Time flies so fast,
In the blink of an eye, the five-day May Day holiday has ended.
Xiaobian saw someone in the circle of friends
Why isn't the May Day holiday 51 days?
ended a short long vacation,
I wonder if you are fully charged and ready for a new challenge?
Now the editor will test you,
In the process of traveling on May Day,
Have you noticed the physics knowledge around you?
(Welcome to cast your precious vote~)
The secret of walking – the ubiquitous friction
When it comes to friction, everyone is certainly familiar with it, it is not only one of the first forces we came into contact with when we were students, but also plays an important role in our daily lives, we walk, drive, twist bottle caps, are inseparable from friction. If there is no friction, not only will the laces not be tightened, but even the chopsticks will be useless.
A long, long time ago, in Aristotle's time, it was generally believed that force is what keeps objects moving, because common sense tells people that you can only move an object if you apply a certain force on it, and once you remove this force, the object will stop. For example, if you don't push a box on the ground, it won't move.
But by the time of Galileo, it was discovered that the box would stop if it was not pushed because it was affected by friction, and if there was no friction, the moving object would have been moving in a straight line at a uniform speed. This has changed people's perception of the world. Through the ideal inclined plane experiment, Galileo proposed that force is not the cause of maintaining the motion of an object, but the force is the cause of changing the state of motion of an object.
Ideal Slope Experiment | Image source network
Friction is divided into static friction, rolling friction, and sliding friction. Static friction occurs when there is pressure between two objects with rough contact surfaces and there is a tendency to move relative to each other. When there is relative motion between these two objects, the static friction is converted into kinetic friction.
The magnitude of the sliding friction is directly proportional to the pressure between the contact surfaces and the roughness of the contact surfaces. It is expressed by the formula as:
The maximum static friction is generally greater than the sliding friction.
Rolling friction is the frictional force produced when an object rolls on top of another object, rolling friction is actually a static friction force. For example, a small ball placed on an inclined plane. As the ball rolls downward, the point of contact between the ball and the inclined plane is constantly changing, and there is static friction between each contact point and the inclined plane.
Friction is not necessarily a drag, and many times it is friction that provides the force for an object to move forward.
For example, when we are walking, we push the ground backwards, and the static friction between the feet and the ground provides forward momentum, and when we ride a bicycle, the rolling friction between the wheels and the ground also provides forward momentum.
If the coefficient of friction is small and the static friction becomes sliding friction, the scene will look like this:
Source: Bilibili
That's why snow chains are installed in cars when the road is icy, in order to increase the coefficient of friction between the contact surfaces, which in turn increases the rolling friction.
Blue sky, white clouds and sunset – the scattering of light
During the trip, I always like to pick up my mobile phone or camera to take pictures of the cloudless blue sky, or the gorgeous sunset on the horizon, have you ever wondered why the blue sky, white clouds and sunset show different colors?
Just what we come into contact with when we open our eyes is commonplace in our lives, but the study of it has gone through a long history, and the question of whether light is a particle or a wave has been debated for hundreds of years.
Now we know that light has wave-particle duality.
Light propagates in a straight line in a homogeneous medium, and when it is blocked by an inhomogeneous medium or an object, it will undergo phenomena such as reflection, refraction, scattering, interference, and diffraction. Scattering is the phenomenon in which light rays propagate in all directions as light beams pass through a medium with inhomogeneous optical properties. A very intuitive example is that in a dense foggy forest, we can see the trajectory of direct sunlight, which is the scattering of light.
To explain why the sky is blue, Rayleigh studied the scattering of fine particles, and he found that when the scale of the scattered object is smaller than the wavelength of light, the intensity of the scattered light is inversely proportional to the fourth power of the wavelength, but when the scale of the scattered object is comparable to the wavelength of the light, Meter and Debye found that the scattering intensity has no obvious relationship with the wavelength.
The reason why the sky is bright during the day is because of the scattering of sunlight by the atmosphere, and if there is no atmosphere, because the light travels in a straight line, we would see a bright sun hanging in a dark background, just like we see in space.
Electromagnetic spectrum | Image source network
When light passes through the atmosphere, the scattering of molecules in the atmosphere due to density fluctuations is much smaller than the scale of the wavelength of light, and according to the principle of Rayleigh scattering, the scattering intensity of blue-violet light with a shorter wavelength in white light is much greater than that of other colored light, so the sky appears pale blue. In the morning or evening, the light passes through the atmosphere at a large angle, and the thickness of the atmosphere is much greater than when it is perpendicular incidence, and the blue-violet light is basically scattered, so the sunrise and sunset we see are often red or yellow.
