Levers in technology, everyday life and nature. Simple mechanisms around us - write to Antoshka Using leverage
Since ancient times, simple mechanisms have often been used in complex, in a variety of combinations.
The combined mechanism consists of two or more simple. This is not necessarily a complex device; many fairly simple mechanisms can also be considered combined.
There are many types of simple mechanisms. This is a lever, and a block, and a wedge, and an inclined plane, and many others.
In physics, simple mechanisms are called devices that serve to transform forces.
The use of simple mechanisms is very common both in production and in everyday life.
For example, in a meat grinder there is a gate (handle), a screw (pushing meat) and a wedge (knife-cutter).
An inclined plane that helps roll in or pull heavy objects up is also a simple mechanism.
Arrows wrist watch rotated by the system gear wheels different diameter that are in contact with each other. One of the most famous simple combined mechanisms is a jack. The jack is a combination of screw and collar.
Most often, simple mechanisms are used in order to obtain a gain in strength, that is, to increase several times the force acting on the body.
A lever in physics is a simple mechanism
In physics, a lever is a rigid body that can rotate around a fixed support.
A crowbar, a board, and the like can be used as a lever.
There are two types of levers. For a lever of the first kind, the fulcrum O is located between the lines of action of the applied forces. At the lever of the second kind, the fulcrum is located on one side of them. That is, if we are trying to move a heavy object with a crowbar, then the lever of the first kind is a situation when we put a bar under the crowbar, pressing down on the free end of the crowbar. fixed support in this case, we will have a bar, and the applied forces are located on both sides of it. And the lever of the second kind is when we, having slipped the edge of the crowbar under the weight, pull the crowbar up, thus trying to turn the object over. Here, the fulcrum O is located at the point where the crowbar rests on the ground, and the applied forces are located on one side of the fulcrum.
Using leverage allows you to get a gain in strength. So, for example, the worker shown in the left figure, applying a force of 400 N to the lever, will be able to lift a load weighing 800 N. Dividing 800 N by 400 N, we get a gain in force equal to 2.
The law of balance of forces on the lever
Using the lever, we can gain strength and lift a heavy load with our bare hands. The distance from the fulcrum to the point of application of the force is called the shoulder of the force. Moreover, the balance of forces on the lever can be calculated using the following formula:
F 1 / F 2 \u003d l 2 / l 1,
where F 1, F 2 - forces acting on the lever,
and l 2 , l 1 are the shoulders of these forces. (In the figure above, OB and OA are lever arms)
This law was established by Archimedes in the third century BC. It follows from this that a smaller force can balance a larger one. To do this, it is necessary that the shoulder of the smaller force be greater than the shoulder of the greater force. And the gain in strength obtained with the help of a lever is determined by the ratio of the shoulders of the applied forces.
Nowadays, levers are widely used both in production (for example, cranes, a gearbox in a car) and in everyday life (scissors, wire cutters, scales, spanners etc.).
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.
gate- uh then two wheels connected together and rotating around the same axis, for example, well gate with a handle.
Winch- a design consisting of two gates with intermediate gears in the drive mechanism.
Inclined plane used to move heavy objects for more than high level without directly lifting them.
Such devices include ramps, escalators, conventional stairs and conveyors.
Wedge- one of the varieties of a simple mechanism called "inclined plane". The wedge consists of two inclined planes, the bases of which are in contact. It is used to obtain a gain in strength, that is, with the help of a smaller force to counteract a larger force.
When chopping firewood, to facilitate the work, a metal wedge is inserted into the crack of the log and beaten on it with the butt of an ax.
Screw- an inclined plane wound on an axis. The thread of a screw is an inclined plane repeatedly wrapped around a cylinder. The ideal gain in strength given by the wedge is equal to the ratio of its length to the thickness at the blunt end. The real payoff of a wedge is difficult to determine.
Due to the large friction, its efficiency is so small that the ideal gain has no special significance. Depending on the direction of rise of the inclined plane, the screw thread can be left or right.
Examples simple devices with a screw thread - a jack, a bolt with a nut, a micrometer, a vice.
