Re: Why did my bottle explode?

You both are on the right track with this one, the non-circular bottle shape certainly will cause a problem every time! The key, as you both surmised, is in the pressure. In the case of the exploding bottle, there are two types of pressure at work: external and internal. Your student was talking about external pressure and was completely correct. The external pressure is the force per unit area that the liquid exerts on the bottle. This force is distributed equally to all points on the surface of the bottle.

The reason the bottle explodes is because of the internal pressure. This pressure is the bottle's response to the external forces acting upon it. This pressure (called stress) is generally not uniform throughout the bottle. The non-uniform stress distribution is controlled by the shape of the bottle. For bottles that are circular in shape, the stress is distributed uniformly throughout the interior of the bottle. If the bottle has corners, the stresses near the corners will generally be much higher than the stresses in the circular bottle. The bottle that breaks first will be the first one, under a given pressure, to reach the bottle's failure stress. If the bottles are made of the same material and have the same thickness, the triangular bottle will win that race, and therefore be the first to break.

I'm familiar with the bottle openers and I don't believe that they produce enough pressure to shatter a bottle under normal circumstances. I think that it's much more likely that there was a small crack or chip in the bottle. Stresses in the vicinity of chips and cracks are concentrated even more than those around corners in the bottles. These cracks may be too small to even see, but they could certainly explain why your triangular bottle exploded and not the circular one; the triangular one may have had a small crack in it.

In short, your student was right; the pressure from the opener was distributed equally on the surface of the bottle. To your credit, you were mostly right. Because of Newton (every action has an equal but opposite reaction) the internal pressure (stress) must be equal on the surface. In irregularly shaped bottles (non-circular), stress in points away from the surface may (and usually do) vary.

Great debate, especially for an English class!