Clouds are made up of water droplets in the atmosphere, and the size of the water droplets is much larger than the light wave, Rayleigh scattering does not apply, when the light passes over the cloud, the color light of various wavelengths is scattered, so the cloud appears white.
There are still many interesting optical phenomena in nature, and we have the opportunity to explore them one by one.
Cars whizzing by – the Doppler effect
The name Doppler effect may sound unfamiliar, but the effects it produces must be familiar to everyone.
While waiting on the side of the road, you will hear the cars whizz by, yes, you read that right, "hear". If you pay attention, you'll notice that when a car passes you, the sound of the vehicle goes from low to high-pitched and back to low. This is because when a vehicle is coming towards you, the frequency of the sound it emits becomes louder, and when it is driving away, the frequency of the sound becomes lower, which is known as the Doppler effect. Like this:
In 1842, the Austrian mathematician and physicist Christian Doppler proposed this effect, the main content of which is that when the wave source approaches the observer, the wavelength becomes shorter and the frequency becomes larger due to the compression of the wave, and when the wave source is far away from the observer, the wavelength becomes longer and the frequency becomes lower. In 1845, Bayes Barot proved this hypothesis using sound waves.
Bayes Barot's Experiment | Source: Wikipedia
In astronomy, the observation of the planet's motion can be judged by the redshift and blueshift to determine whether the planet is moving away or closer to us. According to the spectrum of electromagnetic waves, we can see that when the frequency becomes low, the redshift will occur, combined with the Doppler effect, we can know that the planet is moving away from us at this time, on the contrary, when the frequency is high, the blueshift occurs, indicating that the planet is approaching us.
There are also many application examples of the Doppler effect in life, for example, in mobile communication, it is necessary to consider the impact of the Doppler effect when the base station and the user move relative to each other. The Doppler effect is also widely used in the medical field, and color ultrasound is the application of this principle.
The cordon of the train platform – Bernoulli's principle
Everyone must have noticed that there is a yellow cordon on the platforms on both sides of the tracks when you take the train, and when you try to cross this cordon, there will be a staff member with a loudspeaker shouting at you (try not to try, Taishe is dead).
This is because, when the train passes, if the train passes without stopping, then the speed of the train will be very fast, and if you stand within the yellow line, the huge pressure generated by the fast-moving train is likely to push people towards the tracks, which is very dangerous.
This is Bernoulli's principle. This principle was proposed by the Swiss physicist Daniel Bernoulli in 1726. Generally speaking, when flowing at a constant height, the pressure is small where the flow velocity is large.
Bernoulli's principle is used to describe the motion of an ideal fluid and is not applicable to all fluids. However, in many cases, the fluids involved can be approximated as ideal fluids and can therefore be explained by Bernoulli's principle.
Bernoulli's principle can be understood in terms of conservation of energy. To put it simply, in an ideal fluid without viscous force, the sum of kinetic energy, potential energy and internal energy at each point in the flow is a constant, which is Bernoulli's equation.
Therefore, when the fluid flow velocity increases, its kinetic energy increases, and the corresponding internal energy and potential energy will decrease, and this potential energy includes the potential energy generated by the static pressure, that is, the flow velocity increases, and the pressure decreases.
This phenomenon is often encountered in our lives. I believe we all have this experience, when we take the high-speed train, when the two trains meet, the body will become unstable, this is because the two trains moving in opposite directions at high speed, the air flow between them is very fast, resulting in a huge pressure difference between the two sides of the train.
There is also the design of the wing of the aircraft, which makes use of this principle. When the airflow passes, the upper airflow travels a long distance and the speed is fast, so the pressure is low, so that the pressure difference between the upper and lower sides of the aircraft is formed, and the upward lifting force is generated.
We can also intuitively feel Bernoulli's principle. Pick up two vertically placed A4 sheets and blow into the middle, and you will notice that the two sheets will not be separated, but will be brought closer together.
To be continued - waiting for you to discover the beauty of physics
Physics is a science that explains the world, and its development is closely related to our perception of life and the world.
Sound, light, heat, force, and electricity are precisely in the process of understanding the world, and the construction of this discipline system has been improved step by step.
If you pay attention, you can see all kinds of physical phenomena around you. If you have any other physics knowledge around you, please leave a message in the comment area to add!
(Cover picture pick-up)
Resources:
1. Friction
2. Doppler effect
3. Bernoulli's principle
4、Bernoulli principle
5. Zhao Kaihua, Optics, Peking University Press
Editor: Ah Bai