The lever will be in equilibrium provided. Lever arm. Leverage balance. Moment of power. II. Homework check phase

Sections: Physics

Lesson type:a lesson in mastering new material

Lesson objectives:

• Educational:
  • acquaintance with the use of simple mechanisms in nature and technology;
  • develop skills in analyzing information sources;
  • to establish experimentally the rule of balance of the lever;
  • to form the ability of students to conduct experiments (experiments) and draw conclusions from them.
• Developing:
  • develop the ability to observe, analyze, compare, generalize, classify, draw up diagrams, formulate conclusions based on the material studied;
  • develop cognitive interest, independence of thinking and intelligence;
  • develop competent oral speech;
  • develop practical skills.
• Educational:
  • moral education: love of nature, a sense of comradely mutual assistance, ethics of group work;
  • education of culture in the organization of educational work.

Basic concepts:

• mechanisms
• lever arm
• shoulder strength
• block
• gate
• inclined plane
• wedge
• screw

Equipment:computer, presentation, handouts (work cards), a lever on a tripod, a set of weights, a laboratory set on the topic "Mechanics, simple mechanisms».

DURING THE CLASSES

I. Organizational stage

1. Greetings.
2. Definition of the absent.
3. Checking the readiness of students for the lesson.
4. Checking the readiness of the classroom for the lesson.
5. Organization of attention .

II. Homework check phase

1. Revealing the fact that the whole class has done homework.
2. Visual check of assignments in the workbook.
3. Finding out the reasons for the failure of the assignment by individual students.
4. Questions about homework.

III. The stage of preparing students for the active and conscious assimilation of new material

"I could turn the Earth with a lever, just give me a fulcrum"

Archimedes

Guess riddles:

1. Two rings, two ends, and a stud in the middle. ( Scissors)

2. Two sisters swayed - they sought the truth, and when they achieved, they stopped. ( Libra)

3. Bows, bows - comes home - stretches out. ( Ax)

4. What kind of miracle giant?
Reaches out to the clouds
Engaged in labor:
Helps build a house. ( Hoisting crane)

- Look again carefully at the answers and name them in one word. "Tool, machine" in translation from Greek means "mechanisms".

Mechanism - from the Greek word "???? v?" - cannon, construction.
Car - from the Latin word " machina " – construction.

- It turns out that an ordinary stick is the simplest mechanism. Who knows what it's called?
- Let's formulate the topic of the lesson together:….
- Open notebooks, write down the number and topic of the lesson: “Simple mechanisms. Lever equilibrium conditions. "
- What goal should we set with you today in the lesson ...

IV. The stage of assimilation of new knowledge

"I could turn the Earth with a lever, just give me a fulcrum" - these words, which are the epigraph of our lesson, Archimedes said more than 2000 years ago. And people still remember them and pass them on from mouth to mouth. Why? Was Archimedes right?

- Levers began to be used by people in ancient times.
- What do you think, what are they for?
- Of course, to make it easier to work.
- The first person to use the lever was our distant prehistoric ancestor, who used a stick to move heavy stones from their place in search of edible roots or small animals hiding under the roots. Yes, yes, because an ordinary stick, which has a fulcrum around which it can be turned, is a real lever.
There is a lot of evidence that in ancient countries - Babylon, Egypt, Greece - builders widely used levers when lifting and transporting statues, columns and huge stones. At that time they had no idea about the law of the lever, but they already knew well that the lever in the right hands turns a heavy load into a light one.
Lever arm - is an integral part of almost every modern machine, machine tool, mechanism. The excavator digs a ditch - its iron "hand" with a bucket acts as a lever. The chauffeur changes the speed of the car using the gearshift lever. The pharmacist weighs the powders on the pharmaceutical very precise scales, the main part of these scales is the lever.
Digging up the beds in the garden, the shovel in our hands also becomes a lever. All kinds of rocker arms, handles and collars are all levers.

- Let's get acquainted with simple mechanisms.

The class is divided into six experimental groups:

1st is studying the inclined plane.
2nd examines the lever.
3rd studies block.
4th examines the gate.
5th studies the wedge.
6th is studying the screw.

The work is carried out according to the description offered to each group in the working card. ( Appendix 1 )

We draw up a diagram based on the answers of the students. ( Appendix 2 )

- What mechanisms have you met ...
- What are simple mechanisms for? ...

Lever arm - a solid body that can rotate around a fixed support. In practice, a stick, board, crowbar, etc. can play the role of a lever.
The arm has a fulcrum and a shoulder. Shoulder - this is the shortest distance from the fulcrum to the line of action of the force (i.e. the perpendicular dropped from the fulcrum to the line of action of the force).
Usually, the forces applied to the lever can be considered the weight of the bodies. We will call one of the forces the resistance force, the other - the driving force.
On the picture ( Appendix 4 ) you see an equal arm that is used to balance the forces. An example of this use of a lever is a balance. What do you think will happen if one of the forces doubles?
That's right, the scales will go out of balance (I show on a normal scale).
Do you think there is a way to balance more power with less?

