Physics questions from the first to the second year of junior high school, and one physics question

Useful work w=100n*2*3m=600j

The total work of the first method w1 = (100 + 400 + 20) n * 2 * 3m = 3120 J The total work of the second method w2 = (100 + 20 + 10) n * 2 * 3m = 780 J The total work of the third method w3 = (100 + 10 + 5) n * 2 * 3m = 690 J efficiency N1 = 600 3120

Efficiency n2 = 600 780

Efficiency n3 = 600 690

The third method is efficient because it does less useless work.

Let's read the book on pediatrics and do it myself.

Figure out the definition of useful work: the work done on an object when you lift it! Therefore, the useful work is the work done by lifting 6 meters (two floors high) of 100n of sand w=gh=100n, 6m=600j

In the first method, overcoming one's own weight and doing work on the bucket is additional work;

The second type of extra work is the work done on the barrel and the moving pulley;

In the third method, extra work is done on pockets and moving pulleys.

It is easy to see from the data that the third method has the highest mechanical efficiency, calculated as the extra work w=g h=(10n+5n) 6m=90j, the total work is 600j+90j=690j, and the mechanical efficiency =600j 690j=87%.

It's so simple, you're embarrassed to ask?

When I saw this question, I wanted to choose D first

In fact, the signal from the satellite can be received in any direction.

However, in fact, the interference and weakening effect of the signal is considered here, probably because if it is placed horizontally, or facing downward, it will receive a weakened signal passing through the earth itself and surrounding buildings or other electromagnetic wave divergence devices, which will affect the reception effect.

Therefore, putting the concave side up can effectively receive a better signal...

Then you can only choose bbbbbbbbbbb.

It should be that the B satellites are all in the sky, and the main investigation here is light, and the electromagnetic waves are propagating in a straight line, so they can only be obliquely upward.

c This is like the refraction of light, imagined as a concave mirror.

b right. A satellite antenna, often referred to as a cauldron, is a metal parabola that is responsible for reflecting satellite signals into the feed and high-frequency head located at the focal point. The role of a satellite dish is to collect the weak signals from the satellite and remove noise as much as possible.

Most antennas are usually parabolic, but there are also some multifocal antennas that are a combination of spherical and parabolic surfaces. The satellite signal is reflected by the parabolic antenna and concentrated to its focal point.

The satellite is diagonally above the sky, so the spherical or parabolic plane should be pointed diagonally upward.

B should be chosen, because the satellite is only fixed in one position, so D is not right. A is obviously not right. C should be the transmitted signal, so B should be chosen

Answer: 4n, 1000pa

Analysis: The calculation of the pressure of water on the bottom of the cup is to find the pressure first:

P water gh, h 8 cm

Pressure: fps, s=50cm2

i.e.: f=800

The total weight of the cup and water is:

g mg according to: p f s, f = 6n, s 60 cm2 6

Select B to move the code to the left to balance again, indicating that the weight mass is greater than the mass of the object, and it should be reduced.

Dude, the answer is wrong, the answer is to do it according to "After balancing the beam, I found that the code is exactly at the rightmost end of the ruler, and the maximum scale of the scale is 5g," to do it.

Select B to adjust the balance when the yard is in the middle, put the object and the weight and then adjust the yard to the left, so apply the weight quality minus yard scale.

The answer is B. It is the phenomenon of condensation. (There is also water vapor in the air below 0) the general substance is expanded when exposed to heat, and the water division only expands abnormally from 0 to 4, and the temperature of the ice is lower than 0 in two years, and it will not expand when it is cold, so A is wrong.

There is no water column in the air, and even if the water that was originally attached to the icicle solidifies, it only makes the icicle thinner because that water is formed by melting the original icicle. So C is wrong.

Carbon dioxide turned into dry ice requires lower temperatures and cannot be achieved. So d is also wrong.

b should be the condensation process of water vapor in the air from gaseous state to solid state.

Your understanding is nothing but errors. The pressure of the liquid on the container should be calculated first according to p = gh and then f = ps.

1) p A = p B = p C.

2) F A = P A, F B = P B S, F C = P C S So F A = F B = F C.

3) F A g A, F B g B, F C g C.

1 Answer: P A = P B = P C.

Pressure is only related to the density of the liquid, the height of the liquid, and the acceleration due to gravity. That is, p= gh

I just read your understanding and think C should be the biggest, right? Because C has a large mass, but it is the vessel with the largest opening, you can understand it this way, a large part of the pressure is borne by the side wall of C. The same case of a container can be understood that the sidewalls have pressure on the liquid.

2 Answer: Your teacher is right, it is equal, the pressure f=p·s, in this formula, the pressure is the same, the ground area is the same, then the bottom pressure of the three of them is the same.

3 Answer: You know the answer, but I don't know why, let me tell you that your mind is stuck.

There are two concepts to be distinguished, the pressure of the liquid on the bottom of the container, and the pressure of the container on the support.

The pressure of the liquid on the bottom of the container is not necessarily equal to the pressure of the container on the support, as I said earlier, the pressure of the liquid on the bottom of the container is less than the pressure of the container on the support because the side wall of the container bears the pressure.

For small containers and large containers, the pressure of the liquid on the bottom of the container will be greater than the pressure on the support of the container because the side wall of the container actually has pressure on the liquid.

The pressure of the liquid on the bottom of the container is equal to the pressure of the container on the support only in containers that go straight up and straight down.

Let's understand it well.!!

First of all, the pressure generated by the liquid has nothing to do with the shape of the container, but with the density and depth of the liquid.

Therefore, GH is used to determine the pressure of the liquid. The other two, you can see this picture <>

The water inside the dotted line is really pressed on the bottom of the container, and the water outside the dotted line (as shown in Figure A) seems to be pressed on an inclined plane. Hope you understand.

(1) The pressure of the liquid on the bottom of the container is calculated by P=Gh, and the pressure is calculated by F=PS; The pressure of the liquid on the bottom of the container: P A = Gh, P B = Gh; So: pA = p B = p C. The sidewalls are subjected to some pressure.

That's pretty much the case. F A = P AS, F B = P B S, F C = P B S The pressure of the liquid is in any direction, which is due to the fluidity of the liquid. So the specific situation is as shown in the figure.

3) As can be seen from the second question, due to the pressure of the liquid. The pressure of A is actually greater than its gravitational force, and C is the opposite. fA, gA, fB, gB, fC, gC. C.

This is the pressure of the liquid on the bottom of the container, so with p=pgh, not the pressure of the whole container and the liquid on the ground, you should know that the liquid has pressure on each side of the container.

The water directly above acts on the side wall, and when doing the experiment, the liquid has a characteristic that the pressure of the liquid is in any direction, which is due to the fluidity of the liquid.

Let's break it down first.

You can get a clear image on the light screen.

There are only three cases.

1.The object distance is outside of two times the focal length.

2.The object distance is at double the focal length.

3.The object distance is between two times the focal length and one time the focal length.

However, the object distance is greater than the focal length and the only one can get a clear image, so you can get: 40cm 2f f 20cm 2f is a 15cm