The bones connected by the joints act as levers when the muscles contract. In biomechanics, levers are distinguished:
the first type - the lever of force - if the shoulder of application of muscle force is longer than the shoulder of resistance (gravity); an example is the foot during a toe lift.
the second type - the lever of speed - the shoulder of application of muscle force is shorter than the shoulder of resistance, where gravity is applied, counteracting; an example is the elbow joint in flexion.
lever of the first kind or "balance lever", two shoulders - points of resistance and application of muscle force are located on opposite sides of the fulcrum. an example is the connection of the spine with the skull.
lever of the second kind, one-arm - both forces are applied on the same side of the fulcrum, at different distances from it, two types are distinguished depending on the location of the point of application of the force and the point of action of gravity:
12 Fascia and cellular spaces of the neck.
Fascia of the neck according to V.N. Shevkunenko:
Superficial fascia of the neck (subcutaneous muscle).
Superficial sheet of the own fascia of the neck (sternocleidomastoid and trapezius muscles).
Deep sheet of the own fascia of the neck (sternum-thyroid, sternohyoid and scapular-hyoid muscles).
Intracervical fascia (trachea, esophagus, thyroid gland, neurovascular bundle) - parietal leaf, visceral leaf.
Prevertebral fascia (scalenus anterior).
13 Topography of the neck (triangles, prescalene and interscalene spaces).
prescalene space located between the edges of the sternothyroid, sternohyoid and anterior scalene muscles, contains the subclavian vein in front, and behind it the phrenic nerve and the subclavian lymphatic trunk.
Interstitial space lies between the anterior and middle scalene muscles, bounded from below by the 1st rib; in it is located in front of the subclavian artery and behind it the trunks of the brachial plexus (supraclavicular part).
Front the area or anterior triangle of the neck is limited on the sides by the anterior edges of the sternocleidomastoid muscles, above - by the chin, base and branches of the lower jaw, mastoid processes, below - by the jugular notch of the sternum.
The anterior midline from the chin to the jugular notch divides the area into medial triangles: right and left. In each medial triangle, they distinguish at the top: submandibular triangle, bounded by the anterior and posterior bellies of the digastric muscles and the lower jaw. It contains the submandibular salivary gland and a small lingual triangle, described by N. I. Pirogov within the boundaries:
front- the posterior edge of the maxillofacial muscle,
back - lower edge of the posterior belly of the digastric muscle;
top- hypoglossal nerve;
the area of the triangle is occupied by the hyoid-lingual muscle and the underlying lingual artery, for operational access to which N.I. Pirogov this triangle.
The middle of the anterior region is the carotid ( sleepy) triangle, formed in front and below by the upper belly of the scapular-hyoid muscle, from above - by the posterior belly of the digastric muscle, and behind - by the anterior edge of the sternocleidomastoid muscle. In the sleepy triangle pass internal jugular vein, vagus nerve and common carotid artery, which within it is divided at the level of the upper edge of the thyroid cartilage into external and internal. In the lower part of the triangle, the common carotid artery is adjacent to the anterior tubercle of the transverse process of the YI cervical vertebra and is pressed against it (carotid tubercle) when the pulse is felt and the bleeding stops.
The lower part of the anterior region is occupied by scapular-tracheal a triangle within the boundaries: the upper lateral - the upper abdomen of the scapular-hyoid muscle, the posterior inferior - the edge of the sternocleidomastoid muscle, the medial - the anterior midline. In the depths of the triangle lie the trachea and esophagus.
Sternocleidomastoid region corresponds to the muscle of the same name and serves as a good reference point between the lateral and medial triangle. The front edge of the muscle corresponds to the projection line of the carotid artery, internal jugular vein and vagus nerve located between them.
Lateral area neck has an anterior border along the posterior edge of the sternocleidomastoid muscle, a posterior border along the trapezius muscle, and a lower border along the clavicle.
It contains:
Scapular-trapezoid the triangle occupying the upper section is located between the edges of the trapezius, sternocleidomastoid muscles (lateral sides) and the lower abdomen of the scapular-hyoid muscle (lower side). It projects the cervical plexus and its short branches.
Scapular-clavicular the triangle is formed by the clavicle (lower side) and the edges of the sternocleidomastoid, scapular-hyoid (lower abdomen) muscles. Inside it - in the ladder intervals - there is a horizontal neurovascular bundle of the neck in the composition (front and back) of the subclavian vein, artery and trunks of the brachial plexus.
Back area neck has an upper border along the upper nuchal line, lateral borders along the anterior edges of the trapezius muscle, and a lower border along the line of the acromion-spinous process of the YII cervical vertebra. The area is occupied by the multi-layered posterior muscle group described above. Under the back of the head in the back region is suboccipital a triangle bounded by the posterior rectus and oblique muscles of the head.
