Summary of physical, mechanical, and internal energy knowledge in the third year of junior high scho

Summarization and induction should be the student's own business. Learn to learn!

Mechanical energy:

A general term for kinetic energy and potential energy.

Mechanical energy = kinetic energy + potential energy) The unit is: J

Kinetic energy and potential energy can be converted into each other. The ways are: kinetic energy and gravitational potential energy can be converted into each other; Kinetic energy and elastic potential energy can be converted into each other.

Conservation of mechanical energy: Only the mutual transformation of kinetic energy and potential energy, the sum of mechanical energy remains the same.

The artificial earth satellite revolves around the earth, and the mechanical energy is conserved; The perigee kinetic energy is the largest, and the gravitational potential energy is the smallest. The gravitational potential energy at apogee is the largest and the kinetic energy is the smallest. The perigee moves towards the apogee and the kinetic energy is converted into gravitational potential energy.

Mechanical energy and its transformation" is the first time for students to systematically learn the mutual transformation of different energy, the content of this section is closely related to social life, is the compulsory content of the high school entrance examination in various places, the examination question type involves multiple choice questions, fill-in-the-blank questions, experimental questions and question and answer questions, etc., the main test point is the analysis of the mutual transformation of kinetic energy and potential energy and the use of kinetic energy and potential energy mutual transformation to explain some phenomena.

Analysis of kinetic energy and potential energy conversion problem:

1) First of all, analyze the factors that determine the magnitude of kinetic energy, the factors that determine the magnitude of gravitational potential energy (or elastic potential energy) - see how kinetic energy and gravitational potential energy (or elastic potential energy) change.

2) It is also necessary to pay attention to the energy loss and increase in the process of mutual conversion of kinetic energy and potential energy - if there is no other external force to do work except gravity and elastic force (i.e., there is no other form of energy replenishment or no energy loss), the mechanical energy in the process of kinetic energy potential energy conversion remains unchanged.

3) If there is "sliding on a smooth inclined plane", then "smooth" means that there is no energy loss – mechanical energy is conserved; "Constant descent on the inclined plane" indicates that there is a loss of energy – mechanical energy is not conserved.

The work we usually talk about includes additional work and useful work, and the two are collectively called total work. And when we use machinery, people will do no less work than they do directly with their hands; That is: the use of any machinery does not save effort.

Don't be so nervous about the high school entrance examination. I'm completely playing for the high school entrance examination. But the results are very good. Don't memorize].

Mechanical energy:1 If an object is capable of doing work, that object has energy (energy).

2 Kinetic energy: The energy that an object has due to motion is called kinetic energy.

3 The greater the velocity and mass of the moving object, the greater the kinetic energy.

4 Potential energy is divided into gravitational potential energy and elastic potential energy.

5 Gravitational potential energy: The energy that an object has due to being lifted high.

6 The more massive the object is and the higher it is lifted, the greater the gravitational potential energy.

7 Elastic potential energy: The energy possessed by an object due to elastic deformation.

8 The greater the elastic deformation of an object, the greater its elastic potential energy.

9 Mechanical energy: A general term for kinetic energy and potential energy. (Mechanical energy = kinetic energy + potential energy) in joules.

10. Kinetic energy and potential energy can be converted into each other. The ways are: kinetic energy, gravitational potential energy;

Kinetic energy Elastic potential energy.

11 The mechanical energy in nature that can be used by humans in large quantities is wind energy and water energy. Merit.

There are two necessary elements of work: one is the force acting on the object; The second is the distance the object passes in the direction of the force. [A must master].

Calculation of work: Work (w) is equal to the product of the force (f) and the distance (s) that the object passes in the direction of the force. (work = force distance).

The formula for work: w=fs; Unit: W J; f Newton; s m. (1 joule = 1 Nm).

The principle of work: When using machinery, the work done by people is equivalent to the work done directly by hand without using machinery, that is to say, using any machinery does not save work [must be remembered].

Mechanical efficiency: The ratio of useful work to total work is called mechanical efficiency.

Calculation formula: =w useful w total.

Power. p): The work done per unit time (t) (w), which is called power.

Calculation formula: p=w t=fv. [The force that is consistent with the direction of velocity].

Unit: p Watts; w coke; t seconds. (1 watt = 1 joule.) 1 kW = 1000 watts).

