Virtual laboratory is the golden rule of mechanics. The golden rule of mechanics. Motivation and goal setting

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The purpose of the lesson: introduce students to fixed and movable blocks; find out if there is a gain in strength and work when using simple mechanisms; formulate the "Golden Rule" of mechanics.

Equipment for the lesson:

• presentation for the lesson on the topic,
• projector,
• screen,
• a computer,
• equipment for conducting a physical experiment (blocks, lever, thread, weights, tripod, dynamometer).

During the classes

1. Organizational moment.

Hello guys and dear guests. I am glad to welcome you to the physics lesson. Physics, which is loved by many, and in a lesson that they are looking forward to.

2. Motivation and goal setting.

We continue to study simple mechanisms. And the topic of our today's lesson is “Blocks. "Golden Rule" of mechanics. (SLIDE 1) The purpose of our lesson is to get acquainted with another type of simple mechanisms - blocks. And our task is to find out whether there is a gain in strength and work when using simple mechanisms and formulate “ Golden Rule” mechanics. (SLIDE 2)

3. Actualization of knowledge.

To achieve them, we need to remember the material of the last lesson.

Now I’m giving you 7 minutes to write a basic summary in your notebook on the topic “Simple Mechanisms”, and while you write, I will ask one or two students to tell their homework orally at the blackboard.

4. Learning new material.

Let's start learning new stuff.

So what is a block? A block is a wheel with a groove, fixed in a holder. (SLIDE 3)

There are two types:

• fixed - the axis of which is fixed and does not move when the load is lifted;
• movable - the axis of which rises or falls along with the load.

Block demonstration.

Consider first the fixed block. (SLIDE 4)

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

Let's see what the forces will be F 1 and F 2 ?

Demo: Hang 4 weights on one end of a rope and a dynamometer on the other end. It will show a force of 4 N.

Question to the class: What do we see? The forces are the same. Let's try to explain this mathematically.

The block is a circle. Point O is the center of the block through which its axis passes. OA = l one ; OB = l 2 are radii. Consequently, l 1 =l 2. We see that these are the shoulders of forces. Recall the rule of lever balance: if the arms are equal, then the forces will be equal.

Will there be a win in this case? There is no win.

What will we conclude? A fixed block does not give a gain in strength.

Now consider the movable block. (SLIDE 5)

Will it be a lever? Yes, but this lever has different shoulders. What will be the forces F 1 and F 2 ?

Demo: Hang 4 weights on one end of a rope and a dynamometer on the other end. It will show the force 2N.

Question to the class: What do we see now? Strength F1 > F2 2 times, i.e. we win in strength 2 times.

This is the block system. (continuation of SLIDE 5) Make sure the power F2 Will not change.

Demo: Hang 4 weights on one end of a rope and a dynamometer on the other end. It will show a force of 2 N.

Which was to be proven. But let's try to prove mathematically.

So, point O is the center of the block, point A is the fulcrum of the lever. l 1 - shoulder strength F1 , l 2 - shoulder strength F2; AO = l 1 , AB = l 2 , therefore l 2 = 2l 1 .

What conclusion do we draw now? The movable block gives a gain in strength of 2 times.

5. Physical education

And now let's break for a couple of minutes: everyone got up and rested. (SLIDE 6)

Two girlfriends in the swamp -
Two green frogs
Washed early in the morning
Rubbed with a towel
They stamped their feet,
hands clapped,
Right, left leaning
And they returned back.
Here is the secret of health
To all friends - physical education hello!

We continue our lesson. So, we found out that a fixed block does not give a gain in strength, but a mobile one gives 2 times - i.e. applying our force, we can move a load, the weight of which is 2 times greater. But what about work? Will there be a gain at work?

We have a lever (demonstration). We balance it with different forces and turn it counterclockwise. (SLIDE 7) We see that the points of application in the same time went through different paths S 1< S 2 . Измерив эти пути и модули сил, нашли, что . Перемножив крест накрест, получим F 1 * S 1 = F 2 * S 2 , т.е. А 1 = А 2 .

Thus, acting on long shoulder leverage, we win in strength, but we lose just as many times on the road.

What will we conclude? The leverage does not work.

Consider a fixed block. (SLIDE 8) Note the starting position. We know that F 1 = F 2 . Moving the load up, one end of the rope will rise to a height h 1, and the other will fall to a height h 2, while h 1 = h 2. Then F 1 * h 1 = F 2 * h 2, i.e. A 1 \u003d A 2.

Conclusion? The fixed block does not give a gain in work.

The moving block remains. Maybe he will give a win in the work? (SLIDE 9) We have a block. Note the starting position. Let's move it up to the height h, then l will be the length of the free end of the rope. F 1 \u003d 2F 2, l\u003d 2h, A 1 \u003d F 1 * h,

What is the conclusion? The movable block does not give a gain in work.

