for people who want to get questions to answers. "physics problem" is also obvious because this is a website about physics. Please read this meta post about how to write meaningful question titles. $\endgroup$ – DanielSank May 13 '15 at 7:44
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    $\begingroup$ i quite like this question, i don't think it ought to be closed. it is quite counterintuitive that work must be done to raise the COM $\endgroup$ – innisfree May 13 '15 at 8:14
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When you rotate the block, the center of mass (which lies midway on the diagonal) will move until the diagonal is perpendicular. Therefore, the center of mass must rise from the midpoint of the vertical edge of the block to the midpoint of the (now vertical) diagonal.

If you push horizontally on the top edge of the block, you've got a lever equal to the diagonal of the block pivoting on the fulcrum of the edge on which the block rolls. But this will not affect the energy required to roll the block.

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  • $\begingroup$ Nah, it's just a cie a level physics question. $\endgroup$ – Lim LS May 13 '15 at 7:55
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    $\begingroup$ work done will be same, even using levers or ropes and ladders $\endgroup$ – aaaaaa May 13 '15 at 8:09

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