Two questions about physics in the second year of junior high school. Physics problems in the second

25-2)/(25+1)=(60-2)(t+1)t=

95-5) (100-0) = (32-5) (t-0) t = 30 degrees Celsius.

Principle: The ratio of the length of the mercury column is unchanged (proportional).

1: Late, because sound travels slower through the air than the speed of light.

Since the propagation speed of light is 3*10 8m s, it is negligible because the value of the speed of light is too large. The speed of sound in the air at room temperature is 340m s, so according to the time = distance Speed = 100m 340m s, so it is later than when you see smoke.

It is more accurate to watch the smoke timekeeping, because the speed of light is much greater than the speed of sound, so the error in the timing is minimal.

2: The speed of light is 3*10 8m s, because the distance between the sun and the earth is so according to time = distance Speed = minutes 20 seconds.

Because it is 12 noon, the time of daylight emitted by the sun = 12 o'clock - 8 minutes 20 seconds = 11:51 minutes 40 seconds.

The relationship between A and the standard temperature is: A 1 scale = 1 standard temperature * 25 23, and the relationship between B and the standard temperature is: B 1 scale = 1 standard temperature * 25 26, so when A temperature shows 60 degrees, B means number = 60 * (25 23) * (26 25) = degrees.

When the title is properly (95+5) 2=50 degrees, the actual temperature is equal to the indication, and the actual temperature = (50-5) 50 32 = degrees.

The head you spoke of earlier is the Word. According to Archimedes' law. The magnitude of the buoyancy of the object.

Calculation method: is the density of the liquid multiplied by g and multiplied by the volume of the liquid dislodged by the object. So.

The greater the volume of the liquid expelled by the object. The more buoyant it is. So.

An object that is completely submerged in a liquid experiences more buoyancy than a partially submerged liquid. So the buoyancy experienced by an object floating on the surface of the liquid is less than the buoyancy experienced by him when he is fully submerged in the liquid. So far, it's completely correct.

The first why? Why is the buoyancy greater than the gravitational force when an object is floating on the water. This statement is wrong.

When an object floats on water, the buoyant force is equal to the gravitational force. At this time, the two forces are balanced. a: Gravity and buoyancy are equal in magnitude and opposite in direction.

When levitating, the gravitational force is equal to the buoyant force. The buoyancy of the object when it sinks to the bottom is less than the c-gravity force.

The three gravity forces here, I used the three letters a, b, and c to distinguish them. Because these are supposed to be three different objects. While floating and levitating.

Silent in liquids. The volume of objects is not the same. So.

It is not possible for the same object to exhibit two different states. It is both capable of floating. It is also able to levitate.

Explain. A floating object must have a portion of its volume exposed. and not parallel to the surface of the water).

So. Suppose the gravity of the three objects you are talking about does not change. So.

Their volume varies. The floating volume is the largest. He didn't need to be completely submerged in the buoyancy generated by the rest of the water, it was already equal to its gravity.

The volume that sinks to the bottom of the water is the smallest. Because it produces less buoyancy than his gravitational force. So much so that he also needs the bottom of the container to give him support.

Friend, the heavy noise of the object does not change the force, then under the same liquid conditions, in the absence of external force, its relationship with the liquid is certain. Masaeda: Stress analysis, draw a draw, and then analyze. Remember that when sinking to the bottom, there is the support of the bottom of the water to raise the stove.

Go for the same quality brine.

Place two pieces of wood of equal mass in salt water.

Look at the height at which the block floats.

The density is so dense that it floats greatly.

1. It should be that the closer the object is to the convex lens, the smaller the image.

2.Concave lenses have a converging effect.

Because the concave lens is upright and narrows the virtual image, it is consistent with the situation seen by the cat's eye3The virtual image is not necessarily large (the virtual image made of the concave lens is reduced), the virtual image made of the plane mirror is equal to the large, the virtual image made of the concave lens is reduced, and the virtual image made of the convex lens is magnified.

1.All three people run in the same direction (using the ground as a reference).

2.There is wind, because the speed of B is the same as the wind speed, so it feels that there is no wind; A's speed is smaller than the speed of the wind, so it is thought to be a tailwind; C's speed is greater than the speed of the wind, so it is considered to be a headwind.

3.The wind is in the same direction as the trio is moving.

Because C feels headwinded.

I take it by all means.

A downwind means that he is running in the same direction as the wind.

Or if his direction is opposite to the direction of the wind and his speed is greater than the wind speed, B has no wind indicating that his speed is the same as the wind speed.

In the opposite direction, the direction of the C run must be opposite to the direction of the wind.

And the speed is less than the wind direction, question 3.

The wind direction is the same as that of the first run.

The opposite of ethyl-propylene is the opposite.

ko!@

1) Imaging with a convex lens, when forming a virtual image, the closer the object is to the convex lens, the smaller the magnification of the image.

2) Because the concave lens is upright and narrows the virtual image, it is consistent with the cat's eye.

3) The virtual image can be large or small, the virtual image made of the plane mirror is equal to the large, the virtual image made of the concave lens is reduced, and the virtual image made of the convex lens is magnified.

