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Name: Billy
Status: Student
Grade: 6-8
Location: DC
Country: United States
Date: September 2008


Question:
How does the shape of a disposable water bottle (Dasani, Eco-Shape Poland Spring, or Vitamin Water) affect the amount of pressure it can withstand?



Replies:
Hi Billy,

Interesting question! The shape of the water bottle has little effect on the pressure it can hold. The most important thing is that the bottle must be round. Whether it is straight-sided (like a cylinder), or it gets "skinny" in the middle (like a Coke bottle), for example, makes no significant difference to its ability to hold pressure.

If the bottle is not round, things get more complicated. Take a square bottle for example. If you try to pressurize it, the flat sides will bulge outward, and if there is enough pressure it will actually start to look round! Water and pop bottles derive almost all their strength (and ability to resist pressure) by being perfectly round. They can increase or decrease in diameter at different points along their length without affecting their strength, provided all sections are still round.

Regards,

Bob Wilson


The pressure inside a container is the SAME IN ALL DIRECTIONS regardless of the shape of the container. These bottles are not meant to withstand high pressure so they should not be pressurized. Failure of the container occurs at the weakest point of the container. Typically this is a seam (if there is one), a "thin" spot, or where there is a large curvature such as the base.

Vince Calder



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