The common point is that they are all energy, the difference is that the work is the amount of process, and the mechanical energy is the state quantity, that is to say, when the object is moving, the work is to describe how much work is done in the process of motion, and the mechanical energy is the energy of the object when it moves to what state, and the time and time point are the same reason, and the mechanical energy is kinetic energy plus potential energy, and the potential energy is not 1, but the gravitational potential energy and elastic potential energy, I hope you can understand.

What else does physics need to summarize? It's a liberal arts subject, as long as you don't yawn in class and do it yourself in the physics workbook every time, you don't have to worry about not being able to cope with the exam. As for the question type, you can go back and review the questions you have done wrong in the workbook!

Liberal arts don't require you to work hard to memorize, the key is to understand and look at types. The example questions in the workbook can be said to be classics.

Analysis: The pull force of 60N here is not gravity, but friction. In other words, at this time, the tensile force is balanced with the sliding friction force, and the tensile force has nothing to do with gravity. The direction of gravity is vertically downward, perpendicular to the direction of motion, so gravity does not do work.

So the first empty:

w=fs=60n×5m=300j

Second space: g s

g is not short, this blank fills 0j

If you shoot bullets, if it is an ordinary gun, the work done cannot be calculated. Because the bullet moves forward by inertia.

If the title states that it is a long gun, and also states the distance of movement of the bullet in the chamber s s and the thrust f f, w = fs is fine.

In the case of an inclined plane, you first calculate it as an ideal inclined plane, that is, frictionless. Then there is f's=gh

f' is the force required for an ideal inclined plane. Then use the formula =f'/f×100%。However, this formula is only suitable for fill-in-the-blank multiple-choice questions, which is used for simple calculations, and it is better to use =gh fs 100% for calculation problems.

If the title says it's a [smooth] bevel, then use this formula directly).

Because bullets and bevels don't have examples, it's not easy to explain, but if there are questions, it's better to explain. Do you understand the work that the front pull did to do? If you don't understand, please feel free to ask.

w=fs=100*5=500

w'=mgh=0 h=0

He also has a horizontal upward support of 100n

So to be able to pull.

1.The work done by the pulling force is equal to 500j

2.The work done by gravity is equal to 0j

3.It can be pushed or pulled with a pulley block or an inclined plane.

Starting from the definition, work: has two factors, one is the force and the other is the displacement in the direction of the force.

The magnitude of the tensile force is 60n, the direction is horizontal, and the distance traveled in the horizontal direction is 5m, therefore, the work done by the tensile force is 300j

The magnitude of gravity is 100n, the direction is vertical, and the distance traveled in the vertical direction is 0m, therefore, the work done by gravity is 0j.

To analyze the force, first consider gravity, and the direction of gravity is always straight downward.

Then consider the elastic force, such as the support force of the desktop, the elastic force of the spring, etc., the direction of the elastic force is the direction of the deformation of the object to the opposite, ** there is oppression, there is resistance, the direction of elastic force is generally the vertical direction of the contact surface, note, the difference between vertical and vertical.

Finally, consider the frictional force, which is in the opposite direction to the direction in which the object is moving, or where there is a tendency to move, generally along the contact surface.

I don't know if it's clear, haha.

300 j

The force of 0j60n is horizontal, and whether it can be pulled or not has nothing to do with the weight of the object, but with the amount of friction between the object and the horizontal plane.

You can ask me about bullet fighting.

The tensile force has been done 60x5=300j, although the direction of gravity is strong, but there is no distance, so the work done by gravity is 0j, and the theoretical basis is: work = force x distance.

(1) Because gravity is perpendicular to the direction of displacement of the object, gravity does not do work (there is no displacement in the vertical direction).

2) It is known that f=100n, s=50m, and w=fs=100*50j=5000j

Hello.

1.There will be no energy loss. From the perspective of work, the force of the rope on the ball is always in the same straight line as the rope, while the ball moves in a circular motion, and its direction of motion is always perpendicular to the rope.

So even if the rope is blocked, no work is done on the ball. So in the case where only gravity does the work, the mechanical energy of the ball is conserved.

2.Frictionless. Because after the rope touches the nail, there is neither relative motion nor relative motion tendency with the nail. The answer at home is that c is mainly because of air resistance, which does negative work on the ball.

Air resistance does negative work and mechanical energy is not conserved.