Levers are widespread in everyday life. It would be much more difficult for you to open a tightly screwed water faucet, if it did not have a 3-5 cm handle, which is a small but very effective lever. The same applies to a wrench, which you use to unscrew or tighten a bolt or nut. The longer the wrench, the easier it will be for you to unscrew this nut, or vice versa, the tighter you can tighten it. When working with especially large and heavy bolts and nuts, for example, when repairing various mechanisms, cars, machine tools, wrenches with a handle up to a meter are used.
Another a prime example leverage in Everyday life- the most common door. Try to open the door by pushing it near the hinges. The door will give in very hard. But the farther from door hinges the point of application of force will be located, the easier it will be for you to open the door.
Naturally, levers are also ubiquitous in technology. The most obvious example is the gear lever in a car. The short arm of the lever is the part that you see in the cabin. The long arm of the lever is hidden under the bottom of the car, and is about twice as long as the short one. When you shift the lever from one position to another, a long arm in the gearbox switches the corresponding mechanisms. Here you can also very clearly see how the length of the arm of the lever, the range of its travel and the force required to shift it correlate with each other.
Levers can be found at the construction site: excavator, crane, wheelbarrow, scrap.
An example of a lever that gives a gain in strength is paper scissors, wire cutters, metal shears, a shovel.
Levers different kind available on many machines: handle sewing machine, bicycle pedals or handbrake, piano keys are all examples of levers. Libra is also an example of a lever.
An example of a lever that gives a loss in strength is an oar. This is necessary to get a gain in distance. The longer the part of the oar lowered into the water, the greater its radius of rotation and speed.
Thus, we can make sure that the lever mechanism is very widespread both in our daily life and in various mechanisms.
We have the right to say without exaggeration that each person is much stronger than himself, that is, that our muscles develop a force much greater than that which is manifested in our actions.
Is such a device appropriate? At first glance, as if not, we see here the loss of strength, which is not rewarded in any way. However, let us recall the old Golden Rule» mechanics: what is lost in strength is gained in movement. This is where the gain in speed comes in: our hands move 8 times faster than the muscles that control them. The way in which muscles are attached, which we see in animals, provides the limbs with agility of movement, more important in the struggle for existence than strength. We would be extremely slow creatures if our hands and feet were not arranged according to this principle.
"I could turn the Earth with a lever, just give me a fulcrum"
Archimedes
Lever arm one of the most common and simple types mechanisms in the world, present both in nature and in the man-made world.A lever is a rigid body that can rotate around some axis. A lever is not necessarily a long and thin object.
The human body as a lever
In the skeleton of animals and humans, all bones that have some freedom of movement are levers, for example, in humans - the bones of the limbs, the lower jaw, the skull, the phalanges of the fingers.
Let's take a look at the elbow joint. The radius and humerus are connected together by cartilage, and the muscles of the biceps and triceps are also attached to them. So we get the simplest lever mechanism.
If you hold a 3 kg dumbbell in your hand, how much effort does your muscle develop? The junction of the bone and muscle divides the bone in a ratio of 1 to 8, therefore, the muscle develops a force of 24 kg! It turns out that we are stronger than ourselves. But the lever system of our skeleton does not allow us to fully use our strength.
An illustrative example of more successful application The advantages of leverage in the musculoskeletal system of the body are reversed hind knees in many animals (all kinds of cats, horses, etc.).
Their bones are longer than ours, and the special structure of their hind legs allows them to use the strength of their muscles much more efficiently. Yes, of course, their muscles are much stronger than ours, but their weight is an order of magnitude greater.
The average horse weighs about 450 kg, and at the same time can easily jump to a height of about two meters. To perform such a jump, you and I need to be masters of sports in high jumps, although we weigh 8-9 times less than a horse.
Since we remembered the high jump, consider the options for using the lever, which were invented by man. The pole vault is a very good example.
With the help of a lever about three meters long (the length of the pole for high jumps is about five meters, therefore, the long arm of the lever, starting at the bend of the pole at the time of the jump, is about three meters) and the correct application of effort, the athlete takes off to a dizzying height of up to six meters.
Pick up a pen, write something or draw, and watch the pen and the movement of your fingers. You will soon discover that the handle is a lever. Find a foothold, evaluate your shoulders and make sure that in this case you lose in strength, but gain in speed and distance. Actually, when writing, the friction force of the stylus on the paper is small, so that the muscles of the fingers do not strain too much. But there are such types of work when the fingers must work to the fullest, overcoming significant forces, and at the same time make movements of exceptional accuracy: the fingers of a surgeon, a musician.
