These two lessons were conducted according to the textbook by S.V. Gromova, N.A. Homeland Physics Grade 7. M. Enlightenment 2000

The peculiarity of the lessons is that they use the technology of a programmed survey for classes with a occupancy of less than 15 people. The technology consists in offering several answers to a question. Thanks to this, it is possible to simultaneously repeat the previous material, highlight the main thing in the topic covered, control the assimilation of the material by all students in the class. As practice shows, it takes no more than 17 minutes to interview the entire class. For young teachers, an important point will be the rapid development of skills to determine the level of assimilation of knowledge by students. Subsequent control and independent work invariably confirm the grades received by students during the programmed survey.

All interviews are oral. Children show answers on cards or on fingers, for which it is necessary that the number of answers does not exceed five. The results of the survey are displayed on the board immediately in the form of pluses, minuses and zeros (there is an opportunity to refuse to answer). This form of survey allows you to relieve tension during the survey, conduct it impartially, publicly and at the same time psychologically prepares the student for the tests.

The programmed survey has many disadvantages. To nullify them, it is necessary to alternate it reasonably with other forms of knowledge control.

Lesson #1 Blocks.

The purpose of the lesson: to teach children to find the gain in strength given by the block system.

Equipment: blocks, threads, tripods, dynamometers.

During the classes:

1. Organizational moment

II. New material:

The teacher asks the following question:

Daniel Defoe's book "Robinson Crusoe" tells the story of a man stranded on a desert island and managed to survive in harsh conditions. It says that once Robinson Crusoe decided to build a boat to sail away from the island. But he built the boat far from the water. And the boat was too heavy to be lifted. Let's imagine how you would deliver a heavy boat (say 1 ton in weight) to the water (at a distance of 1 km).

Students' solutions are briefly written on the board.

Usually they offer to dig a channel, move the boat with a lever. But in the work itself it is said that Robinson Crusoe began to dig a canal, but calculated that he would need his whole life to complete it. And the lever, if you calculate, will be so thick that you won’t have enough strength to hold it in your hands.

Well, if someone offers to make a winch, use a chain hoist, blocks or gates. Let this student tell what kind of mechanism it is and why it is needed.

After the story, they begin to study new material. If none of the students offers a solution, the teacher tells himself.

Blocks are of two types:

see fig 54 (page 55)

See fig 55 (page 55)

A fixed block does not give a gain in strength. It only changes the direction of force application. And the movable block gives a gain in strength by 2 times. Let's take a look in more detail:

(Reading the material §22 derivation of the formula F=P/2;)

In order to add up the action of several blocks, a device called a chain hoist (from the Greek poly - "many" spao - "pull") is used.

To raise the lower block, you need to pull up two ropes, that is, lose in distance by 2 times, therefore, the gain in strength of this chain hoist is 2.

To raise the lower block, you need to cut 6 ropes, therefore, the gain in strength of this chain hoist is 6

III. Consolidation of new material.

Practice Poll:

1. How many ropes are being cut in the picture?

One,
Four,
Five,
Six,
Another answer.

2. The boy can lift 20 kg. And you need to lift 100. How many blocks does he need to make a chain hoist?

Four,
Five,
Eight,
Ten,
Another answer.

3. Do you think it is possible to get an odd number of times with the help of blocks, for example, 3 or 5 times?

Answer: Yes, for this it is necessary that the rope connects the load with the upper block three times. Approximate solution in the figure:

III.1. Solution of problem 71.

III.2. Solution of the Robinson Crusoe problem.

To move the boat, it was enough to assemble a chain hoist or winch (a mechanism that we will study in the next lesson).

Hungarian admirers of Daniel Defoe even conducted such an experiment. One person moved a concrete slab 100 m with a makeshift pulley carved from wood.

III.3. Practical work:

Assemble from blocks and threads, first a fixed block, then a movable block and a simple chain hoist. Measure the gain in strength in all three cases with a dynamometer.

IV. Final part

Summary of the lesson, explanation of homework

Homework: §22; task 72

Lesson number 2. Gate. Winch.