Hey, there are so many problems, but my brother is happy today and still helps you solve one: first of all, to verify that light travels in the air in a straight line, you must set up a controlled experiment to observe the propagation of light in other media. Step 1:

Observe the propagation of light in the smoke Wrap a paper funnel in a white cardboard paper and go deep into the Coke bottle Then light the mosquito coil with a match Collect the smoke Finally, shoot the laser through the bottle Observe its propagation Step 2: Shoot the laser directly through the glass brick Observe its propagation Step 3: Wrap the cardboard into a cylinder Shoot the laser from one end into the other to be observed.

Two: the principle of point-and-shoot camera is designed and manufactured according to the human eye When the object is very close to the lens, the image will become larger, and the image will become larger on the retina after corneal magnification, and in the long run, the cornea will become thicker, and the image will be said to be in front of the retina, and the image must be presented to the retina with concave lenses of different powers.

Three: degree d = 100 f

f is the focal length. The focal length unit is meters, so f=100 d algebra data get meters four:1: lips are prone to splitting due to dryness; 2：

Clothes dry faster Five: Although the amount of gasoline contained in plastic barrels is small, the volume of the barrel is small From the principle of ** It can be seen that the rapid combustion of combustibles in a limited space is very easy to cause** Therefore, for the sake of prevention and safety, it is forbidden to use plastic barrels for gasoline.

1、b.Both A and B are positively charged (because triboelectric is one object that loses electrons and becomes positively charged, and another object gains electrons that become negatively charged, it is impossible for both to be positive).

2、b.Mutual repulsion (rubbing the fur against the rubber rod, after rubbing, the rubber rod is negatively charged while the fur is positively charged; The glass rod rubbed by silk is also positively charged, so the fur is repelled when it is close to the glass rod).

3. There is both a positive charge and a negative charge on the glass rod, when the silk rubs the glass rod, the glass rod is easy to lose electrons (electrons are negatively charged), and the negative charge is less than the glass rod shows a positive charge, and at the same time, the silk gets negatively charged electrons from the glass rod, so its negative charge increases, so the silk shows a negative charge. In short, triboelectric charging is not caused by the generation of electric charges, but by the transfer of electric charges between objects rubbing against each other.

There are 3 possibilities for this question, 1 A and B did not give electrons to each other, nor did they lose electrons, (electrons are negatively charged,) so they are not charged.

2 A loses electrons, and A, who originally had an equal amount of positive and negative charge, has a positive charge because he loses his negatively charged electrons, and B has a negative charge because he gains electrons. 3 means that A gains electrons and B loses electrons.

b The rubber rod rubbed against the fur has a negative charge, then the fur belt rubbed against the rubber rod has a positive charge, and the silk rubbed glass rod has a positive charge, so the same charge repels each other.

Question 1 A Triboelectric Electrification Question 2 A Heterogeneous charges attract each other Atoms are composed of nuclei and electrons, electrons (negatively charged) move outside the nucleus (positively charged), different atoms have different electrons, and some electrons in the same atom are close to the nucleus and some are far away from the nucleus. You are only in the second year of junior high school can understand the loss of electrons in this way, because the electrons are negatively attracted to the nucleus, the nucleus attracts the electrons of other atoms to gain electrons, and the electrons of the atom are attracted away by other nuclei to lose electrons.

, you can use the method of elimination, first of all, a can be obtained by common sense, for example, if you take two books and rub them, you will not be electrified; If you choose one of the CDs, then the other one should also be selected, but because it is a single choice, you can't choose both, so the CDs are all wrong, so choose B.

However, it can be seen from the common sense of physics that if an object appears to be positively charged, its electrons move to another object, making it negatively charged, and it is impossible for two objects to have the same kind of charge after friction;

Since the fur belt is negatively charged and the glass rod is positively charged, they should be attracted to each other3This memory is not too clear, it has to do with the transfer of electrons. And there are no shortcuts, you only have to meet one and remember one.

b b When the silk rubs the glass rod, because the silk has a strong ability to bind the electrons, the electrons on the glass rod are transferred to the silk, so it is positively charged.

The essence of the dotted object is the transfer of electrons. Either one loses electrons, one electrons, or both objects lose electrons at all. It is impossible for both objects to lose electrons, that is, to be positively charged.

The fur belt that has been rubbed against the rubber rod is positively charged, and the silk rubbed glass rod is positively charged. The same kind of charge repells each other.

b The friction of two objects is the process of seizing electrons from each other, that is, the relationship between being deprived of electrons (now positively charged) and seizing electrons (now negatively charged), and the two objects may have the same binding force on electrons by atomic nuclei, so they cannot take each other's electrons from each other, and they are not charged; And when the nucleus of A has a stronger binding force on electrons than B, A is negatively charged, and vice versa.

The second course, a textbook foundation, how to learn.

3. If the binding force of A's nucleus to the electron is stronger than B's nucleus, A will take electrons from B, and the electrons are now negatively charged, and if there is more, it is negatively charged, on the contrary, B loses electrons, and the charged characteristics of the nucleus are manifested, and it is positively charged (the nucleus is positively charged, and the electrons are negatively charged).

3. The specific characteristics of which two materials are positive and which are negatively charged after rubbing against each other is determined by the properties of the materials, and the level of the second year of junior high school cannot be understood.