Brief analysis

1. Sound needs media to spread. Sound cannot travel in a vacuum. The astronauts who landed on the moon had to talk by wireless phone even if they were very close.

2. human hearing has a certain frequency range, that is, the frequency is 20~20000Hz, and the sound waves below 20Hz are called infrasound waves. For example, the sound waves generated during tsunami and earthquake are infrasound waves; Sound waves with a frequency higher than 20000Hz are ultrasonic waves, such as B-ultrasound in hospitals. For ultrasonic waves and infrasound waves, people can't hear them.

3. In addition to the frequency, the condition that the human ear hears the sound is also related to the distance of the sound generator. If it is too far away from the sound generator, it can not cause the vibration of the eardrum or hear the sound after it is introduced into the human ear through the air.

2. Objects whose density is greater than water "may not" sink into the water.

Brief analysis

There are three situations when an object with density greater than water is placed in water, namely, sinking, suspending and floating. What kind of state it is in depends on the gravity and buoyancy of the object when it is completely immersed in water:

1, sinking. According to f float =Vρ water g and G=Vρ object g, because ρ water

2. Suspension, that is, when the gravity of the object caused by the hollowness inside the object is equal to the gravity of the boiled water discharged when it is immersed in water. (In the process of hollowing out, buoyancy remains unchanged and gravity gradually decreases)

3, floating, when the object is hollow, the object floats. (The hollowed-out part is large, so that the buoyancy is greater than gravity, and the object floats all the way out of the water, and the buoyancy is equal to gravity), such as a ship made of steel.

When the temperature of an object rises, "not necessarily" means that it absorbs heat.

Brief analysis

The increase in the temperature of an object can only show that the irregular thermal motion of molecules inside the object is accelerated and the internal energy of the object is increased. There are two ways to increase the internal energy of an object.

1, let the object absorb heat (heat transfer);

2. The outside world does act on objects.

For example, if the temperature of the saw blade rises, it may be baked in the furnace to absorb heat; It may also be that it has just sawed the wood, that is, by overcoming friction to do work, the internal energy will increase and the temperature will rise.

4. The object absorbs heat, and the temperature "does not necessarily" rise.

Brief analysis

When an object absorbs heat, the most direct change is that the internal energy of the object increases, but we know that the internal energy is the sum of all kinetic energy and potential energy of the molecules in the object.

1, if the state of an object remains unchanged after absorbing heat, that is, the molecular potential energy remains unchanged and only the kinetic energy of the molecule is changed, then the temperature of the object will rise. For example, if the iron block is heated, the temperature of the iron block will rise;

2. If the state of an object changes after absorbing heat, such as the melting of crystals and the boiling of liquids, although they are constantly absorbing heat, the temperature does not rise and remains unchanged. When amorphous absorbs heat, the kinetic energy and potential energy of molecules are changing, so the temperature rises with the change of state.

5. When an object is stressed, its motion state may not necessarily change.

Brief analysis

First, the force has two functions, 1, to change the shape of an object; 2. Change the motion state of the object. Therefore, when an object is subjected to a force, its motion state may not necessarily change.

Second, even if the function of force is to change the motion state of an object, the change of motion state is determined by the same effect of force on the object. 1. When an object is subjected to unbalanced force, its motion state will inevitably change (the magnitude or direction of motion speed changes). 2. When an object is subjected to a balanced force, its motion state shall not change (static or uniform linear motion).

6. A strong force acts on the object, and this force "does not necessarily" do work on the object.

Brief analysis

When a force does work on an object, two conditions must be met at the same time:

1, strong effect on objects; 2. The object moves a distance along the direction of force, and both are indispensable.

According to the formula W=F.S, it is concluded that the force has no distance and does no work. The most common phenomenon is "pushing without moving"; The most common phenomenon is that an object moves due to inertia, and the direction of the object's movement is perpendicular to the direction of the force.

7. Small magnetic needles attract each other near steel bars, and steel bars are not necessarily magnetic.

Brief analysis

There are two kinds of magnetic attraction: 1, and different magnetic poles attract each other; 2. Magnets have the property of attracting iron, cobalt, nickel and other substances. So it may be that iron, cobalt, nickel and other substances attract each other near the magnet, or it may be a magnet.

8. The actual power of "PZ220V40W" lamp is "not necessarily" 40W.

Brief analysis

1, when U real =U forehead =220V, the actual power of the light bulb prereal = p forehead =40W, and the light bulb normally emits light at this time;

2. When U is a real number < U, the actual power of the bulb is a real number < P, and the bulb is dim and cannot work normally.

3. When U is greater than U, the actual power of the bulb is greater than P. At this time, the bulb emits strong light, which shortens its life and is easy to burn.

9. An object immersed in water is "not necessarily" affected by buoyancy.

Brief analysis

Buoyancy is the difference between the upward and downward pressures of an object immersed in a liquid. Because the lower surface is deeply immersed in liquid, the pressure is always greater than the upper surface, so the direction of buoyancy is always vertical upward.

When the bottom of the object is closely combined with the bottom of the container without gaps (that is, it is equivalent to sticking together), the object is not affected by the pressure of the upward liquid, so it is not affected by buoyancy.

For example, one-third of the big stones immersed in mud at the bottom of the river are immersed in water, but the stones are not affected by buoyancy.

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

Brief analysis

The formula P=F/S is a general formula for calculating the pressure, and P=ρgh is a formula specially used for calculating the pressure generated by liquid. It can be seen from P=ρgh that when the density of the liquid is constant, the pressure generated by the liquid is only related to the depth h of the liquid. Then, according to F=PS, it is not difficult to see that the pressure generated by the liquid on the bottom of the container is determined by the density, depth and bottom area of the liquid.

That is, the pressure generated by the liquid at the bottom of the container: f = rhoghs. But only cylindrical container G liquid = mg = ρ VG =ρGHS = F. There are many shapes of containers. As long as they are not cylindrical containers, the volume of liquid inside is v≠hs, so F≠G liquid.

The container is filled with liquid, and the relationship between the pressure f of the liquid at the bottom of the container and the weight g of the liquid is: 1, the cylindrical container: F=G liquid 2, and the non-cylindrical container: F≠G liquid (wide-mouth container: F < G liquid necking container: F > G liquid).