Lesson objectives: to consider the remaining simple mechanisms - a winch, a gate and an inclined plane; to get acquainted with the methods of finding the gain in the force given by the winch and the inclined plane.

Equipment: gate model, large screw or screw, ruler.

During the classes:

I. Organizational moment

II. Programmed survey on the previous material:

1. Which block does not give a gain in strength?

Mobile,
Fixed,
No.

2. Is it possible to get a 3x win in strength with the help of blocks?

3. How many ropes are being cut in the picture?

One,
Four,
Five,
Six,
Another answer.

4. The boy can lift 25 kg. And you need to lift 100. How many blocks does he need to make a chain hoist?

Four,
Five,
Eight,
Ten,
Another answer.

5. The carpenter, repairing the frames, could not find a strong rope. He came across a twine that could withstand 70 kg to break. The carpenter himself weighed 70 kg, and the basket in which he climbed - 30 kg. Then he took and assembled the mechanism shown in Figure 1. Will the rope hold up?

6. After work, the carpenter was going to have lunch and attached the rope to the frame to free his hands, as shown in figure 2. Will the rope hold up?

III. New material:

Recording terms in a notebook.

The gate consists of a cylinder and a handle attached to it (show the model of the gate). Most often used for lifting water from wells (Fig. 60 p. 57).

Winch - a combination of a gate with gear wheels of different diameters. This is a more advanced mechanism. When using it, you can achieve the greatest strength.

Teacher's word. Legend of Archimedes.

Once Archimedes came to a city where the local tyrant had heard about the miracles performed by the great mechanic. He asked Archimedes to demonstrate some kind of miracle. "All right," said Archimedes, "but let the blacksmiths help me." He made an order, and two days later, when the car was ready, in front of the astonished public, Archimedes alone, sitting on the sand and lazily turning the handle, pulled the ship out of the water, which was barely pulled out by 300 people. Now historians think that it was then that the winch was first used. The fact is that when using a chain hoist, the actions of individual blocks add up, and in order to achieve a 300-fold increase in strength, 150 blocks are needed. And when using a winch, the actions of individual gears are multiplied, that is, when two gears are connected, one of which gives a gain in strength of 5 times and the other also 5 times, we get a total gain of 25 times. And if you apply the same transfer again, then the total gain will reach 125 times. (Instead of 15, as with simple addition).

Thus, to create this winch, it was enough to make a mechanism similar to a device (Fig. 61 p. 58). With the dimensions indicated, the upper gate gives a gain in strength of 12 times, the gear system 10 times, and the second gate 5 times. The winch gives a 60-fold gain in strength.

The inclined plane is a simple mechanism that many of you are familiar with. It is used to lift heavy objects, such as barrels into a car. How many times we gain in strength when lifting, the same number of times we lose in distance. For example, we can roll a 50 kg barrel. And you need to lift 300 kg by 1 meter in height. What length board should I take?

We solve the task:

Since we must win in strength by 6 times, therefore, the loss in distance must also be at least 6 times. This means that the board must have a length of at least 6 meters.

As examples inclined plane nuts and screws, wedges and a variety of cutting and piercing tools (needle, awl, nail, chisel, chisel, scissors, wire cutters, tongs, knife, razor, cutter, ax, cleaver, planer, jointer, selector, cutter, shovel, chopper, scythe, sickle, pitchfork, etc.), working bodies of machines for tillage (ploughs, harrows, brush cutters, cultivators, bulldozers, etc.)

Let's take "grouse" as an example. This is a blind wedge in the hammer that holds the handle. By spreading the fibers of the wood, this wedge, like a press, pushes the handle apart in the hole and securely fixes it.

But what if we do not need the nail to push the fibers apart. For example, you need to hammer a nail into a thin board. If you hammer a regular nail into it, it will simply crack. To do this, carpenters specifically blunt nails and hammer already blunt ones. Then the nail simply crushes the fibers of the wood in front of it, but does not push them apart like a wedge.

In ancient times, many simple mechanisms were used for military purposes. These are ballistae and catapults (Figure 62, 63). How do you think they work?

