With the help of a moving block, strength is gained. Movable block. Solving problems with moving and fixed blocks
Movable block differs from a stationary one in that its axis is not fixed, and it can rise and fall along with the load.
Figure 1. Movable block
Like the fixed block, the moving block consists of the same wheel with a groove for the cable. However, one end of the cable is fixed here, and the wheel is movable. The wheel moves with the load.
As Archimedes noted, the movable block is essentially a lever and works on the same principle, giving a gain in strength due to the difference in the shoulders.
Figure 2. Forces and forces in the moving block
The moving block moves along with the load, as if it were lying on a 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 axis, 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 moment rule in this case will look like:
$$mgr = F \cdot 2r \Rightarrow F = mg/2$$
Thus, the movable block gives a double gain in strength.
Usually in practice a combination of a fixed block and a movable one is used (Fig. 3). The fixed block is used for convenience only. It changes the direction of the force, allowing, for example, to lift a load while standing on the ground, and the movable block provides a gain in force.
Figure 3. Combination of fixed and moving blocks
We examined ideal blocks, that is, those in which the action of friction forces was not taken into account. For real blocks, it is necessary to introduce correction factors. The following formulas are used:
Fixed block
$F = f 1/2 mg $
In these formulas: $F$ is the applied external force (usually the force of a person’s hands), $m$ is the mass of the load, $g$ is the gravity coefficient, $f$ is the resistance coefficient in the block (for chains approximately 1.05, and for ropes 1,1).
Using a system of movable and fixed blocks, the loader lifts the toolbox to a height of $S_1$ = 7 m, applying a force of $F$ = 160 N. What is the mass of the box, and how many meters of rope will have to be removed while the load is lifted? What work will the loader do as a result? Compare it with the work done on the load to move it. Neglect friction and mass of the moving block.
$m, S_2 , A_1 , A_2$ - ?
The movable block gives a double gain in strength and a double loss in movement. A stationary block does not provide a gain in force, but changes its direction. Thus, the applied force will be doubled less weight load: $F = 1/2P = 1/2mg$, from where we find the mass of the box: $m=\frac(2F)(g)=\frac(2\cdot 160)(9.8)=32.65\ kg $
The movement of the load will be half as much as the length of the selected rope:
The work performed by the loader is equal to the product of the applied force and the movement of the load: $A_2=F\cdot S_2=160\cdot 14=2240\ J\ $.
Work performed on the load:
Answer: The mass of the box is 32.65 kg. The length of the selected rope is 14 m. The work performed is 2240 J and does not depend on the method of lifting the load, but only on the mass of the load and the height of the lift.
Problem 2
What load can be lifted using a moving block weighing 20 N if the rope is pulled with a force of 154 N?
Let's write down the moment rule for the moving block: $F = f 1/2 (P+ Р_Б)$, where $f$ is the correction factor for the rope.
Then $P=2\frac(F)(f)-P_B=2\cdot \frac(154)(1,1)-20=260\ N$
Answer: The weight of the load is 260 N.
Research assignment report
“Study of a system of blocks that give a gain in strength of 2, 3, 4 times”
7th grade students.
Secondary school No. 76, Yaroslavl
Topic: Studying the system of blocks that give a gain in strength of 2, 3, 4 times.
Purpose of the work: Using block systems, get a gain in strength of 2, 3, 4 times.
Equipment: movable and fixed blocks, tripods, legs with couplings, weights, rope.
Work plan:
Study theoretical material on the topic “Simple mechanisms. Blocks";
Collect and describe the installations - systems of blocks that give a gain in strength of 2, 3, 4 times.
Analysis of the experiment results;
Conclusion
"A little about blocks"
In modern technology, lifting mechanisms are widely used, which are indispensable components which can be called simple mechanisms. Among them are the oldest inventions of mankind - blocks. The ancient Greek scientist Archimedes made man's work easier by giving him a gain in strength when using his invention, and taught him to change the direction of force.
