MACHINES AND MECHANISMS, mechanical devices that facilitate labor and increase its productivity. Machines can be of varying degrees of complexity - from a simple one-wheeled wheelbarrow to elevators, cars, printing, textile, computers. Energy machines convert one form of energy into another. For example, hydroelectric generators convert mechanical energy falling water in electrical energy. Engine internal combustion converts the chemical energy of gasoline into thermal energy, and then into the mechanical energy of the car's movement. THERMAL ENGINE; TURBINE). The so-called working machines transform the properties or state of materials (metal-cutting machines, transport machines) or information (computers).

Machines consist of mechanisms (motor, transmission and executive) - multi-link devices that transmit and transform force and movement. A simple mechanism called a chain hoist ( cm. BLOCKS AND POLYSPATS) , increases the force applied to the load, and due to this, allows you to manually lift heavy objects. Other mechanisms make work easier by increasing speed. For example, a bicycle chain that engages with a sprocket converts slow pedaling into fast rear wheel rotation. However, mechanisms that increase speed do so by decreasing force, while those that increase force do so by decreasing speed. It is impossible to increase both speed and strength at the same time. Mechanisms can also simply change the direction of the force. An example is a block at the end of a flagpole: to raise the flag, the cord is pulled down. A change in direction may be combined with an increase in strength or speed. So, a heavy load can be lifted by pushing the lever down.

BASIC PRINCIPLES OF OPERATION OF MACHINES AND MECHANISMS

The basic Law.

Although the mechanisms allow you to get a gain in strength or speed, the possibilities of such a gain are limited by the law of conservation of energy. As applied to machines and mechanisms, it says: energy can neither arise nor disappear, it can only be converted into other types of energy or into work. Therefore, the output of a machine or mechanism cannot be more energy than the input. In addition, in real machines, part of the energy is lost due to friction. Since work can be converted into energy and vice versa, the law of conservation of energy for machines and mechanisms can be written as

Input work = Output work + Friction loss.

This shows, in particular, why a perpetual motion machine is impossible: because of the inevitable loss of energy for friction, it will stop sooner or later.

Gains in strength or speed.

Mechanisms, as mentioned above, can be used to increase strength or speed. The ideal or theoretical gain in force or speed is the rate of increase in force or speed that would be possible in the absence of energy loss due to friction. The ideal gain is unattainable in practice. The real gain, for example in force, is equal to the ratio of the force (called load) that the mechanism develops to the force (called force) that is applied to the mechanism.

mechanical efficiency.

Coefficient useful action machine is called the percentage ratio of the work at its output to the work at its input. For a mechanism, the efficiency is equal to the ratio of the real gain to the ideal one. Lever efficiency can be very high - up to 90% or even more. At the same time, the efficiency of the chain hoist due to significant friction and the mass of moving parts usually does not exceed 50%. The efficiency of the jack can be only 25% due to the large contact area between the screw and its body, and therefore high friction. This is approximately the same efficiency as a car engine. Cm. CAR PASSENGER.

Efficiency can be increased within certain limits by reducing friction due to lubrication and the use of rolling bearings.

SIMPLE MECHANISMS

The simplest mechanisms can be found in almost any more complex machines and mechanisms. There are six of them: lever, block, differential gate, inclined plane, wedge and screw. Some authorities argue that in fact we can talk about only two simple mechanisms - a lever and an inclined plane - since it is easy to show that the block and gate are variants of the lever, and the wedge and screw are variants of the inclined plane.

Lever arm.

It is a rigid rod that can rotate freely about a fixed point called the fulcrum. An example of a lever is a crowbar, a split hammer, a wheelbarrow, a broom.

Levers are of three kinds, differing mutual arrangement points of application of load and effort and fulcrum (Fig. 1). The ideal gain in leverage is equal to the ratio of the distance D E from the point of application of force to the fulcrum to the distance D L from the point of application of the load to the fulcrum. For a lever of the first kind, the distance D E usually more D L, and therefore the ideal gain in strength is greater than 1. For a type II lever, the ideal gain in strength is also greater than one. As for the lever of the third kind, the value D E less for him D L, and therefore, the gain in speed is greater than unity.

Block.

This is a wheel with a groove around the circumference for a rope or chain. Blocks are used in lifting devices. The system of blocks and cables, designed to increase the carrying capacity, is called a chain hoist. A single block can be either with a fixed axle (levelling) or movable (Fig. 2). A block with a fixed axle acts as a Class I lever with a fulcrum on its axle. Since the force arm is equal to the load arm (block radius), the ideal gain in strength and speed is 1. The movable block, on the other hand, acts as a type II lever, since the load is located between the fulcrum and the force. The load arm (block radius) is half the force arm (block diameter). Therefore, for a moving block, the ideal gain in strength is 2.

Equalizing and moving blocks can be combined in different ways to increase the gain in strength. In one clip, you can install two, three or more blocks, and the end of the cable can be attached either to a fixed or to a movable clip.

Differential gate.

These are, in essence, two wheels connected together and rotating around the same axis (Fig. 3), for example, a well gate with a handle.

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 meet general position simple mechanisms 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:

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:

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

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

Similarly leverage, inclined planes reduce the force required to lift bodies. For example, it is quite difficult to lift a concrete block weighing 45 kilograms by hand, but it is quite possible to drag it up an inclined plane. The weight of a body placed on an inclined plane is decomposed into two components, one of which is parallel and the other is perpendicular to its surface. To move the block up the inclined plane, a person must overcome only the parallel component, the value of which increases with an increase in the angle of inclination of the plane.

inclined planes very different in terms of design. For example, a screw consists of an inclined plane (thread) that spirals around its cylindrical part. When screwing a screw into a part, its thread penetrates into the body of the part, forming a very strong connection due to high friction between the part and the threads. The vise converts the action of the lever and rotary motion screw into a linear compressive force. The jack used to lift heavy loads works on the same principle.

Forces on an inclined plane

In a body located on an inclined plane, the force of gravity acts parallel and perpendicular to its surface. To move a body up an inclined plane, a force is required equal in magnitude to the gravity component parallel to the surface of the plane.

Inclined planes and screws

The relationship of the screw with the inclined plane is easy to trace if you wrap the cylinder with a diagonally cut sheet of paper. The resulting spiral is identical in location to the screw thread.

Forces acting on the screw

When a screw is turned, its thread creates a very large force applied to the material of the part into which it is screwed. This force pulls the screw forward if it turns clockwise and backward if it turns counterclockwise.

Weight Lifting Screw

The rotating screws of the jacks develop tremendous force, allowing them to lift objects as heavy as cars or trucks. When the central screw is turned with a lever, the two ends of the jack are pulled together, producing the necessary lift.

Inclined planes for splitting

The wedge consists of two inclined planes connected by their bases. When driving a wedge into a tree, the inclined planes develop lateral forces sufficient to split the strongest lumber.

Strength and work

Although the inclined plane may make the task easier, it does not reduce the amount of work required to complete the task. Lifting a 45 kg concrete block (W) 9 meters vertically upwards (far right) requires work of 45 x 9 kilograms, which corresponds to the product of the weight of the block and the amount of displacement. When the block is on a 44.5° inclined plane, the force (F) required to pull the block in is reduced to 70 percent of its weight. Although this makes it easier to move the block, now, in order to raise the block to a height of 9 meters, it must be dragged along a plane of 13 meters. In other words, the gain in strength is equal to the height of the lift (9 meters) divided by the length of travel on an inclined plane (13 meters).