What are the important knowledge points from Grade 4 Unit 1 to Unit 8?

The important knowledge from unit 1 to unit 8 in grade four is the weather, which refers to the atmospheric conditions in the area where we live at a certain moment. Weather information is often described by weather characteristics such as cloud cover, precipitation, temperature, wind direction and wind speed.

A weather calendar is a table that records various weather phenomena every day. In our weather calendar, we usually record the date and time, temperature, cloud cover, precipitation, wind direction and wind speed. Observing the weather should be at the same time and place every day. Temperature refers to the outdoor temperature in a cool and ventilated place, and the temperature should be measured at the same time every day. The outdoor temperature in a cool and ventilated place can reflect the local temperature.

Knowledge interpretation

Sound is produced by the vibration of objects. Everything that happens is vibrating. Sound travels in waves. When sound waves meet an object, it will make the object vibrate. This is how sound travels from one place to another through various substances. The propagation speed is generally solid-liquid-gas. The level of sound can be described by pitch.

Pitch is determined by the speed at which the object vibrates. The faster the object vibrates, the higher the sound. The slower the object vibrates, the lower the sound. The unit of pitch is hertz. Air is an important substance for transmitting sound, and sound cannot be transmitted in a vacuum. On the moon, because there is no air, that is, there is no substance that can transmit vibration, two people can't talk to each other even if they are not far apart, so they must use radio equipment.

The human ear consists of outer ear, middle ear and inner ear. The vibrating object vibrates the surrounding air, and the auricle of the external ear transmits the collected sound to the eardrum through the ear canal, causing the eardrum to vibrate.

The ossicle of the middle ear transmits vibration to the inner ear filled with fluid, and the fluid vibration caused by it stimulates the auditory nerve to generate signals, which are accepted by the brain, and we feel the sound. The eardrum is very thin and elastic, even if it is very light, it will vibrate.