A block is a wheel with a groove around its circumference for a rope or chain, the axis of which is rigidly attached to the wall or ceiling beam. Lifting devices usually use not one, but several blocks. A system of blocks and cables designed to increase load capacity is called a chain hoist.
In physics lessons we study movable and stationary blocks. Using a fixed block, you can change the direction of the force. And the movable block - reducing it gives a 2-fold gain in strength.Fixed blockArchimedes considered it as an equal-armed lever. The moment of force acting on one side of a stationary block is equal to the moment of force applied on the other side of the block. The forces that create these moments are also the same. And Archimedes took the moving block for an unequal-armed lever. Relative to the center of rotation, moments of forces act, which in equilibrium must be equal.
Block drawings:
2. Assembling installations - systems of blocks that give a gain in strength by 2, 3 and 4 times.
In our work we use a load,whose weight is 4 N (Fig.3).
Rice. 3
Using movable and fixed blocks, our team assembled the following installations:
A block system that gives a 2x increase in strength (Fig.4 and Fig.5).
This pulley system uses a movable and a fixed pulley. This combination doubles the strength. Therefore, a force equal to half the weight of the load must be applied to point A.
Fig.4
Fig.5
The photograph (Fig. 5) shows that this installation gives a 2-fold gain in force, the dynamometer shows a force approximately equal to 2 N. There are two ropes coming from the load. We do not take into account the weight of the blocks.
A block system that gives a 3x increase in strength . Fig.6 and Fig.7
This pulley system uses two movable and fixed pulleys. This combination gives a threefold gain in strength. The principle of operation of our installation with a multiplicity of 3 (a gain in strength of 3 times) looks as shown in the figure. The end of the rope is attached to the platform, then the rope is thrown over a stationary block. Once again - through a moving block that holds the platform with the load. Then we pull the rope through another fixed block. This type of mechanism gives a gain in strength of 3 times, this is an odd option. We use simple rule: how many ropes come from the load, such is our gain in strength. In the length of the rope we lose exactly as many times as the gain in strength.
Fig.6
Fig.7
Fig.8
The photograph (Fig. 8) shows that the dynamometer shows a force of approximately 1.5 N. The error is determined by the weight of the moving block and platform. There are three ropes coming from the load.
A block system that gives a 4x increase in strength .
This pulley system uses two movable and two fixed pulleys. This combination gives a fourfold gain in strength. (Fig.9 and Fig.10).
Rice. 9
Fig.10
The photograph (Fig. 10) shows that this installation gives a 4-fold gain in force, the dynamometer shows a force approximately equal to 1 N. There are four ropes coming from the load.
Conclusion:
A system of movable and fixed pulleys, consisting of ropes and pulleys, allows you to gain effective strength while losing in length. We use a simple rule - the golden rule of mechanics: how many ropes come from the load, such is our gain in strength. In the length of the rope we lose exactly as many times as the gain in strength. Thanks to this golden rule of mechanics, you can lift large loads without exerting much effort.
Knowing this rule, you can create systems of blocks - chain hoists, which allow you to win in strength in nth quantity once. Therefore, blocks and block systems are widely used in various areas of our lives. Pmoving and fixed blocks are widely used in automobile transmission mechanisms. In addition, the blocks are used by builders to lift large and small loads (For example, when repairing the external facades of buildings, builders often work in a cradle that can be moved between floors. Upon completion of work on a floor, workers can quickly move the cradle to the floor above using and only own strength). The blocks have become so widespread because of the ease of their assembly and the ease of working with them.
For now, 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, we can consider the tension force of the cable to be the same in all its parts. In addition, we will assume that the cable is inextensible and its mass is negligible.
Fixed block
A stationary block is used to change the direction of a force. In Fig. 24.1, and shows how to use a stationary 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 wish.
Draw a diagram of the use of a stationary block that can be used to rotate the direction of a force by 90°.
