Junior high school physics: 10 "not necessarily" in junior high school physics, do you know?

Junior high school physics: 10 "not necessarily" in junior high school physics, do you know?

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1. Objects are vibrating, we "not necessarily" can hear the sound

1. The dissemination of the sound requires a medium. The sound in the vacuum cannot be spread. Even the astronauts who board the moon must rely on wireless telephone to talk even if they are close.

2. Human hearing has a certain frequency range, that is, 20 ~ 20000Hz, the sound waves with frequency below 20Hz are called secondary sound waves. If the tsunami occurs when the tsunami occurs, the sound waves generated during the earthquake are secondary sound waves; Such as the B -ultrasound in the hospital. It is impossible to hear the ultrasonic and secondary ears.

3. In addition to the condition of human ears, in addition to the frequency, it is also related to the distance of the echo. If the sound is too far away, it cannot cause the vibration of the tympanic membrane behind the human ear to pass through the air.

Second, the density is greater than the water, "not necessarily" sinking in the water

There are three cases of objects with density than water, sinking, suspended, and floating. What kind of state is in which state is related to the gravity and buoyancy that the object is immersed in the water:

1. Sinking. According to F floating = v 根据 根据 g and g = V 根据 g, because the water <ρ 根据, F floating, and the object sinks. At this time, the object is solid. For example: the iron block is sinking in the water.

2. Suspension, the object is suspended when the gravity of the object is soaked in the water in the water that is not drained in the water. (In the process of hollowing up, the buoyancy remains unchanged, and the gravity gradually decreases)

3. Floating, when the inside of the object is hollow and hollow, the object floats. (The hollowing part is larger, making the buoyancy greater than gravity, and the objects float until the surface is surfaced, and the buoyancy is equal to gravity again.) For example, the ship made of steel.

Third, the temperature of the object has risen, "not necessarily" is to absorb the calories

The temperature of objects has risen, and it can only show that the non -regular heat movement of the internal molecular inside the object has accelerated, and the internal energy of the object can increase. There are two ways to increase the objects in the object.

1. Make objects absorb heat (heat transmission);

2. The outside world does work (work).

For example: The temperature of a sawing side increased, and it might bake it with a stove to absorb the heat; it may also be seen that the internal can increase by overcoming the friction and increase the temperature.

Fourth, the object absorbs heat, the temperature "not necessarily" increases

The most direct change in the absorption of objects is to increase the energy in the object, but we know that the internal energy is the sum of all molecular kinetic energy inside the object and the potential energy.

1. If the state of the object does not change after absorbing the heat, that is, the molecular potential energy is unchanged, and only the molecular kinetic energy is changed, the temperature of the object will increase, such as heating the iron block, the temperature of the iron block increases; the temperature of the iron block increases;

2. If the state of the object changes after absorbing heat, such as the crystal melting, the liquid boiling, although the heat is constantly absorbing the calories, the temperature does not rise, and the temperature remains unchanged. When non -crystal absorbs heat, the dynamics and potential energy of the molecule are changing, so while the state changes, the temperature also increases

Fifth, the effect of the object's force, the "not necessarily" of the movement of the movement changes

First, force has two effects,

1. Change the shape of the object;

2. Change the motion of the object. Therefore, the effect of the object's force does not necessarily change the state of exercise.

Second, the effect of the power is to change the state of movement of the object. The change in the state of movement is determined by the common effect of the object.

1. When the object is influenced by non -balanced force, the state of exercise must be changed (the size of the exercise speed or direction is changed).

2. When the object is affected by the balance, the motion state must not change (static or uniform straight movement).

6. Powerful effect on objects, the force "not necessarily" does work on objects

For the force on objects, two conditions must be met at the same time:

1. strong effect on objects;

2. The distance moved in the direction of the object, and the two were indispensable.

According to the formula W = F.S: powerful, no distance, no merit, so -called labor and no merit, the most common phenomenon is "pushing and unmoved"; The phenomenon is when the object is perpendicular to the direction of the object and the direction of the object movement.

7. Small magnetic needles are close to the steel rod attracting each other, and the steel rod is "not necessarily" magnetic

There are two situations in the attraction in magnetic phenomena:

1. Magnetic poles attract each other;

2. Magnetic has the nature of attracting iron, cobalt, nickel and other substances. Therefore, it may be iron, cobalt, nickel and other substances that are close to each other, or maybe the magnet

8. "PZ220V40W" electric light, actual power "not necessarily" is 40W

1. When U actual = u quota = 220V, the actual power of the light bulb is real = p quota = 40W, at this time the light bulb is glowing normally;

2. And U solid

3. When U actual> U, the actual power of the light bulb is real> P querous. At this time, the light bulb emits strong light, and the life span is shortened.

Nine, objects immersed in water, "not necessarily" the role of buoyancy

The buoyancy is the difference between the objects immersed in the liquid by the liquid to the upward and downward pressure of the object, because the lower surface is immersed in the liquid

The liquid is deeper, and the pressure is always greater than the surface, so the direction of buoyancy is always vertical upward.

When the bottom of the object is closely bonded with the bottom of the container, when seamless gap (that is, it is equivalent to sticking together), the object is not pressured by the upward liquid, so it is not affected by the effect of buoyancy. For example, the large stones in the mud in the bottom of the river, one -third of the exposed mud is immersed in the water, but the stone is not affected by buoyancy.

10. The pressure on the bottom of the container is not necessarily equal to the gravity of the liquid in the container

Formula P = f/s is a general formula for calculating pressure, and P = ρGh is a formula for seeking liquid to produce pressure. From p = ρGh, we can see that when the density of the liquid is certain, the liquid generates generated by the liquid. The pressure is only related to the depth H of the liquid, and then it is not difficult to see that the pressure generated by the liquid to the bottom of the container is determined by the density of the liquid to the bottom of the container.

That is, the pressure of liquid to the bottom of the container: f = ρGhs. However, only the columnic gyulizer = mg = ρvg = ρGhs = f. There are many types of containers. As long as it is not the volume of the internal liquid of the column, the volume of the liquid of the pillar, so F 而 G liquid.

The relationship between the liquid in the container, the pressure of the liquid to the bottom of the container and the liquid Global is:

1. Pillar describer: F = G liquid

2. Non -columnic describer: F 2 G liquid (wide mouth container: FG liquid)

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