The term "block" means some mechanical device, which is a roller, which is fixed on a perpendicular axis. This roller can either move freely, or vice versa - it is rigidly fixed. Let's simplify the definition - if the axis of rotation of the roller moves in space, then the block is movable. The roller has a groove into which a rope or cable is inserted. The picture below demonstrates appearance block.
If the roller is fixed, for example, on the ceiling, it is a fixed block. If the roller moves with the load, it is a moving block. In a general sense, the only difference is this.
The meaning of using a movable block is to gain strength when lifting or moving loads and physical bodies. A fixed block does not give a gain, however, it often greatly simplifies the movement of the body and is used in systems in conjunction with a movable block.
The use of movable and fixed blocks
The block system is ubiquitous. These are cranes and various devices for moving goods in the garage, and even drive belts V modern car. Often the block is used even without a clear understanding that this is the same mechanism.
Surely on construction sites you have seen movable wheels fixed on the upper floors of a house under construction. A rope or chain is thrown over such a wheel and the worker, fixing the bucket on the first floor, raises it to the upper floor, moving the rope. This is a simple example of using a fixed block. If you add one more wheel to the bucket, you get a system of blocks - movable and stationary.
Another more rare example of using a fixed block. When a person pulls a car out of the mud, wrapping tow rope around the tree trunk. This is done for greater convenience, since the towing winch will easily catch on the small end of the cable wrapped around the trunk. There is no gain from such a block itself, and since the tree does not rotate around its axis, the resistance force increases the load.
Examples of using these simple mechanisms there are a lot around us.
The most famous device that works on the principle of blocks is the chain hoist. It is actively used in lifting mechanisms. Block system reduces strength and general work is reduced by 4-8 times.
Solving problems with moving and fixed blocks
In physics problems, it is often necessary to determine what total gain in strength will be obtained when using blocks. The student is offered complex scheme where several blocks of different types are connected in a row.
Key to the solution of such tasks lies in the ability to understand the interaction of these devices. Each block is calculated separately and then added to general formula. The calculation formula for the entire task is compiled according to the diagram that the student drew while reading the condition.
For a better understanding of such problems, it should be remembered that block is a kind of lever. The force gained gives a loss in distance (in the case of a moving block).
The calculation formula is very simple.
For fixed block F=fmg, where F is the force, f is the drag coefficient of the block, m is the mass of the load, g is the gravitational constant. In other words, F is the force that must be applied to lift, for example, a box from the ground using a fixed block. As you can see, the dependence is direct and there is no coefficient.
For moving block we have a double gain in strength. Calculation formula F=0.5fmg, where letter designations similar to the formula above. Accordingly, when using a movable block, such a box with a mass m will be twice as easy to lift with a block than using only one's own back.
note that drag coefficient- this is the resistance that occurs in the block when the rope moves along it. Usually these values are specified in the problem statement or are a tabular value. Sometimes in school tasks these coefficients are completely omitted and not taken into account.
Moreover, one must not forget that if the force is applied at an angle, then you need to use the standard method for calculating the triangle of forces. If the task says that a person is pulling a load by a rope that is at 30 degrees to the horizon line, then this should certainly be taken into account and indicated on the design diagram.
Lifting machines are designed to help a person lift something heavy to a height. At the heart of most lifting mechanisms is simple system blocks - chain hoist. He was still familiar to Archimedes, but now about this brilliant invention many do not know. Remembering the physics course, find out how such a mechanism works, its structure and scope. Having understood the classification, you can proceed to the calculation. To make it work - your attention to the instructions for constructing a simple model.
The invention of the chain hoist gave a huge impetus to the development of civilizations. The block system helped to build huge structures, many of which have survived to this day and are bewildering to modern builders. Shipbuilding was also improved, people were able to travel great distances. It's time to figure out what it is - a chain hoist and find out where you can find application for it today.
Simplicity and efficiency of the mechanism
The structure of the lifting mechanism
The classic chain hoist is a mechanism that consists of two main elements:
- pulley;
- flexible connection.
