Give 20 points! Physics questions in the second year of junior high school, physics problems in the

Analysis: The volume of water v=sh=700cm*30cm=21000cm

The mass of water m = v = 1g cm * 21000cm = 21000g = 21kg

The pressure of water on the bottom of the barrel p= gh = (1000*10* (1g cm = 1000kg m).

So the pressure of the bucket on the ground p'=4000pa

So the total gravity g=f=p's=4000pa*700*10^-4 m²=280n

Total mass m = g g = 28kg

So m water = 28kg - 1kg = 27kg

When the pressure of the bucket on the ground is 1800pa, the pressure f'=ps=1800pa*700*10^-4 m²=126n

So the pulling force f=g-f'=154n

A: The mass of the water in the bucket is 27kg, and the size of f is 154n

The pressure of water on the bottom of the bucket is gh=1000*10*Pa.

Then the pressure of the bucket on the ground is 3000+1000=4000Pa.

The bottom area of the bucket is 700cm =

The pressure of the bucket on the ground is 4000*

So the weight of the bucket + water is 280 10=28 kg, so the weight of the water is 28-1=27 kg

When the child applies an upward force f to the bucket.

The pressure of the barrel on the ground is 280n-f=1800*The solution gives f= 280-126=154 n

Let the quality of the water in the bucket be m,1By this sentence:"As shown in Figure 8a, the pressure of water on the bottom of the bucket is 1000pa less than the pressure of the bucket on the ground"An equation can be listed:

It can be solved: m = 27 kg

2.By this sentence:"When Xiao Ming lifts the bucket with a vertical upward force f, but does not lift it, as shown in Figure 8B, the pressure of the bucket on the ground is 1800pa (g takes 10n kg)."An equation can be listed:

27+1)g-f=1800*7*10 -2 can be solved: f=406 n

3.The first 4Infrared.

56.Parallel connection.

Series short circuit. 7.Amplify the effect.

The louder the louder, the greater the amplitude.

8.Make the sunlight shine parallel to the main light axis on the lens, and place the white paper underneath to find the smallest and brightest point to measure its distance.

It is not possible to choose C, because there is no reduction in the upright image in the convex lens imaging) as long as the switch is on the dry road) 3Former.

When the convex lens becomes a real image, the law followed is that the near image of the object and the distant image become larger)4Infrared (which should be in this textbook) microamps.

The advance rate is 1000).

of the sixth power. Microampere = 5000mA = 5a

6.Parallel, series, short circuit.

7.Function: Amplify sound waves (that is, amplify the vibration of sound-emitting objects) Phenomenon: The shaking amplitude of the ping pong ball increases, and the sound emitted by the tuning fork will also be.

Note: The magnitude of the amplitude determines the loudness of the sound.

8.Equipment: white paper, reading glasses.

In the sunlight, the lens is facing the sunlight and the other end is covered with white paper. Move the white paper until the brightest and smallest spot appears on the white paper, and the distance from the spot to the center of the lens is the focal length (unit: meters).

So reading glasses is equal to (1 f) 100 Note that the focal length is measured in meters!

degrees (with formula (1 f) 100).

10.(1) The more salt, the faster it solidifies.

2) This question is similar to this one.

Regarding the question of "which one freezes faster, pure water or salt water", Xiaohong made the following conjecture. Pure water may freeze faster than salt water. Using the refrigerator and other equipment at home, please design an experiment to test Red's conjecture.

1) Take two identical cups and put the same water, one of which is salted and one is unsalted (2) Put both cups of water in the refrigerator at the same time, and observe the effect every 5min.

put on the trunk road).

The positive and negative poles of the ammeter are reversed.

b and c (depending on whether the range of the ammeter is 3).

A is the sum of b and c.

The trunk current is equal to the sum of the branch circuits12

The power of the TV is:

p=ui=220v*

1)w=pt=66w*1h=

2) W=PT= is in degrees.

As long as the object distance is determined, the image distance and the image situation (image position, size, virtual and real, positive and inverted, etc.) are also determined.

Relationship between image distance and object distance:

When u>2f f when u=2f v=2f

When f2f (u stands for object distance, v stands for image distance, f stands for double focal length, 2f stands for double focal length) the formula between image distance and object distance:

1/u+1/v=1/f

In the above equation, u represents object distance, v represents image distance, and f represents focal length. In the case of a concave lens, the focal length of a concave lens is expressed as a negative numerical value because it has a divergent effect on the light, and the intersection of the reverse extension line of the divergent light is called the imaginary focus of the concave lens. In the same way, the image distance of the virtual image should also be expressed as a negative number.

Note: The image distance understood by the landlord is incorrect. The so-called image distance refers to the distance between the image and the optical center, not the distance between the optical screen and the optical center.

Such as: <>

It is to be satisfied at the same time. As for determining the distance, you can sketch and calculate the scale.

The imaging situation is determined by the relationship between object distance and focal length!

Once both the focal length and object distance are determined, the imaging situation (including the location of the imaging and the nature of the imaging) is determined.

There is a problem with your understanding, what you said about "image distance" refers to the distance between the image screen and the convex lens during the experiment, which is not the real image distance!

Solution: Known: The pressure on the road surface when the truck is full of goods:

f=g=mg

2 103kg+10 103kg) 10n kg=force area: s=6 200 10-4m2=

Pressure on the road surface: p card = f s = =

P card P accept.

will damage the road surface

Therefore, when the car is driving on the road, it will cause harm to the road

12 10 10) (power of 6 8 power of 10, so there is danger.)

