Common sense of life (1)

Why do high jumpers run?

High jumpers can jump over the crossbar with the help of the inertia force of run-up and the supporting reaction force of take-off and landing. Because the direction of inertia force is horizontally forward and the supporting reaction force is vertically (or nearly vertically) upward, the body center of gravity after take-off moves along a parabolic trajectory. The height of this parabolic trajectory depends on the initial take-off speed and take-off angle, that is, the initial take-off speed and take-off angle are the key to increase the height of high jump. Generally speaking, these two values should be increased as much as possible. The maximum takeoff angle is 90 degrees. However, because the high jump is not a simple vertical upward movement, it must have forward strength to cross the crossbar; Furthermore, we must make full use of the horizontal speed to improve the initial takeoff speed, so the takeoff angle should be less than 90 degrees. As for the initial speed of take-off, it is closely related to the quality and technical proficiency of athletes. The higher the initial speed of Teng, the higher the jump. When the takeoff angle is constant, the initial takeoff speed plays a decisive role.

Why can you "drink" soda through a straw?

This is a common phenomenon in life. When the mouth has not inhaled from the tube, the liquid level inside and outside the tube is flat. At this point, the gas pressure on the liquid surface inside and outside the tube is equal; When the nozzle draws air from the pipe, the gas in the pipe decreases and the pressure on the liquid level in the pipe also decreases. At this time, the gas pressure acting on the liquid level inside the tube is less than the atmospheric pressure acting on the liquid level outside the tube. So we say that the reason for this phenomenon is the result of atmospheric pressure. When drinking soda, you should first insert the tube into the soda. When the mouth inhales, part of the gas in the tube will be sucked into the mouth, resulting in a larger volume of the remaining gas in the tube and a smaller pressure, which is less than the atmospheric pressure outside the tube. Therefore, under the action of atmospheric pressure outside the pipe, soda will rise along the pipe and be sucked into the mouth.

Why can thermos bottles keep warm?

There are three ways of heat transfer: thermal convection, thermal conduction and thermal radiation. Thermal convection mainly occurs between liquid and gas. The heat flow rises, the cold flow falls, and the dynamic balance is achieved through continuous circulation. The conduction of heat occurs on the thermal conductor, and the heat is conducted from the high temperature end to the low temperature end. The radiation of heat does not need medium, but transfers heat to low temperature through radiation. There is air between the bottle liner and the thermos shell. Air is a poor conductor of heat, which will reduce heat conduction. The inside of the bottle liner is as smooth as a mirror, which reduces radiation, so the thermos can keep warm.

Boiled eggs are easy to shell when soaked in cold water?

To understand this problem, we must first know what role water plays in this process. Most of the substances we encounter, except a few, have the physical property of "expanding with heat and contracting with cold". However, the degree of expansion and contraction of various substances is different. An egg consists of a hard shell, a soft protein and a yolk, and their expansion and contraction are different. When the temperature changes slightly or evenly, it doesn't show anything, but when the temperature changes sharply, the pace of protein and eggshell is different. When boiled eggs are suddenly soaked in cold water, the temperature of cold water changes greatly. The eggshell suddenly contracted. Protein is still at its initial temperature, and its size has not shrunk. At this time, a part of protein was squeezed into the short position of the egg by the eggshell, and then the protein gradually contracted due to the gradual decrease of temperature. Because protein, eggshell and yolk have different contractions, it forms the separation of protein and yolk. Therefore, if you peel it off, neither the eggshell nor the meat will come down.

Common sense of life (2)

Special feeling on the elevator

"Overweight" and "weightlessness" are two physical phenomena, and everything on the earth is affected by gravity. If an object is forced to accelerate upward against gravity, it will be overweight. If an object accelerates downward along gravity, it will show weightlessness.

Is the person who got an electric shock "sucked" by electricity?

It is often said that when you get an electric shock, people will be attracted by it and can't pull it out.