Does a stationary block provide a gain in strength? Let's look at this using the example shown in Fig. 24.1, a. The cable is tensioned by the force applied by the fisherman to the free end of the cable. The tension force of the cable remains constant along the cable, therefore, from the side of the cable, a force of the same magnitude acts on the load (fish). Therefore, a stationary block does not provide a gain in strength.
When using a stationary block, the load rises by the same amount as the end of the cable to which the fisherman applies force is lowered. This means that by using a stationary block, we neither win nor lose along the way.
Movable block
Let's put experience
Lifting a load from with the help of the lung moving block, we will notice that if friction is low, 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 2-fold gain in strength.
Rice. 24.3. When using a moving block, we gain 2 times in strength, but lose the same number of times on the way
However, for a double gain in strength, you have to pay with the same loss along the way: to lift the load, for example, by 1 m, you need 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 proven without resorting to experience (see the section below “Why does a moving block give a double gain in strength?”).
A movable block differs from a stationary one in that its axis is not fixed, and it can rise and fall along with the load.
Figure 1. Movable block
Like the fixed block, the moving block consists of the same wheel with a groove for the cable. However, one end of the cable is fixed here, and the wheel is movable. The wheel moves with the load.
As Archimedes noted, the movable block is essentially a lever and works on the same principle, giving a gain in strength due to the difference in the shoulders.
Figure 2. Forces and forces in the moving block
The moving block moves along with the load, as if it were lying on a 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 axis, 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 moment rule in this case will look like:
$$mgr = F \cdot 2r \Rightarrow F = mg/2$$
Thus, the movable block gives a double gain in strength.
Usually in practice a combination of a fixed block and a movable one is used (Fig. 3). The fixed block is used for convenience only. It changes the direction of the force, allowing, for example, to lift a load while standing on the ground, and the movable block provides a gain in force.
Figure 3. Combination of fixed and moving blocks
We examined ideal blocks, that is, those in which the action of friction forces was not taken into account. For real blocks, it is necessary to introduce correction factors. The following formulas are used:
Fixed block
$F = f 1/2 mg $
In these formulas: $F$ is the applied external force (usually the force of a person’s hands), $m$ is the mass of the load, $g$ is the gravity coefficient, $f$ is the resistance coefficient in the block (for chains approximately 1.05, and for ropes 1,1).
Using a system of movable and fixed blocks, the loader lifts the toolbox to a height of $S_1$ = 7 m, applying a force of $F$ = 160 N. What is the mass of the box, and how many meters of rope will have to be removed while the load is lifted? What work will the loader do as a result? Compare it with the work done on the load to move it. Neglect friction and mass of the moving block.
$m, S_2 , A_1 , A_2$ - ?
The movable block gives a double gain in strength and a double loss in movement. A stationary block does not provide a gain in force, but changes its direction. Thus, the applied force will be half the weight of the load: $F = 1/2P = 1/2mg$, from where we find the mass of the box: $m=\frac(2F)(g)=\frac(2\cdot 160)(9 ,8)=32.65\ kg$
The movement of the load will be half as much as the length of the selected rope:
The work performed by the loader is equal to the product of the applied force and the movement of the load: $A_2=F\cdot S_2=160\cdot 14=2240\ J\ $.
Work performed on the load:
Answer: The mass of the box is 32.65 kg. The length of the selected rope is 14 m. The work performed is 2240 J and does not depend on the method of lifting the load, but only on the mass of the load and the height of the lift.
Problem 2
What load can be lifted using a moving block weighing 20 N if the rope is pulled with a force of 154 N?
Let's write down the moment rule for the moving block: $F = f 1/2 (P+ Р_Б)$, where $f$ is the correction factor for the rope.
Then $P=2\frac(F)(f)-P_B=2\cdot \frac(154)(1,1)-20=260\ N$
Answer: The weight of the load is 260 N.
Bibliographic description: Shumeiko A.V., Vetashenko O.G. Modern look on a simple “block” mechanism, studied in physics textbooks for grade 7 // Young scientist. 2016. No. 2. P. 106-113..07.2019).