The simplest scheme: 1 - movable block, 2 - fixed, 3 - rope
A pulley is a metal wheel that has a special groove for the cable along the outer edge. As a flexible connection, a conventional cable or rope can be used. If the load is heavy enough, synthetic fiber cables or steel ropes and even chains are used. In order for the pulley to rotate easily, without jumps and jamming, use roller bearings. All elements that move are lubricated.
One pulley is called a block. Polyspast is a system of blocks for lifting loads. Blocks in the lifting mechanism can be fixed (rigidly fixed) and movable (when the axis changes position during operation). One part of the chain hoist is attached to a fixed support, the other to the load. The movable rollers are located on the side of the load.
Fixed block
The role of the fixed block is to change the direction of the rope movement and the action of the applied force. The role of mobile is to gain a gain in strength.
Movable block
The principle of operation - what is the secret
The principle of operation of a chain hoist is similar to a lever: the force to be applied becomes several times less, while the work is performed in the same volume. The rope plays the role of a lever. In the work of the chain hoist, the gain in strength is important, so the resulting loss in distance is not taken into account.
Depending on the design of the chain hoist, the gain in strength can be different. The simplest mechanism of two pulleys gives approximately two times the gain, three - three times, and so on. The increase in distance is calculated according to the same principle. For the operation of a simple chain hoist, a cable is needed twice as long as the lifting height, and if a complex of four blocks is used, then the length of the cable increases in direct proportion to four times.
The principle of operation of the block system
In what areas is the block system used?
Polyspast is a faithful assistant in a warehouse, in production, in transport sector. It is used wherever you need to use force to move all kinds of goods. The system is widely used in construction.
Despite the fact that most of the hard work is done by construction equipment ( crane), the chain hoist found a place in the design of load-handling mechanisms. The block system (polyspast) is a component of such lifting mechanisms as a winch, a hoist, construction equipment (cranes of various types, a bulldozer, an excavator).
In addition to the construction industry, chain hoists received wide application in the organization of rescue operations. The principle of operation remains the same, but the design is slightly modified. Rescue equipment is made of durable rope, carabiners are used. For devices of this purpose, it is important that the entire system is quickly assembled and does not require additional mechanisms.
Polyspast as part of a crane hook
Classification of models according to different characteristics
There are many versions of one idea - a system of blocks, united by a rope. They are differentiated depending on the method of application and design features. Get to know different types lifts, find out what their purpose is and how the device differs.
Classification depending on the complexity of the mechanism
Depending on the complexity of the mechanism,
- simple;
- complex;
- complex polyspasts.
An example of even models
A simple chain hoist is a system of rollers connected in series. All mobile and fixed blocks, as well as the load itself are combined by one cable. Differentiate even and odd simple chain hoists.
Those are called even lifting mechanisms, whose end of the cable is attached to a fixed support - the station. All combinations in this case will be considered even. And if the end of the rope is attached directly to the load or the place where the force is applied, this construction and all derivatives from it will be called odd.
Scheme of an odd chain hoist
A complex chain hoist can be called a chain hoist system. In this case, not individual blocks are connected in series, but whole combinations that can be used on their own. Roughly speaking, in this case, one mechanism sets in motion another similar one.
The complex chain hoist does not belong to either one or the other type. His distinguishing feature- rollers moving towards the load. The composition of the complex model can include both simple and complex chain hoists.
Combining a two-fold and six-fold simple chain hoist gives a complex six-fold option
Classification according to the purpose of the lift
Depending on what they want to get when using the chain hoist, they are divided into:
- power;
- high-speed.
A - power option, B - high-speed
Power option used more often. As the name implies, its task is to ensure a gain in strength. Since a significant gain requires an equally significant loss in distance, a loss in speed is inevitable. For example, for a 4:1 system, when lifting a load one meter, you need to pull 4 meters of cable, which slows down the work.
The high-speed chain hoist, in its principle, is a reverse power structure. It does not give a gain in strength, its goal is speed. It is used to speed up work to the detriment of the applied effort.
Multiplicity - the main characteristic
The main indicator that is paid attention to when organizing the lifting of goods is the multiplicity of the chain hoist. This parameter conditionally indicates how many times the mechanism allows you to win in strength. In fact, the multiplicity shows how many branches of the rope the weight of the load is distributed.
