Source: Shanghai Science and Technology Museum
Do you like to eat this canned fruit in glass bottles? In the past, when the material was not abundant, this kind of canned food was a rare delicacy for children. To this day, many people still love it, and the taste of canned fruit has become a kind of shared childhood memory and spiritual comfort.
Legend of the rivers and lakes "canned yellow peaches will bless every child"
Do you have a taste of it in your memory?
Source: Little Red Book
Canned food is delicious, but the lid is difficult to unscrew, especially for children with little strength, so they can only ask adults for help. I have the impression that when adults come across a can that cannot be unscrewed, they usually turn the can upside down, pat the bottom, and then "pop" and the cap is loosened.
It's easier to open with a pat on the bottom of the can
Source: sohu.com
While enjoying the delicious taste of canned food, have you ever wondered when you were a child, why cans that can't be unscrewed can be easily opened by patting the bottom? Today we're going to reveal the secret.
The power of water should not be underestimated
With the high-speed camera, we can clearly see what happened at the moment of the shot. Here, we take a glass bottle filled with water and hit the mouth of the bottle to simulate the scene of slapping the can.
Observe the moment of the shot under a high-speed camera
图源：网络视频@ Beyond Slow Motion
With a quick hit, the bottle will move down a short distance. However, due to the inertia of the object, the water in the bottle does not follow the bottle downward at first, but stays in its original position.
At this time, due to the separation of the bottom of the bottle from the water, a low-pressure area close to a vacuum is briefly created at the bottom of the bottle. The water then flows rapidly downwards under the action of the upper atmospheric pressure, filling this area of low pressure. When the fast-moving water hits the bottom of the bottle, the instantaneous deceleration creates a huge impact force that shatters the bottom of the bottle.
From a different angle, you can clearly see the water hitting the bottom of the bottle forward
图源：网络视频@ Beyond Slow Motion
Therefore, it is not the slap that really opens the cap of the can, but the flow of water. Water that looks weak can produce enough power to shatter thick glass. This is not unusual, and it can often be seen in other parts of life. For example, if you quickly turn off the faucet on a water pipe, you can find that the water pipe will shake violently or even make a dull sound.
If the valve is closed suddenly, the pipe will vibrate as if it had been struck with a hammer
Source: bilibili@ Popular Science
It's like a water hammer effect of a chain of rear-end collisions
Just like slapping cans and turning off faucets, if the rapid flow of water is suddenly cut off by some external reason, it is difficult to buffer the water by itself due to the poor compressibility of the water, so it will produce a lot of pressure to the outside world. We call this type of phenomenon the water hammer effect. It's like on the highway, the car in front suddenly brakes, 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 series of rear-end collisions caused by sudden braking
Source: bilibili@ Popular Science
In a water pipeline, a sudden closure of the valve can trigger a strong pressure that can reach tens to hundreds of times 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 intense vibration of the pipeline, and damaging other pipeline devices such as valves, endangering the entire pipeline system.
If the valve is opened suddenly, the pressure in the pipe will drop rapidly while releasing the flow of water, which may cause the pipe to collapse under the action of the outside atmospheric pressure. This is called negative water hammer. In short, whether it is positive or negative water hammer, it needs to be avoided when the pipeline is in use.
Nowadays, in many places, a water hammer eliminator is connected to the pipeline, and its body consists of a piston and an air cavity. 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 cavity, so that the air pressure in the air cavity and the pressure in the pipe form a dynamic balance, thereby weakening the impact and vibration on the pipeline body.
Water hammer eliminators can be used to prevent damage to the line due to pressure fluctuations
Source: bilibili@ Leng Popular Science
The water hammer effect is not useless, and sometimes people need to use it, such as water hammer pumps. It uses the strong pressure generated by water hammer to pump water from low to high places. 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 cleaning, fountains, and other scenarios.
Water hammer pumps can lift water to a high place for use
图源：baijiahao
After today's introduction, the next time you encounter a canned bottle that can't be unscrewed, you should know how to open it elegantly, right? But remember, you should pat the bottom of the bottle, not the cap, otherwise the bottom will be broken but the lid will not be opened, which will be embarrassing......
If you shoot the can in the wrong direction, it's a sad story
Source: Weibo @humor Jun Yaoyao