Discuss student responses with the whole class.

Especially large quantity inventions made famous by Archimedes. (If there is free time, the teacher talks about the inventions of Archimedes).

IV. Fixing new material

Practical work:

1) Take a large screw or screw and use a ruler to measure the circumference of its head. To do this, you need to attach the screw head to the divisions of the millimeter ruler and roll it along the divisions.

Screw head circumference l= 2R = ….mm

2) Now take a measuring compass and a millimeter ruler and measure with their help the distance between two adjacent protrusions of the screw thread. This distance is called the pitch or lead of the screw.

Screw pitch h = … mm

3) Now divide the circumference of the head by the pitch of the screw, and you will find out how many times we gain in strength using this screw.

V. Additional task: "Stupid" hoists.

Try to guess how many times we win in strength when using the following block systems.

To solve the second and third problems, it is not enough to answer the question "How many segments of the rope will be reduced if you pull" all the way "? The tasks require a non-standard approach. For example, let's solve the second problem. Let a person pull with a force of 10 N. This force is balanced by the tension of the rope 2. So, on the second rope, the traction force is 20 N. But it is balanced by the tension of the rope 3. So, on the third rope, the traction force is 40 N. And on the fourth, 80 N. Therefore, the gain in strength is 8 times.

In modern technology for the transfer of goods at construction sites and enterprises, hoisting mechanisms are widely used, indispensable constituent parts which can be called simple mechanisms. Among them are the most ancient inventions of mankind: block and lever. The ancient Greek scientist Archimedes facilitated the work of man, giving him a gain in strength when using his invention, and taught him to change the direction of the force.

A block is a wheel with a groove around the circumference for a rope or chain, the axis of which is rigidly attached to a wall or ceiling beam.

Lifting devices usually use not one, but several blocks. The system of blocks and cables, designed to increase the carrying capacity, is called a chain hoist.

The movable and fixed block are the same ancient simple mechanisms as the lever. Already in 212 BC, with the help of hooks and grabs connected to blocks, the Syracusans seized the means of siege from the Romans. The construction of military vehicles and the defense of the city was led by Archimedes.

Archimedes considered the fixed block as an equal-armed lever.

The moment of force acting on one side of the block is equal to the moment of force applied on the other side of the block. The forces that create these moments are also the same.

There is no gain in strength, but such a block allows you to change the direction of the force, which is sometimes necessary.

Archimedes took the movable block as an unequal lever, giving a gain in strength by 2 times. Moments of forces act relative to the center of rotation, which should be equal at equilibrium.

Archimedes studied the mechanical properties of the moving block and put it into practice. According to Athenaeus, "to launch the gigantic ship built by the Syracusan tyrant Hieron, they came up with many methods, but the mechanic Archimedes, using simple mechanisms, alone managed to move the ship with the help of a few people. Archimedes came up with a block and through it launched a huge ship" .

The block does not give a gain in work, confirming the golden rule of mechanics. It is easy to verify this by paying attention to the distances covered by the hand and the kettlebell.

Sports sailboats, like the sailboats of the past, cannot do without blocks when setting and managing sails. Modern ships need blocks for lifting signals, boats.

This combination of movable and fixed units on an electrified railway line to adjust the tension of the wires.

Such a system of blocks can be used by glider pilots to lift their vehicles into the air.

SUBJECT: Physics

CLASS: 7

TOPIC OF THE LESSON: Inclined plane. " Golden Rule mechanics".

Physics teacher

LESSON TYPE: Combined.

THE PURPOSE OF THE LESSON: Update knowledge on the topic "Simple mechanisms"

and learn the general position for all varieties of simple

mechanisms, which is called the "golden rule" of mechanics.

LESSON OBJECTIVES:

EDUCATIONAL:

- deepen knowledge about the condition of equilibrium of a rotating body, about blocks moving and stationary;

Prove that the simple mechanisms used in the work give a gain in strength, and on the other hand, allow you to change the direction of movement of the body under the action of force;

Develop practical skills in the selection of reasoned material.