Kinematic multiplicity
The multiplicity is divided into kinematic (equal to the number of bends of the rope) and power, which is calculated taking into account the overcoming of the friction force by the cable and the non-ideal efficiency of the rollers. The reference books contain tables that display the dependence of the power multiplicity on the kinematic for different block efficiency.
As can be seen from the table, the force multiplicity differs significantly from the kinematic one. With a low efficiency of the roller (94%), the actual gain in the pulley block strength of 7: 1 will be less than the gain of a six-fold pulley block with a block efficiency of 96%.
Schemes of chain hoists of different multiplicity
How to make calculations for a chain hoist
Despite the fact that theoretically the design of the chain hoist is extremely simple, in practice it is not always clear how to lift the load using blocks. How to understand what multiplicity is needed, how to find out the efficiency of the lift and each block separately. In order to find answers to these questions, you need to perform calculations.
Single block calculation
The calculation of the chain hoist must be performed due to the fact that working conditions are far from ideal. Friction forces act on the mechanism as a result of the movement of the cable along the pulley, as a result of the rotation of the roller itself, no matter what bearings are used.
In addition, at the construction site and as part of construction equipment a flexible and pliable rope is rarely used. A steel rope or chain is much more rigid. Since additional force is required to bend such a cable when running on a block, it must also be taken into account.
For calculation, the moment equation for the pulley about the axis is derived:
SrunR = SrunR + q SrunR + Nfr (1)
Formula 1 shows the moments of such forces:
- Sbeg - effort from the side of the escaping rope;
- Sraid - effort from the side of the oncoming rope;
- q Sraid - effort for bending / unbending the rope, taking into account its rigidity q;
- Nf is the friction force in the block, taking into account the friction coefficient f.
To determine the moment, all forces are multiplied by the shoulder - the radius of the block R or the radius of the sleeve r.
The force of the incoming and outgoing cable arises as a result of the interaction and friction of the rope threads. Since the force for bending / unbending the cable is significantly less than the others, when calculating the impact on the axis of the block, this value is often neglected:
N = 2 Sraid×sinα (2)
In this equation:
- N is the impact on the pulley axis;
- S run - effort from the side of the oncoming rope (assumed to be approximately equal to S run;
- α is the angle of deviation from the axis.
Pulley block
Block efficiency calculation
As you know, efficiency is the coefficient useful action, that is, how effective the work was done. It is calculated as the ratio of the work performed and the work expended. In the case of a pulley block, the formula is applied:
ηb = Srun / Srun = 1/(1 + q + 2fsinα×d/D) (3)
In the equation:
- 3 ηb – block efficiency;
- d and D - respectively, the diameter of the bushing and the pulley itself;
- q is the coefficient of rigidity of the flexible connection (rope);
- f is the coefficient of friction;
- α is the angle of deviation from the axis.
From this formula it can be seen that the efficiency is affected by the structure of the block (through the coefficient f), its size (through the ratio d / D) and the material of the rope (factor q). The maximum efficiency value can be obtained using bronze bushings and rolling bearings (up to 98%). Plain bearings will give up to 96% efficiency.
The diagram shows all the forces S on different branches of the rope
How to calculate the efficiency of the entire system
The lifting mechanism consists of several blocks. The total efficiency of the chain hoist is not equal to the arithmetic sum of all the individual components. For the calculation, a much more complex formula is used, or rather, a system of equations, where all forces are expressed through the value of the primary S0 and the efficiency of the mechanism:
- S1=ηп S0;
- S2=(ηп)2 S0; (4)
- S3=(ηп)3 S0;
- Sn=(ηп)n S0.
The efficiency of the chain hoist at different multiplicity
Since the efficiency value is always less than 1, with each new block and equation in the system, the value of Sn will rapidly decrease. The total efficiency of the chain hoist will depend not only on ηb, but also on the number of these blocks - the multiplicity of the system. According to the table, you can find ηп for systems with a different number of blocks for different efficiency values everyone.
How to make a do-it-yourself lift
In construction during installation work it is not always possible to adjust the crane. Then the question arises, how to lift the load with a rope. And here a simple chain hoist finds its application. For its manufacture and full-fledged work you need to make calculations, drawings, choose the right rope and blocks.
