It is known that the mechanical efficiency of machine a is 65%, and that of machine B is 80% A. Machine a must save labor than machine B. machine a must save distance than machine B. machine a must do more useful work than machine B. all the above statements are wrong

Mechanical efficiency is the ratio of active work to total work. The larger the proportion of useful work to total work, the higher the mechanical efficiency. The mechanical efficiency of machine B can only show that the proportion of active work to total work of machine B is large, but it can not show that machine a must save labor and distance than machine B, and do more active work than machine B

A and B run from one end of the track to the other at the same time. A runs in the first half of the time and walks in the second half of the time, while B walks in the first half of the distance and runs in the second half of the distance
A first to the finish line
B. B. get to the end first
C arrives at the same time
D can't judge
A is time, B is distance
Please tell me what you think

A
Solution 1 (limit method): if the speed of running is the speed of light, then a can reach the end in an instant, while B must spend half the distance, so a will arrive first
Solution 2: let the walking speed be V1 and V2
T a = s / [0.5 (V1 + V2)]
T b = (0.5s / V1) + (0.5s / V2)
T a 〈 t b
So a gets to the finish line first
(in solving kinematics problems, it is very important to learn to use limit method.)

1. When a bulb marked with "220 V 60 W" is connected to a 220 V circuit to emit light, its electric power ()
A is equal to 60W, B is less than 60W, C is greater than 60W, D is possible in all three cases
2. A bulb marked "220 V 60 W" is compared with a bulb marked "110 V 40 W"
When a works normally, the former is brighter than the latter. When B works normally, the former's current is larger than the latter. When c works normally, the former's resistance is smaller than the latter. When D works normally, their brightness is the same
3. Lamp a is marked with "36V 45W" and lamp B is marked with "220V 45W". If both lamps light normally ()
A a lamp is brighter than B lamp, B lamp is brighter than a lamp, C two lamps are the same brightness, D cannot compare brightness

1. A. because the voltage of 220 V is rated voltage, it can light normally. 2. A. in normal operation, because the rated power of the former is higher than that of the latter, the former is brighter than the latter. A is right and D is wrong. The current of the former is I = P / u = 60 / 220 = 0.272a, and the current of the latter is I = P / u = 40 / 110 = 0.364a

Over the lake with a water depth of 1m, a thrush stops on the branch of a willow tree 5m high from the water and sings gently. In the water 2m away from the bank, a small red carp swims leisurely
A thrush in the water is a virtual image of thrush in the tree reflected by the water
B the thrush in the water is 1m away from the water
The distance between the carp and the water surface is less than 1m
D we can see the thrush and the little carp in the water are virtual images
I think it's a C error. Why is the answer a B error?

First of all, a is correct (the plane mirror is an upright virtual image of equal size, and the line between the image and the object is perpendicular to the reflecting surface, and the distance from the image to the reflecting surface is equal to the distance from the object to the reflecting surface). B a thrush stops on the branch of a willow tree 5m high from the water and sings gracefully

The correct understanding of work and energy is as follows:
A: The little ball hanging on the rope does not work, so it can't
B: An object that does work must have energy
C: The more work a body has done, the more energy it has
D: When an object falls freely from the air, its mechanical energy remains unchanged, indicating that there is no force to do work on it
I don't understand. Is it OK to have each option next to the general?

A wrong, not doing work doesn't mean there is no energy ~ there can be potential energy
B: Yes, according to the conservation of mechanical energy, if you do work on other objects and the energy of other objects increases, then your energy must decrease
C work is only a measure of energy conversion, it can only be used to measure the amount of energy conversion, but can not explain the amount of energy possessed by the transformed object
In D, the mechanical energy remains unchanged because the decrease of gravitational potential energy is equal to the increase of kinetic energy. In the whole process, gravity does work

The mixture of 1 l propane and 2 l ethane is completely burned in 80 l air, and the volume of the gas generated after drying is 1
A85 B75 C73 D70

Choose B
1 methane + 5 oxygen = 3 carbon dioxide + 4 water
2 ethane + 7 oxygen = 4 carbon dioxide + 6 water
After combustion, water is liquid, so the volume of methane before and after combustion is reduced by 3l, and the volume of ethane before and after combustion is reduced by 5L, a total of 8L, so the volume after reaction is 80L + 1L + 2l-8l = 75L

It is known that there are four kinds of isomers of propane dichloro, then the isomers of propane hexachloro are ()
A. 2 kinds
B. 3 kinds
C. 4 kinds
D. 5 kinds
What do you think of this?
Good answer,
Thanks!
"There are four kinds of dichloro substitution, and certainly four kinds of hexachloro substitution."
Why do you say that?

Substitution method
We should find out the implicit equivalent relation in the question and transform the object properly
If propane is C3H8, the dichloro is c3h6cl2, then the exchange of H and CL can be changed to
C3cl6h2 is the hexachloro compound of propane
So hexachloro propane has the same amount of isomers as its dichloro propane

The following substances do not react with phenol
A sodium hydroxide B sodium carbonate C sodium acetate D sodium bicarbonate

The answer is: C and D
A. Phenol reacts with sodium hydroxide to form sodium phenolate and water
B. Phenol can react with sodium carbonate to form sodium bicarbonate and water
C. The acidity of phenol is weaker than that of acetic acid
D. You see B: phenol can react with sodium carbonate to form sodium bicarbonate and water, so it's very clear that phenol can't
It reacts with sodium bicarbonate because phenol is more acidic than sodium bicarbonate

Ask an organic multiple choice question
The mixture of a mol H2 and B mol C2H4 is mixed, and a part of them reacts to form w mol C2H6 under certain conditions. The mixture gas obtained after reaction is completely burned, and the amount of oxygen consumed is ()
A.a+3b
B.(a/2)+3b
C.(a/2)+3b+(7W/2)
D.(a/2)+3b-(7W/2)
And why B?

If you think about this topic with logical thinking, it is said that the topic is complete combustion, then all the hydrogen and carbon will completely react with oxygen. "Under certain conditions, some of them will react to form w mol C2H6" is just a trap set by the topic, and you don't need to pay attention to it, In other words, the problem is how many moles of oxygen a mol H2 and B mol C2H4 can completely react with,
C2H4 + 3O2 → 2CO2 + 2H2O
b 3b
2H2 + O2 →2H2O
a a/2
The result is (A / 2) + 3B

Multiple choice questions for organic synthetic materials?
Are the main materials of the following articles organic synthetic materials?
A blue and white porcelain
B plastic cup
C cotton towel
D stainless steel

Choose plastic cup, plastic is synthetic polymer material (blue and white porcelain belongs to inorganic nonmetal material; cotton towel is natural polymer material, not synthetic; stainless steel is metal material, belongs to inorganic material)

It is known that the mechanical efficiency of machine a is 65%, and that of machine B is 80% A. Machine a must save labor than machine B. machine a must save distance than machine B. machine a must do more useful work than machine B. all the above statements are wrong