KNOW WHY BASE OF PACKAGED WATER BOTTLE IS FLAT !!

The bumps or the corrugations in the foot of the bottle increases the strength of the bottle. Take a sheet of paper and fold it. You can easily fold the paper. Now bend any one side of the sheet and try to fold it. You will now feel some resistance. Now, roll the paper into a pipe and try folding it. It will be little hard to fold the paper pipe. Bending any material increases its strength and rigidity. This is because; rolling the paper increases its moment of inertia around the bending axis.

Soft drinks are served chilled. When a liquid is cooled, its volume changes and the bottle must not buckle because of this change in volume. The corner or the tip of the bump can slightly expand/contract without breaking which takes care of the change in volume of the liquid.

Soft drink bottles have to stand up to pressure because of the carbonation. The 5 point bottoms are designed to hold the pressure better. A 5 point bottom isn't the only way, its just one way that works well and is in common use.Because bottles of carbonated liquid are pressing outward, which would cause a flat bottom to bulge out, become rounded, and then wouldn't stand up right any more -- the five points make a steady base no matter how much pressure is inside.

Bottled water is not carbonated so there is no need for a pointed bottom. A flat bottom uses less plastic, and holds up better in drop testing.