EDUCATIONAL:

To cultivate an intellectual culture in leading students to understand the basic rule of simple mechanisms;

To acquaint with the functions of using levers in everyday life, in technology, in a school workshop, in nature.

DEVELOPMENT OF THINKING:

To form the ability to generalize known data on the basis of highlighting the main thing;

To form elements of creative search based on the method of generalization.

EQUIPMENT: Devices (levers, a set of weights, a ruler, blocks, an inclined plane, a dynamometer), a table "Levers in wildlife", computers, handouts (tests, task cards), textbook, blackboard, chalk.

DURING THE CLASSES.

STRUCTURAL ELEMENTS OF THE LESSON ACTIVITIES OF THE TEACHER AND STUDENTS

STATEMENT OF THE LESSON OBJECTIVE The teacher addresses the class:

Covering the whole world from earth to heaven,

Awakening more than one generation,

Scientific progress is sweeping the planet.

Nature has less and less secrets.

How to use knowledge is the concern of people.

Today, guys, let's get acquainted with the general position of simple mechanisms, which is called "golden rule" of mechanics.

QUESTION TO STUDENTS (GROUP OF LINGUISTS)

Why do you think the rule is called "golden"?

ANSWER: " Golden Rule " - one of the oldest moral commandments contained in folk proverbs, sayings: Do not do to others what you do not want to be done to you, - the ancient eastern sages spoke out.

GROUP OF TOKEN ANSWER: ” Golden” is the foundation of all foundations.

DISCOVERY OF KNOWLEDGE. PERFORMING THE "WORK AND POWER" TEST

(on a computer, test attached)

TRAINING TASKS AND QUESTIONS.

1.What is a lever?

2. What is called the shoulder of strength?

3. The rule of equilibrium of the lever.

4. The formula of the lever balance rule.

5. Find the mistake in the picture.

6. Using the lever balance rule, find F2

d1=2cm d2=3cm

7. Will the lever be in balance?

d1=4cm d2=3cm

A group of linguists performs № 1, 3, 5.

A group of experts perform № 2, 4, 6, 7.

EXPERIMENTAL TASKS FOR THE STUDENT GROUP

1. Balance the lever

2. Hang two weights on the left side of the arm at a distance of 12 cm from the axis of rotation

3. Balance these two weights:

a) one load_ _ _ shoulder_ _ _ see.

b) two weights_ _ _ shoulder_ _ _ see.

c) three loads_ _ _shoulder _ _ _ see.

Counselor working with students

In the world of interesting.

"Leverage in wildlife"

(winner of the Olympiad in biology Minakova Marina speaks)

WORK ON Demonstration of experiments (consultant)

LEARNED No. 1 Applying the law of balance of the lever to the block.

MATERIAL. a) Fixed block.

Updating earlier The students should explain that a fixed block can be learned consider as an equal-arm lever and gain in

knowledge about simple does not give strength

mechanisms. No. 2 The balance of forces on the movable block.

On the basis of experiments, students conclude that the mobile
block gives a gain in strength twice and the same loss in
way.

THE STUDY

NEW MATERIAL. More than 2,000 years have passed since the death of Archimedes, but
today the memory of people keeps his words: “Give me a foothold, and
I will raise the whole world to you." So said the eminent Greek
scientist - mathematician, physicist, inventor, having developed a theory
leverage and understanding its capabilities.

Before the eyes of the ruler of Syracuse, Archimedes, taking advantage of

difficult
device of levers, single-handedly lowered the ship. motto
everyone who has found something new is served by the famous "Eureka!".

One of the simple mechanisms that gives a gain in strength is
inclined plane. Define the work done by
inclined plane.

DEMONSTRATION OF EXPERIENCE:

The work of forces on an inclined plane.

We measure the height and length of the inclined plane and

We compare their ratio with the gain in strength by

F planes.

L A) we repeat the experiment by changing the angle of the board.

Conclusion from experience: inclined plane gives

h gain in strength as many times as its length

More height. =

2. The golden rule of mechanics is also fulfilled for

lever.

When the lever is rotated, how many times

we win in strength, we lose as many times

in movement.

