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Lecture 19: Intro to Physical Simulation (24)
reesiespeesies

why will this structure not resist shearing?

bennyd87708

@reesiespeesies My understanding is that this structure will not resist shearing because there is still room for it to stretch from a force at the corners without any of the individual springs needing to expand any further then they already are. You can imagine how if you pulled the top right and bottom left corners outwards, the whole structure could collapse inwards where the bottom right and top left corner of each square would pull to towards and touch each other, and the top right and bottom left corner of each square could pull away from each other, all without any of the individual line segments in the image changing length at all. I apologize as I was unable to find any animations demonstrating this, but perhaps someone else knows of one or could find one to visualize this structure shearing.

daniel-man

@reesiespeesies Building on what benny287708 said, the primary reason why the structure will not resist shearing is because the springs between each pair of points only care about maintaining the distances between the points, not their relative angles with other springs. As a result, the springs will offer no resistance against shifting from their 90 degree angles in the picture to a near 0/180 degree colinear angle when the two corners (top-right and bottom-left) are pulled diagonally away from each other. This is also because, as mentioned in lecture, there is no concept of collision in this model since the wires have no physical volume.

AlexSchedel

I think the easiest way to think of this is by imagining in your head what it would like like when the two points are pulled. The corner points will go from being in a square configuration to being diamond shaped, meaning they will be longer in one direction. More specifically the distance between the two pairs of opposite corners in the rectangle will be greater than and less than root 2. This holds for the whole mesh, since the springs are all linked up together.

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