Lever in everyday life
Levers are also common in everyday life. It would be much more difficult for you to open a tightly screwed faucet if it did not have a 4-6 cm handle, which is a small but very effective lever.
The same applies to a wrench, which you use to unscrew or tighten a bolt or nut. The longer the wrench, the easier it will be for you to unscrew this nut, or vice versa, the tighter you can tighten it.
When working with especially large and heavy bolts and nuts, for example, when repairing various mechanisms, cars, machine tools, wrenches with a handle up to a meter are used.
Another striking example of leverage in everyday life is the most common door. Try to open the door by pushing it near the hinges. The door will give in very hard. But the farther from the door hinges the point of application of force is located, the easier it will be for you to open the door.
In plants, lever elements are less common, which is explained by the low mobility of the plant organism. A typical lever is a tree trunk and roots. A pine or oak root that goes deep into the ground offers tremendous resistance, so pines and oaks almost never turn upside down. On the contrary, spruces, which often have a superficial root system, tip over very easily.
The "piercing tools" of many animals and plants - claws, horns, teeth and thorns - are shaped like a wedge (a modified inclined plane); the pointed shape of the head of fast-moving fish is similar to a wedge. Many of these wedges have very smooth hard surfaces, which is what makes them so sharp.
Levers in technology
Naturally, levers are also ubiquitous in technology.
A simple "lever" mechanism has two varieties: block and gate.
With the help of a lever, a small force can balance a large force. Consider, for example, lifting a bucket from a well. The lever is a well gate - a log with a curved handle attached to it, or a wheel.
The axis of rotation of the gate passes through the log. The lesser force is the force of the person's hand, and the greater force is the force with which the bucket and the hanging part of the chain are pulled down.
Even before our era, people began to use leverage in construction business.
For example, in the picture you see the use of a lever when constructing a building. We already know that levers, blocks and presses allow you to get a gain in strength. However, is such a gain given "for nothing"?
When using a lever, its longer end travels a greater distance. Thus, having received a gain in strength, we get a loss in distance. This means that by lifting a large load with a small force, we are forced to make a large displacement.
The most obvious example is the gear lever in a car. The short lever arm is the part that you see in the cabin.
The long arm of the lever is hidden under the bottom of the car, and is about twice as long as the short one. When you shift the lever from one position to another, a long arm in the gearbox switches the corresponding mechanisms.
For example, in sports cars, for faster gear changes, the lever is usually set short, and its range is also made short.
However, in this case, the driver needs to make more effort to change gear. On the contrary, in heavy vehicles, where the mechanisms themselves are heavier, the lever is made longer, and its range of travel is also longer than in a passenger car.
A simple "inclined plane" mechanism and its two varieties - wedge and screw
An inclined plane is used to move heavy objects to a higher level without directly lifting them. If you need to lift a load to a height, it is always easier to use a gentle slope than a steep one. Moreover, the lower the slope, the easier it is to do this work.
A body on an inclined plane is held by a force that is ... in magnitude so many times less than the weight of this body, how many times the length of the inclined plane is greater than its height.
A wedge driven into a log acts on it from top to bottom. At the same time, he pushes the resulting halves to the left and right. That is, the wedge changes the direction of the force.
Thus, we can be convinced that the mechanism of the lever is very widespread both in nature and in our daily life, and in various mechanisms.
In addition, the force with which he pushes the halves of the log is much greater than the force with which the hammer acts on the wedge. Consequently, the wedge also changes the numerical value of the applied force.
Woodworking and gardening Tools represented a wedge - this is a plow, adze, scrapers, a shovel, a hoe. The land was cultivated with a plow, a harrow. Harvested with rakes, scythes, sickles.
A screw is a type of inclined plane. With it, you can get a significant gain in strength.
By turning the nut on the bolt, we raise it along an inclined plane and win in strength.
By turning the corkscrew handle clockwise, we cause the corkscrew screw to move down. There is a transformation of movement: the rotational movement of the corkscrew leads to its forward movement.
In this lesson, the topic of which is: simple mechanisms» we will talk about the mechanisms that help us in our work. At construction sites, in production, on vacation - everywhere we need help. Such assistants are levers. Today we will talk about them, as well as solve the problem and analyze some of the most simple examples from life.