IMPROVEMENT Quality tasks.

AND APPLICATION No. 1. Why train drivers avoid stopping trains on

KNOWLEDGE. rise? (a group of linguists answers).

No. 2 The block in position B slides down an inclined

plane to overcome friction. Will it

slide the bar and in position A? (the answer is given

accurate).

Answer: It will, because the valueF friction of the bar on the plane is not
depends on the area of contact surfaces.

Calculation tasks.

No. 1. Find the force acting parallel to the length of the inclined plane, the height of which is 1 m, the length is 8 m, so that a load weighing 1.6 * 10³ N is kept on the inclined plane

Given: Solution:

h = 1m F= F=

Answer: 2000N

No. 2. To keep a sled with a rider weighing 480 N on an ice mountain, a force of 120 N is needed. The slope of the hill is constant along its entire length. What is the length of the mountain if the height is 4 m.

Given: Solution:

h = 4m l =

Answer: 16m

No. 3. A car weighing 3 * 104 N moves uniformly on a slope 300 m long and 30 m high. Determine the traction force of the car if the friction force of the wheels on the ground is 750 N. What work does the engine do on this path?

Given: Solution:

P = 3*104H Force required to lift
Ftr \u003d 750H of the car without friction

l = 300m F= F=

h \u003d 30m The traction force is:

Fthrust-?, A -? Engine operation: A= Fthrust*L

A=3750H*300m=1125*103J

Answer: 1125kJ

Summing up the lesson, evaluating the work of students by consultants using a map of an intradifferentiated approach to activities in the lesson.

HOMEWORK § 72 rep. Section 69.71. With. 197 at. 41 #5

Topics of the USE codifier: simple mechanisms, mechanism efficiency.

Mechanism - a device for the transformation of force (its increase or decrease).
simple mechanisms is a lever and an inclined plane.

Lever arm.

Lever arm is a rigid body that can rotate around a fixed axis. On fig. 1) shows a lever with an axis of rotation. Forces and are applied to the ends of the lever (points and ). The shoulders of these forces are equal, respectively, and .

The equilibrium condition for the lever is given by the moment rule: , whence

Rice. 1. Lever

From this ratio it follows that the lever gives a gain in strength or in distance (depending on the purpose for which it is used) as many times as the larger arm is longer than the smaller one.

For example, in order to lift a load of 700 N with a force of 100 N, you need to take a lever with an arm ratio of 7: 1 and put the load on a short arm. We will win in strength by 7 times, but we will lose by the same amount in distance: the end of the long arm will describe a 7 times larger arc than the end of the short arm (that is, the load).

Examples of a lever that gives a gain in strength are a shovel, scissors, pliers. The rower's oar is a lever that gives a gain in distance. And the usual balance scales are an equal-armed lever that does not give a gain either in distance or in strength (otherwise they can be used to weigh buyers).

Fixed block.

An important type of leverage is block - a wheel fixed in a cage with a groove through which a rope is passed. In most problems, the rope is considered to be a weightless inextensible thread.

On fig. 2 shows a fixed block, i.e. a block with a fixed axis of rotation (passing perpendicular to the plane of the figure through the point).

At the right end of the thread, a weight is fixed at a point. Recall that the weight of the body is the force with which the body presses on the support or stretches the suspension. In this case, the weight is applied to the point where the weight is attached to the thread.

A force is applied to the left end of the thread at a point.

The shoulder of the force is , where is the radius of the block. The weight arm is equal to . This means that the fixed block is an equal-armed lever and therefore does not give a gain either in strength or in distance: firstly, we have equality, and secondly, in the process of movement of the load and the thread, the movement of the point is equal to the movement of the load.

Why, then, is a fixed block needed at all? It is useful in that it allows you to change the direction of effort. Usually a fixed block is used as part of more complex mechanisms.

moving block.

On fig. 3 depicted movable block, whose axis moves with the load. We pull the thread with a force that is applied at a point and directed upward. The block rotates and at the same time also moves upward, lifting a load suspended on a thread.

