1, Q: Why are all the blades of the fan odd, not even?

Answer: If the number of blades is even and symmetrically arranged, it is not only difficult to adjust the balance of the fan itself, but also easy to generate more * * * vibration when the fan runs at high speed, which leads to the fatigue of the blades caused by * * * vibration for a long time and eventually leads to fracture.

Therefore, the design of axial flow fan is mostly asymmetric odd blade design. The same concept is also embodied in the design of propeller helicopters.

2. Q: Why do cats and dogs always curl up in a ball when they sleep in winter?

A: Mathematically, under a certain volume, the object with the smallest surface area is a sphere.

Shrinking into a sphere can reduce the contact area with the outside world, reduce the speed of heat exchange, reduce the speed of heat dissipation in the body, save energy and maintain body temperature.

3. Q: Look at the following two line segments with arrows and guess which one is longer?

A: This is the famous "Mueller leye county illusion", also known as the arrow illusion. When two diagonal lines are added to both ends of a line segment and two diagonal lines are added to another line segment, the former is much longer than the latter.

There is a theory about this illusion, which is called nerve inhibition theory. It believes that when two contours are close to each other, the adjacent nerve groups on the retina will inhibit each other, and as a result, the contours will shift, resulting in illusion.

4. Q: We often say that "the weather is unpredictable". Why do weather forecasts sometimes go wrong?

A: This involves a mathematical definition-"chaos", that is, "extreme instability of initial values".

In general, the weather model basically follows a reasonable process, and the future weather changes can be predicted by several different simulation methods.

However, the weather is a combination of a series of complex factors. Small changes in initial conditions will make the forecast results very different. At this time, the weather has entered a chaotic area. The longer the prediction time, the greater the possibility of reaching the chaotic point, and the more difficult it is to grasp the accuracy.

5. Why do weather forecasts sometimes go wrong?

I have been paying attention to the weather in Xi 'an these days, and I am confidently waiting for the next "blizzard" in Xi 'an. The weather forecast also predicts a "blizzard", but it doesn't snow unless necessary, and there is almost no snow. What's going on here?

Generally speaking, the global weather model basically follows some known reasonable processes. Through several different simulation methods, according to slightly different initial conditions, the weatherman can predict the future weather changes. Here is "the possibility of inference, not absolute".

However, the weather is a combination of a series of complex factors. Small changes in initial conditions will make the forecast results very different. At this time, the weather has entered a chaotic area. The longer the forecast time, the more likely it is to reach the chaotic point, so the more difficult it is to grasp the accuracy of weather forecast. Of course, with the progress of modern science and technology, the accuracy of weather forecast will be higher and higher, that is, the "possibility" will be greater and greater.