A rough judgment about the problem of physics

Light wavelength 10 -7 m

Frequency 10 14m

The bird is 8m above the water

Fish 6m, heartbeat, 72 or 75 beats.

The energy pendulum is long. 50 times.

Whether it is a mechanical problem or an electrical problem, after encountering a calculation problem with a certain degree of difficulty, it is necessary not only to carefully review the problem, but also to draw a diagram, so as to establish an intuitive physical situation.

Find out how to solve the problemTwo methods are generally used to solve physical calculation problems, namely the analytical method and the synthesis method. The former is to use physical formulas to solve from the known to the unknown step by step, and the latter is to list physical equations to solve under specific conditions. There is also a proportional method, which uses the proportional method to solve, and its process is simpler and has the effect of getting twice the result with half the effort.

When actually doing the questions to avoid arithmetic errors, some students can only do simple calculation questions, but they are helpless in the face of flexible questions and comprehensive questions with many level changes. Therefore, candidates should not only learn physics knowledge seriously, but also strive to improve their mathematical reasoning ability. Don't let the correct answer slip by because of a miscalculation.

Generally speaking, there will be some approximate orders of magnitude in textbooks, so let's first go to the textbooks. I used to read a lot when I was in my third year of high school, and many things in physics textbooks were often overlooked, and they were often asked by teachers.

By the usual accumulation of brothers

Judging right or wrong based on the answers, I think it's wrong, and it's wrong on top of wrong, and my method is to read the question and find the useful part of the problem, and in physics, there are formulas, and I often find the useful parts and put them into the formula, and this is a good way to do it, I think it's very useful.

For example, (about the "density of matter") [the question of the second year of junior high school].

1. How many grams of alcohol can a bottle hold up to 500g of water?

Known: m=500g The density of water is equal to one gram per cubic centimeter and the density of alcohol is equal to grams per cubic centimeter.

Ask: Can it be loaded? grams of alcohol.

Solution: From the formula of density (the formula you should know!) To find the volume of water, divide 500g by one gram per cubic centimeter to get 500 cubic centimeters.

Then the exact volume and the volume of water are the same.

Then the density of the alcohol can be calculated.

It is to multiply grams per cubic centimeter by 500 per cubic centimeter equals 400g, then 400g is the answer.

This is followed by the question of the third year of junior high school.

For example, how much heat is absorbed by water?

Formula: Heat = specific heat capacity of water multiplied by the mass of water multiplied by temperature difference.

The second year of junior high school is about the floating and sinking of objects.

1. The float and sink of objects invaded in liquid depend on the gravitational and buoyant force subjected to them.

a. Object floating: The buoyancy of the object is greater than the gravitational force of the object.

b. Sinking of the object: The buoyancy of the object is less than the gravitational force of the object.

c. Object levitation: The buoyancy of the object is equal to the gravitational force of the object, for example: a submarine It can only float and sink by changing its own gravity.

Ouch! No, I make too many notes, I don't have a lot of physical strength to write them out for you one by one, I'm in junior high school, I don't know what time you are, so I'll write the content of junior high school for you, if you don't understand, add even QQ! 979871184

Let the initial velocity be v0, when outdoors, the acceleration is g, so the time taken for the velocity from v0 to 0 and from 0 to v0 is the same, both are seconds. From this, v0-gt=0 is obtained, and v0=15m s is solved

When indoors, the velocity of the ball when it hits the ceiling is v, and the time taken to rise and fall is t1 and t2 respectively, which can be obtained from the formula "end velocity = initial velocity + acceleration * time".

v=v0-gt1

v0=v+gt2

t1+t2=

Substituting the data, the solution is v=12m s

From the conservation of energy, 1 2m (v0 2-v 2) = mgh is solved to obtain h=81 20m=

A is wrong and B is right.

For example, if the elevator rises at a constant speed, the tension and gravity are balanced, and the net force is 0. Hence the kinetic energy does not change, the gravitational potential energy increases, and the mechanical energy increases.

Another example is an object in free fall due to gravity, the net force is not 0, but the reduced gravitational potential energy is converted into kinetic energy, and the mechanical energy is conserved.

A is wrong and B is right. To put it simply, the resultant force has nothing to do with the conservation of mechanical energy.