In this lesson, we will focus on simple mechanisms.
simple mechanisms- these are devices with the help of which work is done only at the expense of mechanical energy. We are surrounded by devices that operate on electricity (see Fig. 1), due to the energy of fuel combustion, but this has not always been the case.
Rice. 1 Electric Kettle
Previously, all the work could actually be done by hand, or with the help of animals, due to the wind or the flow of water (mills), that is, due to mechanical energy (see Fig. 2).
Rice. 2. Old simple mechanisms
And they help in this, facilitate the work, simple mechanisms.
Our forces are limited, and this is a problem. For example, we cannot lift and move a ton of bricks from one place to another at a time. But we can spend more time, go more distance back and forth and move the bricks four at a time, or as many as we can carry. What about a screw that needs to be screwed into a tree? We cannot screw it with our bare hands. Screwing it in piece by piece, like a mountain of bricks brick by brick, is also impossible. You need to use a mechanism, a screwdriver. With it, we have to turn the screw a few turns so that it enters the tree at least a centimeter. But it is incomparably easier than by hand.
Consider such a simple mechanism as, for example, a shovel. Of course, it makes the job easier, it is much easier to dig the ground with it than with your hands. We stuck the shovel into the ground. To raise a clod of earth, you need to press on the handle. Where will you push to make it easier? Experience suggests that it is necessary to press, that is, apply force, closer to the end of the handle (see Fig. 3).
Rice. 3. Choice of the point of application of force
Try to apply force closer to the blade of the shovel, it will become much harder to lift a clod of earth. Applying the same strength, you will not lift anything. That is why shovels with a short handle, for example, sappers, are made with a small canvas: you still can’t lift a lot of earth with a short handle.
The shovel is a lever. A lever is a rigid body with a fixed axis of rotation (most often it is a fulcrum or suspension point). Forces act on it that tend to rotate it around the axis of rotation. In a shovel, the axis of rotation is the fulcrum on the upper edge of the hole (see Fig. 4).
Rice. 4. The axis of rotation of the shovel
A lump of earth, which we lift, acts with some force on the blade of a shovel, and our hands act on the handle, with less force (see Fig. 5).
Rice. 5. Action of forces
Consider another example: everyone rode on a swing-balancer (see Fig. 6).
Rice. 6. Swing-balancer
This is also a lever: there is a fixed axis of rotation around which the swing rotates under the influence of children's gravity.
To outweigh your friend, sitting on the opposite seat, to lift him, you will sit on the very edge of the swing. If you sit closer to the swing support, you can not outweigh it. Then you need to put someone adult and heavy in your place (see Fig. 7).
Rice. 7. The applied force must be greater than at the edge
At such a point of application of force, more force is needed than when the force was applied to the edge of the swing (see Fig. 8).
Rice. 8. Application of forces
As you have already noticed, the farther from the fulcrum we apply force, the less force is needed to do the same work. Moreover, the force needed is as many times less, how many times the lever arm is larger. lever arm- this is the distance from the point of support or suspension of the lever to the point of application of force (see Fig. 9).
Rice. 9. Leverage and Strength
Forces will be applied perpendicular to the lever.
Direction of force acting on the lever
In what direction will you act on the shovel to lift the earth? You will apply force to the shovel so that it wraps around the fulcrum, that is, perpendicular to the handle (see Fig. 10).
If you act along the handle, it will not lift the earth, unless you pull the shovel out of the ground or stick it deeper. If you press on the handle at an angle, the force can be thought of as the sum of two forces: you push perpendicular to the handle and simultaneously push or pull along the handle (see Figure 11).
Rice. 11. Action of force along the handle
Only the perpendicular component will rotate the shovel.
So, we have a lever and two forces that act on it: the weight of the load and the force that we apply to lift this load. We have found that the longer the arm of the lever, the less force is needed to balance the lever. Moreover, how many times the lever arm is greater, the force is so much less. Mathematically, this can be written as a proportion:
It does not matter if the applied forces different sides from the fulcrum or to one side. In the first case, the lever was called the lever of the first kind (see Fig. 12), and in the second - the lever of the second kind (see Fig. 13).
Rice. 12. Lever of the first kind
Rice. 13. Lever of the second kind
Working with a shovel
We looked at how the shovel makes it easier for us to dig the ground. It rests on the edge of the formed hole in the ground, this will be the axis of its rotation. The weight of the earth is applied to the short arm of the lever, we apply force to the long arm of the lever with our hands (see Fig. 14).