Miscellaneous schemes simple and complex lifts
Base preparation - diagram and drawing
Before proceeding with the construction of a chain hoist with your own hands, you need to carefully study the drawings and choose a suitable scheme for yourself. You should rely on how it will be more convenient for you to place the structure, what blocks and cable are available.
It happens that the carrying capacity of the chain hoist blocks is not enough, and there is no time and opportunity to build a complex multiple lifting mechanism. Then double chain hoists are used, which are a combination of two single ones. This device can also lift the load in such a way that it moves strictly vertically, without distortions.
Drawings of a dual model in different variations
How to choose rope and block
critical role in the construction of a chain hoist with their own hands, a rope plays. It is important that it does not stretch. Such ropes are called static. Stretching and deformation of the flexible connection gives a serious loss of work efficiency. For a homemade mechanism, a synthetic cable is suitable, the thickness depends on the weight of the load.
The material and quality of the blocks are indicators that will provide home-made lifting devices estimated load capacity. Depending on the bearings that are installed in the block, its efficiency changes and this is already taken into account in the calculations.
But how to lift the load to a height with your own hands and not drop it? To protect the load from a possible reverse motion, you can install a special locking block that allows the rope to move in only one direction - the desired direction.
Roller on which the rope moves
Step-by-step instructions for lifting a load through the block
When the rope and blocks are ready, the scheme is selected, and the calculation is made, you can start assembling. For a simple double chain hoist you will need:
- roller - 2 pcs.;
- bearings;
- sleeve - 2 pcs.;
- holder for the block - 2 pcs.;
- rope;
- hook for cargo suspension;
- slings - if they are needed for installation.
Carabiners are used for quick connection
Step-by-step lifting of the load to a height is carried out as follows:
- Connect rollers, bushing and bearings. Combine it all in a cage. Get a block.
- The rope is launched into the first block;
- The holder with this block is rigidly attached to a fixed support (reinforced concrete beam, pole, wall, specially mounted extension, etc.);
- Then the end of the rope is passed through the second block (movable).
- A hook is attached to the clip.
- The free end of the rope is fixed.
- They sling the load being lifted and connect it to the chain hoist.
A homemade lifting mechanism is ready to use and will provide a double gain in strength. Now, to lift the load to a height, it is enough to pull the end of the rope. By bending around both rollers, the rope will lift the load without much effort.
Is it possible to combine a chain hoist and a winch
If to homemade mechanism, which you build according to this instruction, attach an electric winch, you get a real do-it-yourself crane. Now you don’t have to strain at all to lift the load, the winch will do everything for you.
Even manual winch will make lifting the load more comfortable - no need to wash your hands on the rope and worry that the rope does not slip out of your hands. In any case, turning the winch handle is much easier.
Chain hoist for winch
In principle, even outside the construction site, the ability to build an elementary chain hoist for a winch in field conditions with a minimum of tools and materials is a very useful skill. It will be especially appreciated by motorists who were lucky enough to get stuck in a car somewhere in an impassable place. Made on hastily the chain hoist will significantly increase the performance of the winch.
It is difficult to overestimate the importance of the chain hoist in the development of modern construction and engineering. Everyone should understand the principle of operation and visually imagine its design. Now you are not afraid of situations when you need to lift a load, but special equipment No. A few pulleys, a rope and ingenuity will allow you to do without involving a crane.
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
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| 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).
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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.
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IN 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.
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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 twofold 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 is a flat, flat surface at some angle to the horizontal. 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).
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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 to be accomplished by the mechanism under ideal conditions without any loss, and 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 constituent elements 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 / A 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.
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There is no acceleration, so the forces acting on the load are balanced:
Projecting on the X axis:
. (1)
Projecting on the Y axis:
. (2)
Besides,
, (3)
From (2) we have:
Then from (3) :
Substituting this into (1) , we get:
The total work is equal to the product of the force F and the path traveled by the body along the surface of the inclined plane:
A full=.
The useful work is obviously equal to:
A useful=.
For the desired efficiency, we get.
A block is a kind of lever, it is a wheel with a groove (Fig. 1), a rope, cable, rope or chain can be passed through the groove.
Fig.1. General form block
Blocks are divided into mobile and fixed.
