Why can a can that can’t be opened be opened by tapping the bottom?

Do you like to eat this kind of canned fruit in glass bottles? In the past, when material was not abundant, this canned fruit was a rare delicacy for children. Even now, many people still love it. The taste of canned fruit has become a kind of common childhood memory and spiritual comfort.

It is said that "canned yellow peaches will bless every child". Do you remember the taste of it? Source: Xiaohongshu Canned food is delicious, but the lid is difficult to unscrew, especially for children with less strength. At this time, they can only ask adults for help. In my memory, when adults encounter cans that cannot be unscrewed, they usually turn the can upside down, tap the bottom, and then with a "pop" sound, the cap will loosen.

It is easier to open a can by tapping the bottom. Image source: sohu.com While enjoying the delicious canned food, have you ever wondered when you were a kid why a can that you can't twist open can be easily opened by tapping the bottom? Today we will reveal the secret.

01 The power of water cannot be underestimated

Under the high-speed camera, we can clearly observe what happens at the moment of hitting. Here, we take a glass bottle filled with water and hit the bottle mouth to simulate the scene of hitting a can.

The moment of the slapping observed under a high-speed camera Source: Internet video @ Beyond Slow Motion Under the rapid impact, the bottle will move downward for a short distance. However, due to the inertia of the object, the water in the bottle will not move downward with the bottle at first, but will stay in its original position. At this time, due to the separation of the bottom of the bottle and the water, a low-pressure area close to vacuum will be briefly generated at the bottom of the bottle. Then, under the action of the atmospheric pressure on the upper side, the water will flow downward rapidly to fill this low-pressure area. When the fast-flowing water hits the bottom of the bottle, the instantaneous deceleration will generate a huge impact force, shattering the bottom of the bottle.

From another angle, you can clearly see the process of water hitting the bottom of the bottle. Image source: Internet video @ Beyond Slow Motion

So, what really opens the can lid is not the slap, but the flow of water.

Water, which looks weak, can generate a strong force that is enough to break thick glass. This is not surprising, and can often be seen in other places in life. For example, if you quickly turn off the faucet on the water pipe, you will find that the water pipe will shake violently and even make a dull sound.

If you suddenly close the valve, the pipe will vibrate as if it were hit by a hammer. Image source: bilibili@科普记

02 Water hammer effect like a chain reaction

Just like tapping a can or turning off a tap, if the rapid flow of water is suddenly stopped by some external reason, water has poor compressibility and cannot be buffered by itself, so it will generate a lot of pressure on the outside world. We call this phenomenon the water hammer effect . It is like the car in front suddenly brakes on the highway, and the car behind has no time to react, so it can only hit it hard and cause a rear-end collision.

The water hammer effect is like a chain reaction caused by sudden braking. Image source: bilibili@科普记

In a water pipeline, the sudden closing of a valve can cause a strong pressure that is tens to hundreds of times higher than the normal pressure, which may cause the pipeline to burst. This is called positive water hammer . At the same time, the shock wave will also propagate back and forth in the pipeline, causing violent vibration of the pipeline and damaging valves and other pipeline devices, endangering the entire pipeline system.

If the valve is suddenly opened, the water flow will be released and the pressure in the pipe will drop rapidly. Under the influence of the external atmospheric pressure, the pipe may collapse. This is called negative water hammer . In short, whether it is positive water hammer or negative water hammer, it needs to be avoided when the pipeline is in use.

Nowadays, many places will connect a water hammer eliminator in the pipeline. Its main body consists of a piston and an air chamber. The change of pressure in the pipeline caused by the water hammer effect will act on the piston, causing it to move up and down in the air chamber, so that the air pressure in the air chamber and the pressure in the pipeline form a dynamic balance , thereby reducing the impact and vibration on the pipeline body.

Water hammer eliminators can be used to prevent pressure fluctuations from damaging pipelines. Image source: bilibili@冷科普

The water hammer effect is not completely useless. Sometimes people need to make use of it, such as a water hammer pump . It can use the strong pressure generated by the water hammer to pump water from a low place to a high place. Compared with pumps with other working principles, it has the advantages of simple structure, low maintenance cost and long service life.

At the same time, the water hammer effect can also be used to generate a jet of high-pressure water for use in cleaning, fountains and other scenarios.

The water hammer pump can lift water to a higher place. Source: baijiahao After today's introduction, next time you encounter a can that can't be opened, you should know how to open it elegantly, right? But remember, you should tap the bottom of the bottle, not the cap, otherwise the bottom will break but the cap won't open, which will be embarrassing...

Author: Wei Anran, Application Engineer, PhD in Mechanics, Shanghai Jiao Tong University

Planning & Editing: Rain

Title image source: sohu.com