Why do people often run counterclockwise?

running

It is probably one of the most popular forms of exercise.

The rubber track during my school days

How much of our youth is left behind

But why

We often run counterclockwise

Instead of clockwise?

Q1

Why does the tone of a violin become fuller after applying rosin to the strings?

by vanitas

answer:

The effect of rosin on the tone should be considered from the static friction of rosin. Rosin is a solid form of resin obtained from pine trees and other plants such as conifers. Some other resins and materials are added during the production process. Rosin produced by different companies has its own exclusive secret formula. Rubbing rosin on the bow hair will leave powder on the bow hair. If you touch it with your hand, you will feel the stickiness. When the bow is pulling the string, the string moves toward the bow due to the stickiness of the rosin between the bow hair and the string until the stickiness breaks and the string rebounds and vibrates.

For the common light, amber, and dark rosins, the dark rosin is softer and suitable for use in cool and dry weather, suitable for cello; while the light rosin is harder and denser, suitable for hot and humid weather and violin and viola. Different rosins produce different static friction, which affects the timbre of the instrument. Some companies add precious metals such as gold, silver, lead silver, and copper to their rosin formulas to change the static friction to produce different timbre qualities. It is said that copper rosin creates a warm, velvety soft tone, gold rosin produces a warm, clear tone, silver rosin creates a concentrated, bright tone, and lead silver rosin produces a fresh playing tone (small static friction changes will affect subtle changes in the tone, perhaps only real musicians or listeners can appreciate the mystery of this 😓).

References:

http://stringsmagazine.com/the-differences-between-dark-and-amber-rosin/

https://en.wikipedia.org/wiki/Rosin

by jita

Q2

Why is it that when the bucket on a water dispenser is full, it takes a long time for many small bubbles to appear when you add water to it, but when the water in the bucket is dry, it takes not long for few large bubbles to appear when you add water to it? Is this related to the water pressure?

by Anonymous

answer:

This is mainly related to the volume of air in the bucket. The larger the remaining air volume, the fewer bubbles there are. Ignoring the collapse of bubbles during the rise, the actual measurement can be one familiar "gurgling" sound as one bubble.

Take the water dispenser using the smart seat as an example. As for the specific structure of the smart seat, I will not introduce it in detail. The result of using the smart seat is that the water dispenser can release water continuously, and when the pressure difference between the air pressure in the barrel and the atmospheric pressure reaches a certain value, it will inhale a proper amount of air from the outside, that is, at this time, it will be released with a "gurgling" sound.

We can look at the relationship between pressure and volume qualitatively using the ideal gas equation:

When there is more water and less air in the bucket (that is, V is small), a slight drop in the water level can cause a significant drop in the air pressure in the bucket, thereby sucking in air multiple times. On the contrary, when the bucket is almost empty, the air occupies most of the volume (that is, V is relatively large), and the water level must drop a lot to cause a significant drop in the gas pressure in the bucket, so more water is needed to suck in a bubble. Although when the air volume is small, the number of times the same amount of water is released and air is sucked in is more than when the air volume is large, the difference between the two is not too large due to the influence of some other complex factors.

References:

[1] Watsons Enterprises Limited. Water dispenser: China, CN200710006311.9[P]. 2007.12.19.

by freelance

Q3

What is the difference between activated carbon and charcoal? Why are some substances "activated"?

by Anonymous

answer:

Activated carbon is a type of carbon that is widely used to filter pollutants from water and air. It is essentially a type of carbon. However, the biggest difference between activated carbon and other ordinary carbon is that it has a larger surface area, similar to the difference between popcorn and corn kernels.

Activated carbon has many nanometer-level tiny pores, and the surface area of ​​one gram of activated carbon can even exceed 3,000 square meters. The surface area of ​​one gram of unactivated charcoal is only 2.0 - 5.0 square meters.

There are two ways to produce activated carbon. 1. Physical activation, which is to put the carbonized raw materials into high-temperature oxidizing gas, and produce pores through the reaction of oxidizing gas with carbon atoms and some impurity atoms. 2. Chemical activation, which is to immerse the carbonized material in chemicals such as acid, strong base, etc. Acid or strong base will react with impurities, thereby corroding the carbonized material and producing many pores.

So why do we call some substances "active"? Literally, "active" means the property of being alive, active and reactive. The "activity" of activated carbon here refers to its strong adsorption capacity due to its large surface area, which makes it easy to adsorb small particles and molecules. Therefore, we consider materials like activated carbon that can easily absorb energy or substances from the external environment to be "active". For example, activated alumina is also loose and porous, with a large surface area, and is not only a good catalyst and catalyst carrier, but also a strong dehydrating agent.

References:

https://en.wikipedia.org/wiki/Activated_carbon

https://baike.baidu.com/item/%E6%B4%BB%E6%80%A7

https://en.wikipedia.org/wiki/Activated_alumina

by YJY

Q4

Why do people usually run counterclockwise instead of clockwise on the playground?

by Anonymous

answer:

The question is very precise. It is indeed "generally" counterclockwise running, not "all" counterclockwise running. Looking back at the track and field events in the Olympics, the 1896 Athens Olympics and the 1900 Paris Olympics both adopted clockwise running directions. By the 1908 London Olympics, the IAAF finally stipulated that counterclockwise running should be adopted and it has been used ever since.