Guys, I suggest you in the course mini experiment deduce the condition of the balance of the lever.

Experiment

There are laboratory levers on the tables. Let's figure out with you when the lever is in balance.
To do this, hang one weight on the hook on the right side at a distance of 15 cm from the axis.

• Balance the lever with one weight. Measure your left shoulder.
• Balance the lever, but with two weights. Measure your left shoulder.
• Balance the lever with three weights. Measure your left shoulder.
• Balance the lever with four weights. Measure your left shoulder.

- What conclusions can be drawn:

• Where the strength is greater, the shoulder is less.
• How many times the strength has increased, how many times the shoulder has decreased,

- Let's formulate lever equilibrium rule:

The lever is in balance when the forces acting on it are inversely proportional to the shoulders of these forces.

- Now try to write this rule mathematically, that is, the formula:

F 1 l 1 \u003d F 2 l 2 => F 1 / F 2 \u003d l 2 / l 1

The balance rule of the lever was established by Archimedes.
This rule impliesthat a smaller force can be balanced by a lever with a larger force.

Relaxation: Close your eyes and cover them with your palms. Imagine a sheet of white paper and try to mentally write your first and last name on it. At the end of the recording, put a full stop. Now forget about the letters and remember only the point. It should seem to you moving from side to side with slow and light swaying. You have relaxed ... remove your palms, open your eyes, we return to the real world full of strength and energy.

V. The stage of consolidating new knowledge

1. Continue the phrase ...

• The lever is ... rigid body that can rotate around a fixed support
• The lever is in equilibrium if ... the forces acting on him are inversely proportional to the shoulders of these forces.
• The shoulder of strength is ... the shortest distance from the fulcrum to the line of action of the force (i.e., the perpendicular dropped from the fulcrum to the line of action of the force).
• Strength is measured in ...
• The shoulder of force is measured in ...
• Simple mechanisms include ... lever and its varieties: - wedge, screw; inclined plane and its varieties: wedge, screw.
• Simple mechanisms are needed for ... in order to gain the strength

2. Fill in the table (by yourself):

Find simple mechanisms in devices

P / p No.	Device name	Simple mechanisms
1	scissors
2	meat grinder
3	saw
4	stairs
5	bolt
6	pliers,
7	libra
8	ax
9	jack
10	power drill
11	a pen sewing machine, bicycle pedal or handbrake, piano keys
12	chisel, knife, nail, needle.

INTERCONTROL

Transfer the assessment after the peer review to the self-assessment card.

Was Archimedes right?

Archimedes was sure that there is no such heavy load that a person could not lift - you just need to use the lever.
And yet Archimedes exaggerated human capabilities... If Archimedes knew how huge the mass of the Earth is, he would probably refrain from the exclamation attributed to him by legend: "Give me a fulcrum and I will raise the Earth!" Indeed, to move the earth by only 1 cm, Archimedes' hand would have to travel 10 18 km. It turns out that in order to move the Earth by a millimeter, the long lever arm must be larger than the short one at 100,000,000,000 trillion. time! The end of this shoulder would have traveled 1,000,000 trillion. kilometers (approximately). And on such a road it would take a man many millions of years! .. But this is the topic of another lesson.

Vi. Stage of information to students about homework, instructions on how to complete it

1. Summing up: what was new learned in the lesson, how the class worked, which of the students worked especially diligently (grades).

2. Homework

All: § 55-56
For those who wish: to compose a crossword puzzle on the topic "Simple mechanisms at my home"
Individually: prepare a message or presentation "Levers in Wildlife", "The Power of Our Hands".

- The lesson is over! Goodbye, all the best to you!

A lever is a rigid body that can rotate around a fixed point.

The fixed point is called the fulcrum.

A well-known example of a lever is a swing (fig. 25.1).

When do two people on a swing balance each other? Let's start with observations. You, of course, noticed that two people on a swing balance each other if they have approximately the same weight and they are approximately the same distance from the fulcrum (Fig. 25.1, a).

Figure: 25.1. The condition for the balance of the swing: a - people of equal weight balance each other when they sit at equal distances from the fulcrum; b - people of different weights balance each other when the heavier one sits closer to the fulcrum

If these two are very different in weight, they balance each other only on the condition that the heavier one sits much closer to the fulcrum (Fig. 25.1, b).

Let us now pass from observations to experiments: we will find experimentally the conditions for the equilibrium of the lever.

Let's put experience

Experience shows that weights of equal weight balance the lever if they are suspended at equal distances from the fulcrum (Fig. 25.2, a).

If the weights have different weights, then the lever is in equilibrium when the heavier load is as many times closer to the fulcrum as its weight is greater than the weight of the light load (Figure 25.2, b, c).