AT this moment time, the fixed point is the point , and it is around it that the block rotates (it would "roll" over the point ). They also say that the instantaneous axis of rotation of the block passes through the point (this axis is directed perpendicular to the plane of the figure).

The weight of the load is applied at the point of attachment of the load to the thread. The leverage is the same.

But the shoulder of the force with which we pull the thread turns out to be twice as large: it is equal to. Accordingly, the equilibrium condition for the load is equality (which we see in Fig. 3: the vector is two times shorter than the vector ).

Therefore, the movable block gives a gain in strength twice. At the same time, however, we lose the same two times in distance: in order to lift the load by one meter, the point will have to be moved by two meters (that is, two meters of thread must be pulled out).

The block in Fig. 3 there is one drawback: pulling the thread up (beyond the dot) is not the most best idea. Agree that it is much more convenient to pull the thread down! This is where the fixed block comes to the rescue.

On fig. 4 shows a lifting mechanism, which is a combination of a movable block with a fixed one. A load is suspended from the movable block, and the cable is additionally thrown over the fixed block, which makes it possible to pull the cable down to lift the load up. The external force on the cable is again indicated by the vector.

Fundamentally, this device is no different from the moving block: with its help, we also get a two-fold gain in strength.

Inclined plane.

As we know, it is easier to roll a heavy barrel along inclined walkways than to lift it vertically. Bridges are thus a mechanism that gives a gain in strength.

In mechanics, such a mechanism is called an inclined plane. Inclined plane - it's flat flat surface located at some angle to the horizon. In this case, they briefly say: "inclined plane with an angle".

Let us find the force that must be applied to the load of mass , in order to uniformly lift it along a smooth inclined plane with an angle . This force, of course, is directed along the inclined plane (Fig. 5).

Let's choose the axis as shown in the figure. Since the load is moving without acceleration, the forces acting on it are balanced:

We design on the axis:

It is this force that must be applied to move the load up the inclined plane.

To evenly lift the same load vertically, you need to apply a force equal to it. It can be seen that since . The inclined plane really gives a gain in strength, and the greater, the smaller the angle .

Widely used varieties of inclined plane are wedge and screw.

The golden rule of mechanics.

A simple mechanism may give a gain in strength or distance, but it cannot give a gain in work.

For example, a lever with a leverage ratio of 2:1 gives a gain in strength twice. To lift a load with a weight on the smaller arm, you need to apply force to the larger arm. But to raise the load to a height, the larger arm will have to be lowered to , and the work done will be equal to:

i.e. the same value as without using the lever.

In the case of an inclined plane, we win in strength, since we apply a force to the load, which is less than the force of gravity. However, in order to raise the load to a height above the initial position, we need to travel along an inclined plane. At the same time, we are doing the work

i.e. the same as for the vertical lifting of the load.

These facts serve as manifestations of the so-called golden rule of mechanics.

The golden rule of mechanics. None of the simple mechanisms gives a gain in work. How many times we win in strength, how many times we lose in distance, and vice versa.

The golden rule of mechanics is nothing more than a simple version of the law of conservation of energy.

mechanism efficiency.

In practice, one has to distinguish between useful work A useful, which needs to be done with the help of a mechanism in ideal conditions of the absence of any losses, and the full work A full,
which is performed for the same purposes in a real situation.

The total work is equal to the sum:
-useful work;
-work done against friction forces in various parts of the mechanism;
-work done to move the constituent elements of the mechanism.

So, when lifting a load with a lever, in addition, work has to be done to overcome the friction force in the axis of the lever and to move the lever itself, which has some weight.

Full work is always more useful. The ratio of useful work to full work is called the coefficient of performance (COP) of the mechanism:

=A useful / BUT full

Efficiency is usually expressed as a percentage. The efficiency of real mechanisms is always less than 100%.

Let us calculate the efficiency of an inclined plane with an angle in the presence of friction. The coefficient of friction between the surface of the inclined plane and the load is .

Let the weight of the mass rise uniformly along the inclined plane under the action of a force from point to point to a height (Fig. 6). In the direction opposite to the movement, the sliding friction force acts on the load.