Actually, this statement is wrong. As we know, whether there is current or not, generally speaking, the positive and negative charges inside the positive conductor and the electrical appliance are equal, and the external electrostatic interaction cancels each other out. Even if there are occasional positive and negative charges in some places, they are not equal, and their electrostatic attraction can be ignored. But the problem has arisen. When people get an electric shock, why don't they sometimes pull their hands back? Don't you want to smoke it back? Obviously, it's stuck. I can't get it back. This problem can be explained by the physiological effect of current.

When people get an electric shock, their hands will change from spasm to paralysis due to the stimulation of current. Even if the command to withdraw the hand is issued, the helpless hand can no longer carry out this command. Investigation shows that the vast majority of people who died of electric shock were electrocuted in the palm of their hands or fingers on the same side as the palm of their hands. When just getting an electric shock, the hand bends due to conditioned reflex, and the bending direction is just for the hand to unconsciously hold the wire. In this way, the electric shock time is prolonged, and the hand will soon spasm and paralysis. At this time, even if you want to loosen your fingers and retract your arms, it is impossible. It looks like you have been "sucked". If the electric shock lasts a little longer, people's central nervous system will be paralyzed, and they will not hold hands at this time. These processes all took place in a relatively short time.

If you get an electric shock on the back of your hand, ordinary civil electricity is not easy to cause death. Experienced electricians, in order to judge whether the electric appliance leaks electricity, don't have a pencil at hand, and sometimes touch the shell of the electric appliance with their forefinger nails. If there is electricity leakage, the index finger will bend due to the reverse situation, and the bending direction is just the direction away from the electrical appliance. This shock time is short and there is no danger. Of course, the voltage is very high, so there will be danger.

Common sense of household electricity saving

Lighting energy-saving fluorescent lamp has the characteristics of high luminous efficiency, soft light, long life and low power consumption. 14 watt energy-saving fluorescent lamp is equivalent to the brightness of 75 watt incandescent lamp, so replacing incandescent lamp with fluorescent lamp can greatly reduce power consumption. Low-power fluorescent lamps can be installed in corridors and bathrooms. When watching TV, only turn on 1 watt energy-saving fluorescent lamp, which not only saves electricity, but also has ideal viewing effect. We should also turn off the lights and destroy the ever-burning lamps.

The brightest state of TV consumes 50-60% more power than the darkest state. The louder the volume, the greater the power consumption. So when watching TV, the brightness and volume should be adjusted to make people feel the best, not too bright and not too loud. This can not only save electricity, but also help to prolong the service life of TV sets. As long as some TV sets are plugged in, the picture tube will preheat, consuming 6 ~ 8 watts. So after the TV is turned off, you have to unplug it from the power outlet.

The refrigerator should be placed in a cool and ventilated place, never close to the heat source to ensure that the heat sink can dissipate heat well. When using, try to reduce the number and time of opening the door. The food in the refrigerator should not be too full, and there should be gaps between the foods to facilitate the convection of cold air. Frozen food to be eaten should be slowly melted in the freezer in advance, which can reduce the temperature of the freezer and save electricity.

The power consumption of the washing machine depends on the rated power of the motor and the length of service time. The power of the motor is fixed, so reducing the washing time appropriately can save electricity. The length of washing time depends on the type of clothes and the degree of dirt. Generally, the time for washing fine clothes such as silk can be shorter, and the time for washing thick fabrics such as cotton and hemp can be slightly longer. If you rinse with a washing machine, you can wring out soapy water or washing powder foam on clothes before rinsing, which not only saves electricity, but also reduces the number of rinsing and achieves the purpose of saving electricity.

Electric fans save electricity. Generally, electric fans with large blades have large electric power and consume a lot of electric energy. The power consumption difference between the fastest gear and the slowest gear of the same electric fan is about 40%, and the power consumption of the fast gear 1 hour can be used for nearly 2 hours in the slow gear. Therefore, slow speed is often used to reduce the power consumption of electric fans.