Figure: 25.2. Experiments on finding the equilibrium condition of the lever

Lever equilibrium condition. The distance from the fulcrum to the straight line along which the force acts is called the shoulder of this force. Let's denote by F 1 and F 2 the forces acting on the lever from the side of the weights (see the diagrams on the right side of Fig. 25.2). The shoulders of these forces will be denoted by l 1 and l 2, respectively. Our experiments have shown that the lever is in equilibrium if the forces F 1 and F 2 applied to the lever tend to rotate it in opposite directions, and the moduli of the forces are inversely proportional to the arms of these forces:

F 1 / F 2 \u003d l 2 / l 1.

This condition for the balance of the lever was established experimentally by Archimedes in the 3rd century BC. e.

The condition of equilibrium of the lever you can learn from experience in laboratory work № 11.

Do you know what a block is? This is such a round contraption with a hook, with the help of which loads are lifted to a height at construction sites.

Does it look like a lever? Hardly. However, the block is also a simple mechanism. Moreover, we can talk about the applicability of the law of equilibrium of the lever to the block. How is this possible? Let's figure it out.

Application of the law of equilibrium

The block is a device that consists of a wheel with a groove through which a cable, rope or chain is passed, as well as a clip with a hook attached to the wheel axle. The block can be fixed and movable. A fixed block has an axle fixed, and it does not move when lifting or lowering a load. The fixed block helps to change the direction of the force. Having thrown a rope over such a block suspended at the top, we can lift the load up, while being at the same time at the bottom. However, the use of a fixed block does not give us a gain in strength. We can imagine a block as a lever rotating around a fixed support - the axis of the block. Then the radius of the block will be equal to the shoulders of the forces applied from both sides - the traction force of our rope with a load on one side and the gravity of the load on the other. The shoulders will be equal, respectively, there is no gain in strength.

The situation is different with the movable unit. The movable block moves with the load, it seems to lie on the rope. In this case, the fulcrum at each moment of time will be at the point of contact of the block with the rope on one side, the impact of the load will be applied to the center of the block, where it is attached to the axle, and the traction force will be applied at the point of contact with the rope on the other side of the block ... That is, the shoulder of the body weight will be the radius of the block, and the shoulder of our traction force will be the diameter. The diameter, as is known, is twice the radius, respectively, the arms differ in length by two times, and the gain in strength obtained with the help of the movable block is equal to two. In practice, a combination of a fixed block with a movable one is used. The fixed block at the top does not give a gain in strength, but it helps to lift the load while standing below. And the movable block, moving with the load, doubles the applied force, helping to lift large loads to a height.

The golden rule of mechanics

The question arises: do the devices used give a gain in work? Work is the product of the path traveled by the applied force. Consider a lever with arms that differ by half the length of the arm. This leverage will give us twice the strength gain, however, twice the shoulder will travel twice the distance. That is, despite the gain in strength, the work done will be the same. This is the equality of work when using simple mechanisms: how many times we gain in strength, how many times we lose in distance. This rule is called the golden rule of mechanics., and it applies to absolutely all simple mechanisms. Therefore, simple mechanisms facilitate the work of a person, but do not diminish the work he does. They simply help to translate some types of efforts into others that are more convenient in a particular situation.

A lever is a rigid body that can rotate around a fixed point. The fixed point is called fulcrum... The distance from the fulcrum to the line of action of the force is called shoulder this power.

Lever equilibrium condition: the lever is in equilibrium if the forces applied to the lever F 1and F 2 tend to rotate it in opposite directions, and the modules of forces are inversely proportional to the arms of these forces: F 1 / F 2 = l 2 / l 1This rule was established by Archimedes. According to legend, he exclaimed: Give me a foothold and I will raise the Earth .

For the lever, "Golden rule" of mechanics (if you can neglect the friction and mass of the lever).

By applying some force to the long lever, you can lift a load with the other end of the lever, the weight of which is much greater than this force. This means that by using leverage, you can gain strength. When leverage is used, gains in strength are necessarily accompanied by similar losses along the way.

Moment of power. Rule of moments

The product of the modulus of force on her shoulder is called moment of power. M \u003d Fl , where M is the moment of force, F is the force, l is the shoulder of the force.

Rule of moments: the lever is in equilibrium if the sum of the moments of forces tending to rotate the lever in one direction is equal to the sum of the moments of forces tending to rotate it in the opposite direction. This rule is true for any rigid body that can rotate around a fixed axis.

The moment of force characterizes the rotating action of the force. This action depends on both strength and her shoulder. That is why, for example, when wanting to open a door, they try to apply force as far as possible from the axis of rotation. With the help of a little force, a significant moment is created and the door opens. It is much more difficult to open it by applying pressure around the hinges. For the same reason, it is easier to unscrew the nut longer wrench, the screw is easier to remove with a screwdriver with a wider handle, etc.

The unit of moment of force in SI is newton meter (1 N * m). This is a moment of force of 1 N with a shoulder of 1 m.