Rice. 14. Application of forces to the shovel
Moreover, how many times the shoulders of the lever differ, the forces applied to these shoulders differ in the same number of times.
So, we raised the clod of earth, but then you need to take the shovel with both hands, lift it completely and move the earth. Where do we take the handle of the shovel with the second hand? Everything is simple when we already know the principle of the lever. The second hand will become the new lever support. It must be positioned so as to again give a gain in strength, it must again divide the lever into short and long arms. Therefore, we will take the shovel as close as possible to the blade of the shovel. Try to lift the shovel by holding the edge with both hands - you may not succeed even with an empty shovel.
The principle by which the lever works is used very often. For example, pliers are a lever of the first kind (see Fig. 15). We act on the handles of the pliers with a force , and the pliers act on a piece of wire, tube or nut with a force much greater in modulus than . So many times more, how many times more:
Rice. 15. An example of a lever of the first kind
Another lever - can-opener, only now the application points are on one side of the fulcrum O. And again we apply force to the handle, and the blade of the opener acts on the tin tin can with a much greater modulus of force (see Fig. 16).
Rice. 16. An example of a lever of the second kind
How many times more than? In the same amount, how many times more than:
The gain in strength can be huge, we are limited only by the length of the lever and its strength.
Let's calculate how long the lever should be so that with its help a fragile girl weighing 50 kg can lift a car weighing 1500 kg by pressing on the lever with all her weight. Let's place the fulcrum of the lever so that the short arm of the lever is 1 m (see Fig. 17).
Rice. 17. Drawing for the problem
The problem describes a lever (see Fig. 18).
Rice. 18. Condition of task 1
We know how many times the gain in strength gives leverage:
Forces are applied on opposite sides of the lever support, so the two arms of the lever will add up to its length:
We have described mathematically the process specified in the condition. In our case, the force acting on the shoulder is the weight of the car, and the force acting on the shoulder is the weight of the girl.
Now it remains only to solve the equations and find the answer.
From the first equation we find the shoulder. The greater force is applied to the smaller lever arm, which means that this is the short arm equal to 1 m.
The length of the lever is:
Answer: 31 m.
How does a shovel dig itself?
Considering the examples, we did not take into account the force of gravity acting on the lever.
Imagine that we stuck a shovel shallowly into the ground. If the shovel is heavy enough, it will be able to lift a small mass of earth without our help, we will not even need to apply any force to the handle. The shovel will rotate around the axis of rotation under the action of gravity acting on the shovel handle (see Fig. 19).
Rice. 19. Turning a shovel around its axis
However, most often the weight of the lever is negligible compared to the forces that act on it, so in our model we consider the lever to be weightless.
In the example of a girl and a car, we saw that with the help of a lever we can do a job that we would never have done without a lever. With the help of a lever, even the Earth could be moved, as Archimedes spoke of (see Fig. 20).
Rice. 20. Assumption of Archimedes
The problem is that there is nothing to rest the lever on, there is no suitable fulcrum. And you, of course, imagine what an unimaginable length such a lever must be, because the mass of the Earth is 5974 billion billion tons.
Everything works too well: we can reduce the force necessary to do the work almost indefinitely. There must be a catch, otherwise with leverage our possibilities would be endless. What's the catch?
Using a lever, we apply less force, but at the same time we make a greater displacement (see Fig. 21).
Rice. 21. Moving increases
We moved the shovel handle to our outstretched hand, but raised the earth only a few centimeters. Archimedes, if he still found a fulcrum, in his entire life would not have had time to turn his lever so as to move the Earth. The less force we apply, the more displacement we make. And the product of force and displacement, that is, work, remains constant. That is, the lever gives a gain in strength, but a loss in movement, or vice versa.
Levers that are used "in reverse"
Leverage is not always used to do work with less force. Sometimes it is important to win in the move, even if it means applying more force. So does the fisherman who needs to pull the fish, move it a long distance. At the same time, he uses the fishing rod as a lever, applying force to its short shoulder (see Fig. 22).
Rice. 22. Using a fishing rod
Our hand is also a lever. The muscles of the arm contract and the arm bends at the elbow. At the same time, she can lift some load, do work. At the same time, muscles and loads act with some forces on the bones of the forearm (see Fig. 23).
