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# What causes an object to topple? Watch

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1. I know an object topples when the line of action of the weight of the object is outside the base of the object, but i dont understand the other point to do with stability which is moment what do you comment on in the exam about the moment?
2. Moment is a turning force. It may cause something to topple. When you calculate moments of an object at rest on a surface, you are free to choose the point of rotation, although only two such points are interesting: the bottom left and right corners of the object in the table, since these will be the axes of rotation if the object topples anticlockwise or clockwise (respectively). So you want to calculate the moments about these two points. In general, there will be two moments about this point: the first due to the object's weight, which acts downwards at the COM, and the second is the reaction of the surface on the object, which acts upwards at the point directly below the COM where the object touches the table (assuming a flat bottom).

If you think about how these moments are calculated, depending on where the COM is relative to the two axes, it should become clear why the object will not topple (and is stable) if the COM lies between the two axes. A diagram of some random object (e.g. a square) may help make this clearer.

Just work out the moments of each of those two forces, as a function of the dimensions of the block (trig needed).

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