At the fixed block, the axle is fixed; when lifting or lowering the load, it does not rise or fall. Let's denote the weight of the load that we lift, P, the applied force, denote F, the fulcrum - O (Fig. 2).
Fig.2. Fixed block
The arm of the force P will be the segment OA (the arm of the force l 1), arm of force F segment OB (arm of force l 2) (Fig. 3). These segments are the radii of the wheel, then the shoulders are equal to the radius. If the shoulders are equal, then the weight of the load and the force that we apply to lift are numerically equal.
Fig.3. Fixed block
Such a block does not give a gain in strength. From this we can conclude that it is advisable to use a fixed block for ease of lifting, it is easier to lift the load up using a downward force.
A device in which the axle can be raised and lowered together with the load. The action is similar to the action of a lever (Fig. 4).
Rice. 4. Movable block
For this block to work, one end of the rope is fixed, we apply force F to the second end to lift a load of weight P, the load is attached to point A. The fulcrum during rotation will be point O, because at each moment of movement the block turns and point O serves as a fulcrum (Fig.5).
Rice. 5. Moving block
The value of the shoulder of the force F is two radii.
The value of the shoulder of force P is one radius.
The arms of the forces differ by a factor of two, according to the lever balance rule, the forces differ by a factor of two. The force that is needed to lift a load of weight P will be half the weight of the load. The movable block gives the advantage in strength twice.
In practice, combinations of blocks are used to change the direction of the applied force for lifting and reduce it by half (Fig. 6).
Rice. 6. Combination of movable and fixed blocks
At the lesson, we got acquainted with the device of a fixed and movable block, dismantled that blocks are varieties of levers. To solve problems on this topic, it is necessary to remember the rule of balance of the lever: the ratio of forces is inversely proportional to the ratio of the shoulders of these forces.
- Lukashik V.I., Ivanova E.V. Collection of tasks in physics for grades 7-9 educational institutions. - 17th ed. - M.: Enlightenment, 2004.
- Peryshkin A.V. Physics. 7 cells - 14th ed., stereotype. - M.: Bustard, 2010.
- Peryshkin A.V. Collection of problems in physics, grades 7-9: 5th ed., stereotype. - M: Exam Publishing House, 2010.
- Class-fizika.narod.ru ().
- School.xvatit.com().
- scienceland.info().
Homework
- Find out for yourself what a chain hoist is and what kind of gain in strength it gives.
- Where are fixed and movable blocks used in everyday life?
- How easy is it to climb up: climb a rope or climb with a fixed block?
For the time being, we will assume that the mass of the block and cable, as well as the friction in the block, can be neglected. In this case, the tension force of the cable can be considered the same in all its parts. In addition, we will assume that the cable is inextensible, and its mass is negligible.
Fixed block
The fixed block is used to change the direction of the force. On fig. 24.1a shows how to use a fixed block to change the direction of the force to the opposite. However, with its help, you can change the direction of the force as you like.
Draw a diagram of the use of a fixed block that can be used to rotate the direction of the force by 90°.
Does a fixed block give a gain in strength? Let's look at this using the example shown in Fig. 24.1, a. The cable is taut by the force applied by the fisherman to the free end of the cable. The tension force of the rope remains constant along the rope, therefore, from the side of the rope, the same force acts on the load (fish). Therefore, a fixed block does not give a gain in strength.
When using a fixed block, the load is raised by the same amount as the end of the cable is lowered, to which the fisherman applies force. This means that by using a fixed block, we don't win or lose along the way.
Movable block
Let's put experience
Lifting the load with the help of a light moving block, we will notice that if the friction is small, then to lift the load it is necessary to apply a force that is approximately 2 times less than the weight of the load (Fig. 24.3). Thus, the movable block gives a gain in strength of 2 times.
Rice. 24.3. When using a movable block, we win in strength by 2 times, but we lose the same amount on the way.
However, for a double gain in strength, one has to pay with the same loss on the way: in order to lift the load, for example, by 1 m, it is necessary to raise the end of the cable thrown over the block by 2 m.
The fact that a moving block gives a double gain in strength can be proved without resorting to experience (see the section "Why does a moving block give a double gain in strength?" below).
