Qianjiang Evening News Hourly News reporter Chen Suping
The Long March 5B rocket launches the core module of the Tianhe
On April 29, 2021, China successfully launched the core module of Tianhe on the Tiangong space station with the Long March 5B carrier rocket. 170 seconds after the Takeoff of the Long March 5B, the four boosters burned out and fell into the Pacific Ocean at a lower speed away from the rocket. The core stage rocket can remain burning for more than 400 seconds, continuing to help the rocket accelerate until it sends the core capsule into an orbit nearly 400 kilometers high.
Structural diagram of the launch of the rocket (left), launch control hall scene (right)
Unlike low-altitude dropped boosters, core rockets and core capsules have high velocities when separated at high altitudes, allowing them to orbit the earth outside the atmosphere for several days before slowing down under the weak resistance of cosmic dust and being sucked back into the atmosphere by the earth.
The separation of the booster at the time of launch, the separated booster itself does not burn
Massy, an astronomer at the non-profit organization "Virtual Telescope", took the first photo of the wreckage of the Long March rocket slowly entering the atmosphere in Italy on the 8th. The wreckage can be seen burning in glow.
The China Manned Space Engineering Office issued a statement on May 9 that, after monitoring and analysis, at 10:24 on May 9, the last stage of the Long March 5B rocket had re-entered the atmosphere, and most of the devices were ablated and destroyed during the re-entry into the atmosphere. The last stage wreckage here is mainly a fuel-depleted core rocket.
According to russia's TASS news agency reported on May 10, the wreckage of the Long March 5 rocket has entered the atmosphere and crashed into the vicinity of the Maldives Islands, most of the rocket was burned during the crash, only a small part fell into the sea, and did not cause any casualties or losses.
The final drop point of the wreckage of the Long March 5B
Why does the final debris burn when it enters the atmosphere? What convenience or trouble does the friction that can generate heat bring to our lives? In this issue of Science Exchange, we invite Dr. Lv Bin, deputy director of the Department of Physics of the School of Science of Zhejiang University of Technology, to answer questions for the students.
Why does the last stage of debris burn when it enters the atmosphere?
Objects passing through the atmosphere burn at high temperatures, which are caused by two factors: the air is compressed by the high-speed impact of the object and causes heating; the friction between the object and the air heats up.
The physical mechanism of the former is that when the speed of moving objects exceeds the speed of sound, the air will be too late to avoid the high-speed moving objects, causing the pressure of air on moving objects to increase sharply, forming a sound barrier. This pressure in turn compresses the air, which is heated up by the compressed air. The reason for the heating up of the compressed gas is similar to using a pump to inflate, and during the process you touch the cylinder and find that it is very hot.
The physical mechanism of the latter is the protagonist of today's article: frictional heat generation.
Sound barrier
friction
After the wreckage enters the atmosphere, what layers of earth does it have to rub against?
Distribution of the atmosphere on the Earth's surface
The rocket wreckage from a height of 400 km, after several days of flight around the ground, landed below 200 km, into the denser air mesosphere, stratosphere and troposphere, friction mainly occurred in these layers.
Magnesium burning
Microscopic illumination of ablative materials based on carbon fibers
Why does such friction cause the wreckage to burn? The temperature that can make an object burn is called the ignition point, and the flash point of different objects is different. When the object is heated to the point of ignition, it can be chemically reacted with oxygen to burn. Middle school teachers should have shown their classmates the combustion of magnesium in oxygen, and the ignition point of magnesium is more than 500 degrees. The ablation material on the surface of the rocket is mainly carbon fiber. As with drilling wood for fire, when the heat generated by friction cannot be dissipated in time, the temperature of the object will rise until the ignition point finally burns with oxygen in the atmosphere.
So why didn't the rocket burn when it lifted off? This is due to the fact that the friction received by the rocket when it lifts off is much less than the friction received when the wreckage lands on the ground.
Rocket liftoff
The escape pod returns to the ground
In general, the friction of air is proportional to the velocity of the object and the density of the air.
When the rocket lifts off, it passes through the troposphere with the highest air density from the surface, but the speed of movement is relatively small at this time, and the speed of movement is the largest when it reaches the warm layer with small air density.
On the contrary, when the wreckage returns, the velocity in the warm layer is small (here relatively speaking, the speed is actually not small, about 7.68km/s, students can use the gravitational and centripetal force formula to calculate it), because it has been accelerated by the earth's gravity, the faster it falls to the place where the height is lower and the air density is greater (students can use uniform acceleration linear motion to roughly calculate, and the landing speed is about 8.2km/s).
