When 1 straw is in the glass of water and the other is outside the glass why doesn't any water come in when sucked on both straws?

Answer 1

First consider just sucking on a straw. You are creating a lower air pressure in your lungs by performing a volume expansion. This internal low pressure doesn't last long because outside air rushes in to equalize the pressure both inside and out. You cannot suck in enough air to serious or even detectiably alter the pressure of the whole atmosphere. Second consider sucking on a straw in a glass of water. You create a low pressure region inside of you by performing a volume expansion. The outside air wants to equalize the pressure both inside and out but there's a big hunk of water in the way. So the outside air pressure pushes the water up the straw by pushing down on the surface of the water. The height that the water goes up to depends on the weight of the column of water in the straw and the difference between the outside and inside air pressure. In other words how high up the water goes depends on how hard you inhale (work). Thirdly consider two straws one inside a glass of water, the other free to the outside air. When you inhale you perform a volume expansion which creates a low pressure region inside of you. The outside air rushes in the straw that is free to the outside air and equalizes the air pressure both inside and outside. Consequently there is no pressure difference that would cause the water to be forced up the straw.