# Kangaroo 1 - Wood and Twine Physics Simulation - Physics Objects

1 Iteration 4 Send.gh (37.5 KB)

I have produced the following model as a physics simulation of a real physical model (image attached), and I am trying to make it more accurate. The real model consists of a length of twine fixed at two points. 5 lengths of twine attach 5 pieces of wood to this, and their placement affects the form of the whole, as everything is connected.

The below file has the 6 pieces of twine acting as catenaries - they are divided, are strings, and I have placed unitary forces acting down upon the division points of each. If you enable the simulation the 6 pieces of twine hang, as in real life.

In this model the 5 pieces of twine attached to the 5 ‘wood geometries’ act as lines dependent on this relationship, ie if you move the ‘wood’ (these are internalised from 3d shapes from Rhino), the length of the string will change accordingly.

With the real world model, if you move the wood the twine becomes more or less taut, as everything is connected, the wood has mass, and the pieces of twine have fixed lengths. I am interested in developing the model to replicate this. Does anyone have any ideas on how to achieve this?

I am thinking I would need to re-define the 5 pieces of wood first in grasshopper, and then include them as physics objects in the model, and geometries. I am wondering if it is possible to fix or lock the length of the twine, but enable its direction to change if the ‘wood geometries’ are moved? Thank you.

I don’t think you need to actually include the geometry of the wood objects in the simulation, and can just consider the points where the string attaches to them as moving on fixed horizontal planes.
I can see a couple of distinct uses for Kangaroo here:

1. If what you want to do is use the computer to form-find positions for the objects and lengths for the strings for which they will be taut, you can position the wood points on the floor however you like, set them and the two fixed ends at the top as anchors, and set target length for the strings to zero. If you use the lengths from this result for your twine, it will be taut. (I think we can assume here that the weight of the string is negligible compared to the wood, and that it is not stretching a significant amount). For this version we do not need to subdivide the string into shorter segments, and can treat each part between the knots and anchors as a single straight line.
2. If instead you want to set as input the lengths of the string and positions of the objects on the floor, and simulate how the string behaves when you move them around in the real world, then you would divide each segment into multiple shorter lines, and apply a small vertical load to the points. Then when you move around the points on the floor, you can see when parts of the string become slack and hang in a curve. Again, I think it makes sense here to consider the twine as inextensible, so would set the strength of the lines very high relative to the load.