Therefore, the friction of the fall is much greater than the friction during liftoff.
What convenience or trouble does friction bring to us?
The friction can not be seen and touched, how strong is it? Let's first do a scientific experiment "chopsticks to rice": insert chopsticks into a bottle filled with rice, compact the rice grains, hold the top of the chopsticks and lift them up, and the whole bottle of rice is also lifted. This is caused by the great friction between chopsticks and rice and between rice and rice.
As another example, people walk by using the friction between shoes and the ground. Usually walking, if the shoes and the ground do not have friction, we will not be able to walk. It can be seen that in daily life, friction is very important to human beings.
In general, frictional forces are divided into sliding friction, static friction and rolling friction.
Related to sliding friction, there is a legend in China that the flint man makes a fire. Fire makes cooked food readily available to humans, reduces disease, and promotes brain development and institutional evolution. Fire also brings warmth to humans, thus expanding the range of human activities. And the way the flint people make fire is to drill wood for fire. They reached the ignition point by drilling wood to make the sliding friction between the woods heat up, so that the wood could burn to obtain a fire.
Static friction is more common. Friction between shoes and the ground when walking, friction between hands and sides of bottle caps when twisting, friction between objects and straps when the conveyor belt transfers objects. Smart students will find that static friction is the friction between two relatively stationary objects. When walking slips, it becomes sliding friction.
Finally, rolling friction, the essence is static friction. The presence of rolling friction allows round objects to roll on the ground. Since the rolling friction is much smaller than the sliding friction, rolling handling is far less laborious than dragging and handling.
Of course, in addition to convenience, friction can also bring some trouble. For example, the loss of energy. When the high-speed rail is in motion, it will be subjected to a lot of air friction, which makes the train have to use a large amount of driving force to resist this resistance to maintain the speed of the train, resulting in additional energy loss.
Interest groups
The secret of friction
Knowing the different classifications of friction, the size and direction of friction, and what is it related to?
Taking static friction as an example, people need to pedal to the left when they go to the right, so at the contact point between the shoe and the ground, the movement trend of the shoe is to the left, so the shoe is subjected to static friction to the right. Knock on the blackboard here, draw an important point: the determination of the direction of friction must carefully observe the relative motion trend of the two objects at the contact point, no matter what type of friction, the direction of the friction force on the object is the opposite of the direction of the object's movement or the direction of motion trend at the contact point.
As for the size of the friction force, according to experimental experience, it is proportional to the friction coefficient of the contact surface of the two objects and also proportional to the pressure between the two objects. Students think about it, when you can't unscrew the bottle cap, what will you do? Will the edge of the cap be pressed tighter with more force? This is to increase the friction by increasing the pressure, thereby unscrewing the cap. But if your hands hurt or you can't unscrew, what can you do? The cap is usually wrapped in a dry rag and unscrewed by increasing the coefficient of friction. If the students still can't do it, then you can go to the father who has a lot of strength to help.
Some students asked curiously: "Is there friction in the vacuum environment?" "Macroscopically, two objects that are in contact with each other have friction, and the objects in the vacuum environment do not come into contact with other substances, so there is no friction.
The reason why the core stage rocket wreckage of the Long March 5B launched this time will eventually return to the ground is because there are still a small number of molecules distributed in orbit at an altitude of 400 km, so the core stage rocket will gradually slow down under weak friction, and when the speed cannot maintain its movement in orbit, it will be attracted into the atmosphere by the gravity of the earth and fall.
Test you
(1) What type of friction is this pair of shoes that are stationary on a wooden board in the picture below, and where is the direction of its friction force?
(2) The bicycle is driven by the rear wheel, that is to say, the rear wheel is the active wheel, and the front wheel is the passive wheel. A student rides to school, the bicycle wheels shown in the picture below rolling clockwise to the right, please determine the direction of rolling friction on the rear and front wheels.
answer:
(1) Static friction. Shoes have a tendency to move along the plank, so the friction is along the plank to the upper right.
(2) The rear wheel is actively rotated clockwise, and the contact point on the ground is a backward movement trend, so the friction of the roller affected by the rear wheel is forward. The front wheel is passive clockwise rotating, if there is no friction, the front wheel at the ground contact point is inclined to slide forward, so the rolling friction of the front wheel is backward. So the friction of the rear wheel is the driving force for the bicycle to move forward, and the friction of the front wheel is the resistance to the forward.
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Source: Original text