Why straws work

What are you doing to cause the liquid to rise up the straw into your mouth? To help you see what is happening, lets try it again, with one difference.

This time, use two straws. Place both straws in the liquid and try drinking through both at once. It still works very well. Two straws work just about as well as one. Now lets make another change. Leave one straw in the liquid and hold the other one beside it, but on the outside of the glass. With one straw in the liquid and the other in the air, try drinking with both straws.

Does it work as well this way? In fact, it does not work at all. As already mentioned in the introductory section, from a certain height to be overcome between the surface of the water and the mouth, one will notice that it is no longer possible to drink from the drinking straw, no matter how hard one tries. In theory, this maximum suction lift is around 10 m for water. This does not have to do with a too weakly developed mouth musculature but has a natural physical cause.

Even the most powerful suction pump in the world would not be able to overcome a height difference of more than 10 metres if a perfect vacuum were created. This maximum suction height will be derived mathematically in the following. The force F p with which the water can be effectively pushed upwards is the difference between the forces acting at the lower end F 0 and the upper end of the water column F 1.

The water column is the water in the straw relative to the water level in the glass. At this level, the ambient pressure p 0 also acts in the water itself since there is no hydrostatic pressure at this point. The immersion depth of the drinking straw plays no role more about this later. The resultant upward force F p results from the difference between the force F 0 at the lower end of the water column which pushes the water upwards by the ambient pressure p 0 and the force F 1 at the upper end of the water column which pushes the water downwards by the internal drinking straw pressure p 1.

The weight of the water column to be pushed up depends on the water level in the straw. The weight increases with increasing height. As the water level in the straw increases, the weight of the water column increases. At some point the weight F g will eventually become as large as the upwards acting force F p. In this state it is no longer possible to push the water further upwards. At given pressures this suction height h is determined as follows:.

In this case, the maximum suction lift h max is obtained by creating a vacuum inside the straw:. For this reason, the maximum drinking height is 10 metres. With a longer drinking straw, the ambient pressure can no longer push the water column upwards. These experiments have not been specifically safety tested for home use but we believe them to be safe if the instructions are followed.

Adult supervision or direction is recommended as appropriate. All experiments are carried out at your own risk. Here you can browse the entire back catalogue of Marvin and Milo physics activities For Resources.

Demonstration: This rough method can produce a difference of mass of at least 8 g. When you suck on a straw that has a hole, you pull air through the hole, instead of removing air from the straw. The liquid stays put, unless you cover the hole. Put the index card on top, making sure that the card completely covers the cup. Put the palm of your hand on top of the card to hold it on and turn the cup over.

When the cup is completely upside down, take your hand away. What happens to the card? To the water? Do this over a tub or sink, just in case! The card remains attached to the cup.