The maximum pressure that a certain horizontal highway pavement can withstand is 8 105pa There is a truck, the weight is 2t, the load is 6t, there are 6 wheels, the contact area between each wheel and the road surface is 200cm2, and the volume of the compartment is that the car is filled with sediment The density of the known sediment is kg m3 (g=10n kg) Find:

Is the car overloaded?

Explain whether the car is harmful to the road surface by calculation

It is known that the distance from Huanglong to Zhangwan is about 24km, if the car is filled with sediment and drives from Huanglong to Zhangwan at a constant speed of 90kw, how much work does the car do and how much time does it take? (The resistance of the car is times the weight of the car) (1) Find the volume of the compartment (the volume of sediment at most), know the density of sediment, use the density formula to find the load capacity of the car, and then compare it with the load of 6t to judge whether it is overloaded;

2) The pressure of the car on the ground is equal to the sum of the gravity of the car and the sediment, according to the formula p=fs

The pressure on the ground can be obtained, and then compared with the maximum pressure that the horizontal highway pavement can bear, it can be judged whether it is harmful to the highway pavement;

3) find the resistance of the car according to the question, the car is in a state of equilibrium when driving at a constant speed, find the traction force according to the two-force balance condition, find the work done by the car according to w=fs, and find the time required according to w=pt Answer: Solution: The volume of sediment is equal to the volume of the compartment, that is:

v = , the cargo capacity of the car is equal to the mass of the sand inside the car:

m Sediment = Sediment v = so this car is overloaded

The pressure of this car on the road:

f = g = (m car + m sediment) g = (2000kg + 10080kg) 10n kg =, whether the car is pressure on the road:

p=f s = 200 10-4m2 =, so it will be harmful to the road

Because the car is driving at a constant speed, the traction and resistance are a balancing force, so the traction of the car:

f=f=car+mSediment)g=,The work done by the car:

t=w p = 90000w ≈1288s．

Answer: The car is overloaded;

The car will cause harm to the road surface;

The work done by the car is about 1288s

BC should be chosen

B is chosen because when the crystal solidifies, when the crystal reaches the freezing point, the object emits heat, but the temperature does not change.

C was chosen because when the amorphous solidifies, the amorphous does not have a fixed freezing point, so when it solidifies, the temperature decreases while releasing heat.

The answer should be b and c. Because the temperature of the crystal does not change from solid to liquid, A and D are excluded, leaving B and C. It should be right.

The emitting of heat from the B and C objects is the solidification process.

The temperature of the object keeps decreasing before solidification The temperature does not change during solidification The temperature continues to decrease after solidification.

b, c because the total heat of the object is determined by two things, one is to do work, the other is to directly absorb (release heat), and an object releases heat, but we do not know its work, so we cannot determine its temperature change.

According to the law of conservation of energy, the temperature may decrease.

dBecause the object emits energy, if it is crystalline, then its temperature does not change, if it is amorphous, then the temperature rises when it melts and decreases when it solidifies. So the first three answers are too absolute and incorrect.

d Because the temperature does not change during the solidification process of the crystal, and the temperature of the non-crystal changes, there are two possibilities.

Look at the picture, and if you can't understand it, you can't find x, I have nothing to say.

GJ is the width of the river, EP is the height of the tree, AB is the height of the tree, and PL is the reflection of the tree.

From the knowledge of geometric similarity:

hg ef=gi fi=gi (gi+fg)= so gi=cd ak=jd jk=jd (jd+dk)= so jd=ef kl=fi ki=(fg+gi) (kd+dj+ji)=so ji=3

So gj=gi+ij+jd=

Hope it works for you!

If you look at Newton's first law, the property of an object to remain in its original state of motion is also called the law of inertia.

The two balls you are talking about have different masses, but they have the same velocity, the car stops, and the two balls have the property of maintaining their original motion (inertia), so they will move at the same speed, and the distance will remain the same.

The greater the mass and the greater the inertia, the so-called large inertia, which means that the state of motion is not easy to change, such as a large stone, and a small stone, a large stone is not easy to push, that is, the inertia is large, and the state of motion is not easy to change, so it is difficult to push.

In this question, we should pay attention to an ideal situation, that is, the plane is very smooth, it will keep the same distance, if it is rough, the quality is small stop first, and ask me if I don't understand.

The greater the inertia, the faster the speed, but the more difficult it is to change the direction, and the title says that all continue to move forward, so the direction does not change, and the car suddenly stops, so both balls will move at the speed of the car, so the two balls will keep the original distance and continue to move forward.

Inertia is a property of the object to keep the original state of motion unchanged, the state of motion here can be understood as speed, there must be conditions in the original question (the car is absolutely smooth), then there is no friction force, the ball will not stop at all (force is the reason for changing the state of motion of the object), naturally has nothing to do with the size of inertia.

Regardless of the mass, the object has inertia, so it will continue to move forward for a while. But in the end, it stopped because of friction.

f=maf is the force, m is the mass, a is the acceleration pushed out, a=f m the force on the big ball is large, and its mass is also large. Then the acceleration of the two balls is the same, so the velocity is the same.

Due to inertia, the ball will remain in its original state of motion.

Of course, it will keep the same movement, but if you experiment, you can't do it, because you have to eliminate air resistance.

It's just that the length of the movement is different.

Newton's first law when the force disappears will keep the object in its original state.

I guess it's because of gravity.

Didn't the question say that the car moves in a straight line at a uniform speed when it walks?

Shouldn't it be further and further.