As for the reason why the running direction in track and field competitions is counterclockwise, it is more of a convention in history. There are many different explanations in biology and physics. Among them, the more influential hypotheses include "right-handedness", "heart on the left", "Coriolis force theory" [1], etc. After careful investigation, I will briefly introduce the "right-handedness theory" that seems to be the most reliable.

Since the beginning of hunting in ancient times, most hunters and soldiers are right-handed, that is, they are used to holding a knife (spear, sword, halberd, axe, hook, fork) in the right hand and a shield/carrying a dead rabbit in the left hand. At this time, if there is an obstacle in front of you, you need to turn counterclockwise (to the left), then the knife in your right hand will be exposed first, followed by your defenseless head and torso. If there is an enemy or a tiger ambushing behind the obstacle, and they will attack as soon as they see something popping out, then the opponent must first get rid of their own weapons before attacking their torso. If you turn clockwise to the right, the obstacles that are exposed in turn are the shield (or a dead rabbit), torso, and knife. When you find an enemy hiding behind the obstacle, you don't have time to draw your weapon to fight, so you can only hold up the shield to defend and give up the initiative in the battle. When the opponent sees the shield exposed in the corner of the wall, he can start the attack in advance and take the lead in close combat. Over time, most people subconsciously prefer to turn counterclockwise, and it has evolved into modern track and field sports, and it has become a natural convention.

As for why we don't talk about the "Coriolis force theory", when did we ever talk about physics in the Second Institute? ! Because the Coriolis force is too small (<0.1N), it will not have a significant effect on exercise. The argument that the Coriolis force is the main factor affecting the direction of running has a flaw, so it's better not to mention it.

References:

https://www.zhihu.com/question/19597778

by Tibetan Chi

Q5

Why can a bicycle maintain balance when being ridden and can still maintain balance for a period of time after a person leaves the bicycle?

by Xiaobai

answer:

After a person leaves the bicycle, the bicycle can still maintain balance at a certain speed. There are many factors that affect the stability of the bicycle, such as gyroscopic effect, mass distribution, positive trajectory, and the geometric structure of the bicycle. With their combined effect, the bicycle can maintain self-balance.

If you carefully observe the behavior of the bicycle at this time, the bicycle is twisting. If the body tilts to the left, the front wheel of the bicycle rotates to the left. If it tilts to the right, the front wheel rotates to the right, causing the tilted bicycle to change its forward direction. The bicycle that was about to tilt and fall stands up again. For example, if you hit the side of the bicycle during the hand-off movement, the bicycle has a tendency to tilt and fall, and the front wheel rotates in the direction of tilting. The forward direction of the bicycle has changed, but the body is still vertically balanced (here are all explained under the condition of a certain speed of the bicycle). When riding a bicycle and turning, such as turning left, we will first turn right a little, let the bicycle tilt a little to the left, let the horizontal component of the force given to the bicycle by the ground point to the left, and then turn the front wheel to the left, and you can turn left smoothly. If you turn left directly, it will be unstable and easy to fall. The key to maintaining balance is the front wheel of the bicycle, and it is difficult to maintain balance with a locked front wheel. The gyroscopic effect plays a role here, helping the front wheel to turn in the tilting direction; the gyroscopic effect can also generate a roll torque during the reverse rotation of the front wheel. As shown in the figure above, when a torque is applied about the green tilt axis (tilt), a reaction torque is generated about the blue steering axis (steer).

Although the gyroscopic effect plays a big role in keeping the bicycle stable, it is not indispensable. Cleverly designed mass distribution and geometric structure of the bicycle can also make the bicycle self-stable. Other factors are also important. The mass distribution of ordinary bicycles makes the center of mass of the steering structure in front of the steering axis, which will also cause the front wheel to turn in the tilted direction under the action of gravity. Bicycles moving forward are more stable. Ordinary bicycles will become extremely unstable when going backwards. This is related to the design of the bicycle. Changing the design can also achieve a stable bicycle when going backwards.

The movement behavior of a bicycle is quite complicated. If you are interested, you can take a look at the references.

References:

https://en.wikipedia.org/wiki/Bicycle_and_motorcycle_dynamics

https://www.bilibili.com/video/BV1VQ4y1e7E2?spm_id_from=444.41.0.0

by jita

Q6

Why do snowflakes sometimes fall as single flakes, and sometimes as small clumps of snow in irregular shapes?

by Anonymous

answer:

A small group of snow is formed by the collision and adhesion of single snowflakes as they fall through the atmosphere. Ice nuclei are formed around mineral or organic particles in a water-saturated air mass. Water molecules in the air are deposited on ice crystals and grow. They can develop from the initial shape (prism, needle, plate, etc.) into many symmetrical snowflakes (the shape depends on the ambient temperature and humidity). If there are more water droplets, the snowflakes can grow to micrometer or millimeter size. In this process, the snowflakes grow regularly. When falling under the action of gravity, a large number of large snowflakes easily collide with each other, thus sticking together to form a small group of snow. It is different from the process of water molecule deposition and growth, and cannot form a regular shape. It can be simply imagined that salt can crystallize into regular crystals in a supersaturated solution, but if many such crystals are randomly piled together, they will not have a regular appearance.

References:

https://en.wikipedia.org/wiki/Snowflake

by jita

#This issue's answering team

jita,freelance,YJY,Zangchi,Paarthurnax

Editor: Mu Zi

Source: Institute of Physics, Chinese Academy of Sciences