Counter-example of a: pulling an object to rise at a uniform speed, the object and the external force are 0, but the mechanical energy increases.

Example of b: Free Fall.

Is motion with only normal acceleration necessarily circular motion?

Answer: Not necessarily. Reason:

When a particle moves in a curvilinear motion, the acceleration along the normal direction of the track is called the normal acceleration. Numerically equal to the radius of curvature divided by the square of the velocity v r, i.e. v 2 r; or the square of angular velocity multiplied by the radius r, i.e. (2)r. Its action only changes the direction of the object's velocity, but does not change the magnitude of the velocity.

The "curvilinear motion" mentioned above includes circular motion, as well as other curvilinear motions.

Simple harmonic vibrations can be represented by rotational vectors, so the two are completely equivalent?

Answer: The two are not equivalent. Reason:

"Simple harmonic vibration" is a mode of motion, and "rotation vector" is a representation, and the two are not the same in nature, so they cannot be equivalent. In addition, there are many scenarios that can be represented by "rotation vector", and all those that meet the change relationship of trigonometric functions can be represented by "rotation vector", such as the instantaneous value of AC voltage and the instantaneous value of AC current.

1.Is motion with only normal acceleration necessarily circular motion?

2.Simple harmonic vibration can be expressed by rotation vector, so the answer of complete equivalence between the two is no, simple harmonic vibration is not disturbed by external forces, but the rotation vector may not be able to achieve this condition.

First question: Not necessarily. The magnitude of the normal acceleration can be varied, and the object is in the current circular motion at any instant, but the circumferential radius can be changed by the magnitude of the acceleration.

In addition, in isokinetic spiral motion (three-dimensional space motion), the normal acceleration does not change, but the object does not move in a circular motion.

The second question: the statement is not correct, the two concepts are essentially different, but they can be used to express, how to talk about equivalence. Ten fingers represent ten people, can they be said to be equal?

1. D, it is impossible to have no harm, such as the military nuclear bomb ** will produce infrasound, harmful *340=1020m (the speed of light is very large, the propagation time is generally ignored) The C option of the first question is right, you can experience it yourself, is it better to shout in the wilderness than in the classroom, this is the truth.

It doesn't make sense to calculate the speed of light, even 3s in the question is "about", if you want to calculate the speed of light, then you have to think about the reaction speed soon!!

Typical junior high school physics should be junior high school physics This is an understanding question The answers upstairs are all correct About c This is easy to understand Similar is that before a summer thunderstorm, there is often lightning and thunder, and after thunder, there is a burst of thunder The reason is that the clouds reflect the thunder so that the sound is superimposed This is you will know if you study high school physics In addition, the big exam will never test this type of question It usually appears in the usual handouts and connections It does not have any impact on your high school entrance examination Moreover, high school physics does not test these concepts Common sense questions are all calculations.

Let the height of the object from the ground h

Conservation by mechanical energy: MV2=EP+EK

And because ep=3ek, the above formula mv 2=ep + ep 3 is brought in: ep=3mv 8

ep=mgh=3mv²/8

h=3v²/8g

If you learn the kinetic energy theorem, you can bring in a formula and you can do it.

Regardless of the elevation angle of the irrelevant constant, the highest point h. is calculated by using the conversion of kinetic energy and potential energyThen the height sought is 3h 4

When the rigid fine straight rod is rotated at 90°, the plane where the ball b is located is 0 potential energy surface, so at the beginning, e(b)=2mgl, and the rigid thin straight rod is rotated 90°: 4mgl+mg*2l= w=root number(3g 2l), then at this time, e(b)= so a works on b.

The answer to the first question is AC, the answer to the second question is (H1-H2) 2 mg, and the answer to the third question is A

Average velocity = total distance time.

The time from the time the situation is noticed to the time the car stops is obvious, that is, seconds;

The distance is like this, the distance in the first second is 30*meters, and the second is 52 meters, a total of 18 + 52 = 70 meters in 5 seconds.

Average speed = 70 5 = 14m s

The total distance is divided by the total time, i.e. (30*.)

After discovering the situation, it took a total of 30* to start braking

52m was walked during braking

A total of 52+18=70m was walked

The total time is. v average = 70 5 = 14m s

Drove before braking, then taxied another 52m, for a total distance of 70m, and the total time was an average speed of 70 divided by 5 equals 14