Quote:
Originally Posted by Kevin Sevcik
I disagree. Adding weights in the right location is the best way to simulate the competition bridge.
Firstly, adding weight DOES make the bridge more stable and less sensitive. The CoM is the weighted average of the bridge's CoM and the batteries CoM. Heavier bridge means the batteries move the overall CoM less. I proved that to another mentor tonight with a scrap pie of plywood adding weight to the bridge and a battery barely balancing it. Pick up the plywood and down goes the bridge.
Secondly, the moment of inertia of the bridge could definitely matter to balancing and knock down mechanisms. A heavier bridge is going to have slower dynamics than a lighter bridge. It will accelerate and decelerate more slowly, which may affect dynamic balancing systems. Bungees add no mass, and thus leave a lighter, more lively bridge system. Similarly, lack of care in locating your added weights could leave you with a more sluggish bridge than you'd face in reality.
Long story short, there's no substitute for the real thing. If you can balance the super sensitive bridge, you'll likely only need a little tweaking at competition, but you should expect tweaking to be necessary.
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I think we need some physics. All that we are given about the real competition bridge is the field drawings (from which we could glean a lot of things) and the "battery test," which I would argue is all that matters. You absolutely can get a wooden bridge to pass this test by playing with weight distribution, but adding weight to each end doesn't accomplish this. In fact, adding equal weight to each end does nothing. When the bridge tips around either of its hinges, you are lifting all the weight on one side of that hinge while letting all the weight on the other side down, by more or less equal amounts. The parts of the bridge OUTSIDE the two hinges cancel, because they are equal (sort of, in the team drawings they aren't really), and they are still equal if you are adding equal weights to each end. It's the weight BETWEEN the hinge points, and to some extent the hinging geometry itself, that affects the "tippiness." So by adding weight between the rock points, you are adding weight that must be lifted when the bridge tips, making it less sensitive. By taking away weight in the middle, you lift less weight to tip the bridge so it tips more easily and more sensitive. I didn't actually do any really involved analysis of this problem (taking into account things like the varying distance of each end from the hinge point, how the cg moves and the weight distribution changes when it tips because of the height of the cg, etc.) but I consider these to be mostly negligible so hopefully my rough approximation for a first pass analysis is good enough.
There are two types of competitors in this world, those who aim to rise above their opponents and those who seek to drag them down. Which group do you want to belong to? Which group is more beneficial to our society? Think about it. - Karthik [more] 
17-01-2012, 09:45
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