What did we get? No mechanism gives a gain in work. How many times we win in strength, how many times we lose in distance. This is the “Golden Rule” of the mechanics of the ancient Greek scientist Heron. (SLIDE 10)

6. Summing up the lesson.

Let's sum up the lesson. Tell us what goals and objectives were set before us and have we achieved them?

7. Reflection.

There are papers on your tables. I ask you to complete the phrases in writing, and I will ask orally. (SLIDE 11)

• Today in class I learned...
• What I knew...
• It was interesting...
• But I didn't understand...

In what mood do you leave the lesson? (SLIDE 13)

8. Homework (SLIDE 12) paragraph 59, 60; OK No. 13

When people began to use blocks, levers, gates, they discovered that the movements made during the operation of simple mechanisms turned out to be associated with the forces developed by these mechanisms.

This rule in antiquity was formulated as follows: what we gain in strength, we lose on the way. This provision is general, but very important, and has received the name of the golden rule of mechanics.

We balance the lever with the help of two forces with different modulus. Shoulder l 1 force is acting F 1 , on the shoulder l 2 force is acting F 2 , under the action of these forces, the lever is in equilibrium. Then we set the lever in motion. At the same time, the point of application of force F 1 will pass the path S 1, and the point of application of force F 2 will pass the path S 2 (Fig. 1).

Rice. one

If we measure the modules of these forces and the paths traversed by the points of application of forces, then we get the equality: .

From this equality, we see how many times the forces applied to the lever differ, the paths made by the points of application of the force will differ in the same number of times inversely.

Using the proportion properties, we translate this expression into another form: - the product of the force F 1 on the path S 1 is equal to the product of the force F 2 on the path S 2. The product of the force on the path is called work, in this case the work is equal to A 1 \u003d A 2. The lever does not give a gain in work, the same conclusion can be drawn about any other simple mechanism.

The golden rule of mechanics: no mechanism gives a gain in work. Winning in strength, we lose on the road and vice versa.

Consider a fixed block. We fix the block in the axis and attach two weights to the ropes of the block, then we move one weight down, the weight moved down went the distance S, and the weight that moved up went the same distance S.

The forces are equal, the paths traveled by the bodies are also equal, which means that the work is also equal, and a fixed block does not give a gain in work.

Consider a moving block. We fix one end of the rope, pass it through the movable block and attach the other end to the dynamometer, hang loads from the block. Note the position of the weights on the tripod, lift the weights to a distance S 1, also mark and return to their original position, now mark the position of the dynamometer hook on the tripod. Again we raise the loads to a distance S 1 and note the position of the dynamometer hook in this case (Fig. 2).

Rice. 2

To lift the load to a height S 1, it was necessary to pull out the rope, which was almost twice as different from the distance that the load had traveled. The mobile block gives a gain in strength, but in work it does not give, how many times we gain in strength, how many times we lose on the way.

Condition. With the help of a movable block, the loader lifted the box with tools to a height S 1 = 7 m, applying a force F 2 = 160 N. What work did the loader A 2 do?

In order to find a job, you need the following: .

S 2 - the amount of movement of the rope.

How many times we win in strength, how many times we lose on the way, therefore, then.

Answer: the work done by the loader, 2.24 kJ.

Centuries-old practice proves that not a single simple mechanism gives a gain in work, it is possible, gaining in strength, to lose on the way and vice versa - depending on the conditions of the problem to be solved.

1. Lukashik V.I., Ivanova E.V. Collection of tasks in physics for grades 7-9 of educational institutions. - 17th ed. - M.: Enlightenment, 2004.
2. Peryshkin A.V. Physics. 7 cells - 14th ed., stereotype. - M.: Bustard, 2010.
3. Peryshkin A.V. Collection of problems in physics, grades 7-9: 5th ed., stereotype. - M: Exam Publishing House, 2010.

1. Home-edu.ru ().
2. Getaclass.ru ().
3. School-collection.edu.ru ().
4. School-collection.edu.ru ().

Homework

1. Why use simple mechanisms if they do not give a gain in work?
2. A 200 kg mass is lifted with a lever. To what height was the load lifted if the force acting on the long arm of the lever did the work 400 J.
3. With the help of a movable block, the load was raised by 3 m. How far did the free end of the rope have to be pulled out?

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Subject: Blocks. The "golden rule" of mechanics.

Conducted by a physics teacher

MOU KSOSH №2

Ryzhova E.V.

Lesson objectives: 1. Give the concept of fixed and movable blocks.

2. Calculate the gain in strength for the moving block.

3. Formulate the "golden rule" of mechanics.

Equipment : computer, presentation,

Physics grade 7.

During the classes:

Lesson plan:

1. Checking homework.

2. Presentation of new material.

a) definition of fixed and movable blocks.

b) no gain in strength for a fixed block.

c) the gain in strength given by the movable block.

d) posing the question of the possibility of obtaining a gain in work.

e) no gain in work when using simple mechanisms.

e) the use of simple mechanisms.

