Brian Posted May 28, 2015 Share Posted May 28, 2015 Say I have a setup like this... ...where I have a 4m pole fixed to the edge of a weight which is either 12.5kg or 25kg. There will be a force, X, which will cause the system to rotate about point P, and to ultimately topple. The weight of the pole can be ignored for now. Toppling will occur, I think, when the CofG of the weight goes to the left of point P as drawn. I struggling to work out a value for X as I keep coming up against the fact that the perpendicular distance between the CofG and point P decreases as the pole rotates and as it does so the force acting against X decreases. I think? Anyone any ideas? What I'm trying to do is compare the system above with using a tripod stand of the same height... ...where, given the dimensions above, I need to know how heavy a stand I need to offer the same system stability for the same value of X. Link to comment Share on other sites More sharing options...
adamharman Posted May 28, 2015 Share Posted May 28, 2015 You don't want the pole to tip, so the only thing you really need to calculate is what force it takes to move it from the vertical position. What you need is moments (i.e. force x distance) about the pivot point. The weight will act at it's centre, so you've got 125 x A in the clockwise direction and 4000 x X anticlockwise. At the point where it's just balanced, 125A=4000X so X=A/32 Then you allow whatever safety factor you need to give you the maximum acceptable X. The 4000X is not 100% accurate as it's at an angle, but with such a small angle it won't be far off. Same principle applies to the tripod, but the line from the pivot point to the point where X acts on the stand is further from vertical so you need to work out that angle and do a bit of trig to work out the force perpendicular to that line. Then do the same with the weight of the tripod in the other direction. Link to comment Share on other sites More sharing options...
kerry davies Posted May 28, 2015 Share Posted May 28, 2015 Adam is correct, once the weighted stand tilts it will continue to tilt unless X reduces so X is a fixed value. CoG is only useful, in this case, when it is static and if CoG passes P then the stand falls down even if X is removed. Tripods are not meant for horizontal loads but in theory their weight is irrelevant. The stability depends on base dimensions and that depends on resultant angle of the three legs to both each other and the upright. Added weight on a tripod is a different matter and CoG and turning moments again come into play but 99 times out of 100 we would start out knowing X rather than working backwards and using a tripod for a vertical force load. Fascinating query nonetheless and made I think. Is it a practical or theoretical one? Link to comment Share on other sites More sharing options...
Brian Posted May 28, 2015 Author Share Posted May 28, 2015 You don't want the pole to tip, so the only thing you really need to calculate is what force it takes to move it from the vertical position. Thanks Adam, that confirms what I'd came to as a conclusion ie once the pole starts to tilt you're screwed in for a world of trouble. ... but 99 times out of 100 we would start out knowing X rather than working backwards and using a tripod for a vertical force load. In this case X is a force caused by wind acting on a flat surface so I was able to work out a worst case figure. Fascinating query nonetheless and made I think. Is it a practical or theoretical one? Practical. Link to comment Share on other sites More sharing options...
adamharman Posted May 28, 2015 Share Posted May 28, 2015 Tripods are not meant for horizontal loads but in theory their weight is irrelevant. The stability depends on base dimensions and that depends on resultant angle of the three legs to both each other and the upright. Added weight on a tripod is a different matter and CoG and turning moments again come into play but 99 times out of 100 we would start out knowing X rather than working backwards and using a tripod for a vertical force load. In normal use with only a downward force from whatever is on the top, yes, but the question here is a purely horizontal force.The weight of the tripod and and whatever's on it is all that prevents it tipping over. Base dimensions are important, and a bigger base is clearly more stable, but if you assume a weightless tripod you have no vertical component to the load and it will tip if X>0 Link to comment Share on other sites More sharing options...
Dave m Posted May 28, 2015 Share Posted May 28, 2015 risking stating the obvious, if in any count I whack a bloody great weight as an anchor on the bottom of a tripod stand to lower the CoG Link to comment Share on other sites More sharing options...
Brian Posted May 29, 2015 Author Share Posted May 29, 2015 risking stating the obvious, if in any count I whack a bloody great weight as an anchor on the bottom of a tripod stand to lower the CoG Out of interest, what's your method for doing that? Link to comment Share on other sites More sharing options...
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