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M2 question - moments

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What would the force diagram look like for this question. You take moments about the point at the edge of the river bank, but it seems as if you completely disregard the reaction force - that would surely have to be there for the net force in the vertical direction to be 0. Surely ou should count the reaction force as it is a force that acts on the plank.
Ok so I got the answer now. But one more question - does the reaction force that acts on the plank have to be on the edge of the river bank?
(edited 8 years ago)
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So does the reaction force have to be at the edge of the river bank? Can it not be anywhere else and if so why not?
Original post by B_9710
So does the reaction force have to be at the edge of the river bank? Can it not be anywhere else and if so why not?


Need to see the whole question.
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I just don't understand why the reaction force has to be at the edge of the river bank
Original post by B_9710
I just don't understand why the reaction force has to be at the edge of the river bank


When the woman is standing as far as possible out from the river bank, the plank will be on the point of tipping, and at that time the reaction will be on the edge of the river bank. Since it's on the point of tipping the reaction everywhere else will be zero.
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Original post by ghostwalker
When the woman is standing as far as possible out from the river bank, the plank will be on the point of tipping, and at that time the reaction will be on the edge of the river bank. Since it's on the point of tipping the reaction everywhere else will be zero.


Is there an intuitive way of understanding this concept? I obviously get it now that it has been pointed out to me.
Original post by B_9710
Is there an intuitive way of understanding this concept? I obviously get it now that it has been pointed out to me.


The key is "on the point of tipping", or equivalent, and in that case the entire reaction is at the pivot - imagine a see-saw.

If it wasn't about to tip, and the reactionary forces could be anywhere, then you can't tell where they are - in general.
(edited 8 years ago)

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