When the bag fails

Oops this is what happens when a bag holding a lot of apples fails – they fall to the ground with a loud bang.

In my new video explaining force I have used the weight of an apple to give the viewer a way to feel the force of 1 newton. It just happens that if you hold a medium sized apple in your hand while standing on Earth (unlikely you would be standing anywhere else) the force you feel in your hand and arm is approximately 1 newton force in the vertical direction. This is due to the weight of the apple.

Knowing this you can then imagine the size of any force by imagining the weight of an equivalent number of apples. For the demonstration in the video the bag contains 700 apples, which is equivalent to my weight of 700 newtons. So if you imagine holding 700 apples in a large bag you can imagine my weight.

One of the points I wish to explain in the video is that if a force exceeds the strength of an object, then it will fail. In this case the 700 newtons force due to the weight of the 700 apples exceeds the strength of the bag, resulting in a spectacular failure.

I modelled this in Blender using the rigid body physics modelling system. This is a great system to model objects falling under simulated gravity. The objects can interact with each other, so each apple can influence the behaviour of the other apples as they colloid together during the simulation. The challenge in this animation sequence was not modelling the apples but modelling the bag holding the apples – particularly how to get the initial shape of the bag with the apples in it.

Blender has a cloth modelling capability and this is what I used for the animation of the bag. Unfortunately the cloth cannot interact with rigid bodies (which in this case is the apples). So how did I model the bulging bag formed as a result of the bunch of apples? In reality this is a very complex structural problem. The shape of the cloth affects the bunched shape of the apples and the bunched shape of the apples affects the shape of the cloth. The solution is a state where everything finds balance. Blender can’t model that yet! However in the animation world we don’t need to model everything precisely, it just needs to look real. Yes that’s right, horror of horrors we animators cheat. As long as it looks real then the animation is good to go.

So the approach I took was to model a bowl shaped object, then to fill it with apples by simulating the apples dropping down into the bowl. This gave me a bowl shaped randomly orientated bunch of apples. I then “froze” the apples in this state as the starting point for the subsequent physics simulation. For the cloth simulation of the bag I turned the bowl upside down and placed it inside a cloth ball. When I ran a physics simulation of the cloth it dropped onto the bowl and formed the shape of the upside down bowl into the cloth. I then “froze” the shape of the cloth bag and rotated it 180 degrees so that it now had the bowl shape at the bottom of the bag. Once the bowl was removed from the scene I could then place the apples into the bag still frozen in the shape of the bowl and I had a perfect match between the shape of the bag and the bunch of apples as shown below.

To get the bag to be like a mesh rather than a continuous cloth I used a feature of Blender which converted the continuous cloth into a mesh resulting in the mesh like bag shown in the image above. This mesh can also have a cloth simulation applied so once the models were ready I ran a rigid body physics simulation of the apples dropping at the same time as a cloth simulation of the mesh bag. One thing I had to do to the bag before running the simulation was to separate a selection of the bottom of the bag in order for it to separate from the upper part. The result was a great looking simulation of apples ripping the bag and falling to the ground.

The animation sequence may be found in my video explaining Force. The video will be released April 2014.

Mac