Rice. 23. Our hand is a lever
The axis of rotation of the forearm is the elbow joint. Our entire musculoskeletal system consists of such levers. And the term "lever arm" itself is named so by analogy with the shoulder of one of the levers in our body - the arm.
Muscles are arranged in such a way that during contraction they cannot shorten by those half a meter by which we need to raise, for example, a cup of tea. You need to win in movement, so the muscles are attached closer to the joint, to the smaller arm of the lever. In this case, you need to apply more force, but for the muscles this is not a problem.
The lever is not the only simple mechanism that makes it easier for us to get the job done.
What simple mechanism do you use when you go up to the first floor? You can jump to the window if you can, and just climb into the room. We are accustomed to doing the same job of moving ourselves home much safer and easier - climbing stairs. So we make a greater path, but apply less force to ourselves. If we make a long gentle staircase, it will become even easier to climb, we will walk almost as if flat surface, but the path will have to be longer (see Fig. 24).
Rice. 24. Gentle staircase
The inclined plane is a simple mechanism. It is always easier not to lift something heavy, but to drag it down a slope.
Consider how an ax splits wood. Its blade is pointed and expands closer to the base, and the deeper the wedge of the ax is driven into the wood, the wider it is distributed and eventually splits (see Fig. 25).
Rice. 25. Wood cutting
The principle of operation of the wedge is the same as for the inclined plane. To push the pieces of wood a centimeter, you would need to apply a huge force. It is enough to apply less force to the wedge, however, it will be necessary to make a greater movement deep into the wood.
Screws work on the same principle of an inclined plane. Let's take a closer look at the screw: the groove along the screw is an inclined plane, only wrapped around the screw shaft (see Fig. 26).
Rice. 26. Inclined plane of the screw
And we also without special efforts we drive the screw to the depth we need. At the same time, as usual, we lose in movement: we need to make many turns of the screw in order to drive it a couple of centimeters. In any case, it is better than pushing the wood and inserting a screw into it.
When we screw in a screw with a screwdriver, we make our work even easier: the screwdriver is a lever. Look: the force with which the screw acts on the tip of the screwdriver is applied to the smaller arm of the lever, and we act on the larger arm with our own hand (see Fig. 27).
Rice. 27. The principle of the screwdriver
The handle of a screwdriver is thicker than the tip. If the screwdriver had handles like a corkscrew, the gain in strength would be even greater.
We use simple mechanisms so often that we don't even notice it. Let's take an ordinary door. Can you name three uses for a simple mechanism in a door?
Notice where the handle is. It is always located at the edge of the door, away from the hinges (see Fig. 28).
Rice. 28. Location of the handle on the door
Try to open or close the door by pushing it closer to the hinges, it will be difficult. The door is a lever, and in order to open the door with as little force as possible, the arm of this force should be as large as possible.
Let's take a look at the handle itself. If it was a bare axle, it would be difficult to open the door. The handle increases the arm to which the force is applied, and we, applying less force, open the door (see Fig. 29).
Rice. 29. Door handle
Let's look at the shape of the key. I think you can answer why they are made with wide heads. And why are the hinges on which the door rests not located next to each other, but about a quarter of the height from the edges of the door? Remember how we took a shovel when we picked it up - the same principle here. And you can also pay attention to the lock tongue cut at an angle, to the screws with which the door is screwed to the hinges (see Fig. 30).
Rice. 30. Door hinges
As you can see, simple mechanisms underlie all kinds of devices - from a door and an ax to a crane. We use them unconsciously when we choose, for example, where to grab a branch to tilt it. Nature itself, when creating a person, used simple mechanisms when it created our musculoskeletal system or teeth with their wedge-shaped shape. And if you pay attention, you will notice many more examples of how simple mechanisms make it easier to perform mechanical work and you can use them even more effectively.
This concludes our lesson, thank you for your attention!
Bibliography
- Sokolovich Yu.A., Bogdanova GS Physics: A Handbook with Examples of Problem Solving. - 2nd edition, redistribution. - X .: Vesta: Publishing house "Ranok", 2005. - 464 p.
- Peryshkin A.V. Physics: Textbook Grade 7. - M.: 2006. - 192 p.
- Virtuallab.by().
- School.xvatit.com().
- Lena24.rf ().
- Fizika.ru ().
Homework
- What is a lever? Give a definition.
- What examples of leverage do you know?
- The length of the smaller lever arm is 5 cm, the larger one is 30 cm. A force of 12 N acts on the smaller arm. What force must be applied to the larger arm in order to balance the lever?