3. Fixing the material.

4. The result of the lesson.

5. Homework.

1. Checking homework.

The teacher checks the students' work. In case of students' difficulties, the teacher comments on the correct execution of the first part of the task.

2. Presentation of the material.

a) definition of fixed and movable blocks.

Blocks is wheel with groove around the circumference for a rope or chain, the axis of which is rigidly attached to the wall or ceiling beam. Blocks are used in lifting devices.
Block system and cables, designed to increase the carrying capacity, is called chain hoist (slide 4).

b) no gain in strength for a fixed block.

Movable and fixed block the same ancient mechanisms as the levers. Already in 212 BC, with the help of hooks and grabs connected to blocks, the Syracusans seized the means of siege from the Romans. construction military vehicles and the defense of the city led Archimedes (slide 5).

Fixed block Archimedes considered it as an equal-armed lever. Moments of forces on both sides of the block are the same=> the forces that create these moments are the same: He does not give a gain in strength, but allows you to change the direction of the force, which is sometimes necessary.

c) the gain in strength given by the movable block.

P movable block Archimedes took for an unequal lever, giving gain in strength 2 times (slide 6)

Moments of forces act relative to the center of rotation, which should be equal at equilibrium.

D) raising the question of the possibility of obtaining a gain in work.

Archimedes studied the mechanical properties of the movable 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 alone managed to move the ship with the help of a few people; Archimedes arranged a block and through it launched a huge ship; he first invented the block device"(slide 7.8)

e) no gain in work when using simple mechanisms.

Sports sailing ships, like those of the past, can't do without blocks kettlebell when setting sails and managing them. Modern ships need blocks for lifting signals, boats (slide 9)

Combination mobile and immovable blocks on the line of the electrified railway to adjust the tension of the wires (slide 10).

FROM block system can be used by gliders for lifting their vehicles into the air (slide 11).

"golden rule" of mechanics (slide 12)

How many times we win in strength, how many times we lose in distance.

f) Use of simple mechanisms.

Gate is two wheels, connected together and rotating around one axis for example, well gate with a handle. Such a complex bulky device of the medieval period - a gate or wheel wheels widely used in mining. They were set in motion by people stepping on the bars of the wheel (slide 13)

The gate can be seen as unequal lever: the gain in strength given by it depends on the ratio of the radii R and r (slide 14)

winch - construction consisting of from two gates with intermediate gears in the drive mechanism. The load capacity of modern winches can be over 100 kN. They work on ropeways, on drilling rigs, perform construction and installation and loading and unloading operations (slide 15)

GEAR- system of being in gear gear wheels (gears) to some extent similar to the gate (slide 16).

Since antiquity, simple mechanisms have often been used complex, in various combinations.
Combined mechanism consists of two or more simple. This is not necessarily a complex device; many fairly simple mechanisms can also be considered combined.
For example, in a meat grinder there are gate(a pen), screw(pushing meat) and wedge(knife-cutter). Arrows wrist watch rotated by the system gear wheels different diameters, which are engaged with each other. One of the most famous simple combined mechanisms - jack. The jack is screw and gate combination.

simple mechanisms- these are workers with work experience of more than 30 centuries, but they have not grown old at all (slide 18-19)

At any construction site, tower cranes- this is a combination of levers, blocks, gates. Depending on the " specialties"faucets have various designs and characteristics (slide 20,21,22).

Simple mechanisms will help move the house to widen the street. Frames are brought under the house, lowered onto rollers laid on rails, and electric winches (slide 23).

Inclined plane (slide 23-28).

3. Fixing the material.

Problem solving (slide 29).

1. A rope is thrown over a fixed block. One end of it is attached to the installer's belt, and he pulls the other down with some force. What is this force if the weight of the worker is 700 N? Ignore the friction in the block and the mass of the rope.

2. As you know, a fixed block does not give a gain in strength. However, when checking with a dynamometer, it turns out that the force holding the load on a stationary block is slightly less than the gravity of the load, and with a uniform rise it is greater than it. What explains this?

3. Why do construction cranes have a hook that carries a load not fixed to end of the cable, but on the clip of the movable block?

Answers (slide 30)

1. The force of gravity acting on the installer is balanced by the sum of the elastic forces of the ends of the rope hanging from the block. Therefore, the tension force of each of them is equal to half the weight of a person. This means that the worker pulls the end of the rope with a force of 350 N.

2. The action of the friction force.

3. This fastening makes it possible to reduce the tension of the cable by half
(If you do not take into account the friction in the blocks).

4. The result of the lesson (slide 31)

1. Which block is called fixed and which is movable?

2. For what purpose is a fixed block used?

3. What gain in strength does the movable block give?

4. Can the fixed and movable blocks be considered as levers?

5. What is the "golden rule" of mechanics?

5. Homework(slide 32)

P. 59, 60; ex. 31(1,2,3).