Physics textbooks for 7th grade, when studying a simple block mechanism, interpret winning in different ways force when lifting a load from using this mechanism, for example: in Peryshkin's textbook A. B. winnings in strength is achieved with using the wheel of the block, on which the forces of the lever act, and in Gendenstein's textbook L. E. the same winnings are obtained with using a cable, which is subject to the tension force of the cable. Various textbooks, various items And different forces - to receive winnings in force when lifting a load. Therefore, the purpose of this article is to search for objects and strength, with through which the winnings are obtained force when lifting a load simple mechanism block.
First, let’s take a look and compare how gains in strength are obtained when lifting a load with a simple block mechanism, in physics textbooks for the 7th grade. For this purpose, we will place excerpts from textbook texts with the same concepts in a table for clarity.
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Peryshkin A.V. Physics. 7th grade. § 61. Application of the lever equilibrium rule to the block, pp. 180–183. |
Gendenshtein L. E. Physics. 7th grade. § 24. Simple mechanisms, pp. 188–196. |
|
"Block It is a wheel with a groove, mounted in a holder. A rope, cable or chain is passed through the block gutter. "Fixed block they call such a block the axis of which is fixed and does not rise or fall when lifting loads (Fig. 177). A fixed block can be considered as an equal-armed lever, in which the arms of forces are equal to the radius of the wheel (Fig. 178): OA=OB=r. Such a block does not provide a gain in strength (F1 = F2), but allows you to change the direction of the force." |
“Does a stationary block give you a gain in strength? ...in Fig. 24.1a the cable is tensioned by a force applied by the fisherman to the free end of the cable. The tension force of the cable remains constant along the cable, so from the side of the cable to the load (fish ) a force of the same magnitude acts. Therefore, a stationary block does not provide a gain in strength. 6.How can you gain strength using a fixed block? If a person lifts yourself, as shown in Fig. 24.6, then the person’s weight is distributed equally into two parts of the cable (on opposite sides of the block). Therefore, a person lifts himself by applying a force that is half his weight." |
|
“A moving block is a block whose axis rises and falls along with the load (Fig. 179).
Figure 180 shows the lever corresponding to it: O is the fulcrum of the lever, AO - arm of force P and OB - arm of force F. Since the OB arm is 2 times larger than the OA arm, then the force F is 2 times less than the force P: F=P/2. Thus, the movable block gives a gain offorce 2 times". |
"5. Why does a moving block give a win inin forcetwice? When the load is lifted uniformly, the moving block also moves uniformly. This means that the resultant of all forces applied to it is zero. If the mass of the block and the friction in it can be neglected, then we can assume that three forces are applied to the block: the weight of the load P, directed downwards, and two identical tension forces of the cable F, directed upwards. Since the resultant of these forces is zero, then P = 2F, that is the weight of the load is 2 times the tension force of the cable. But the tension force of the cable is precisely the force that is applied when lifting the load with the help of a movable block. Thus we have proven that the movable block gives a gain in force 2 times". |
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“Usually in practice they use a combination of a fixed block and a movable one (Fig. 181). The fixed block is used for convenience only. It does not give a gain in strength, but it changes the direction of the force, for example, it allows you to lift a load while standing on the ground.
Fig. 181. A combination of movable and fixed blocks - a chain hoist." |
“12.Figure 24.7 shows the system blocks. How many movable blocks does it have and how many fixed ones? What gain in strength does such a system of blocks give if friction and can the mass of the blocks be neglected? .
Fig.24.7. Answer on page 240: “12. Three moving blocks and one fixed; 8 times." |
Let’s summarize the review and comparison of texts and pictures in textbooks:
The proof of obtaining a gain in strength in the textbook by A. V. Peryshkin is carried out on the wheel of the block and the acting force is the force of the lever; When lifting a load, a stationary block does not provide a gain in strength, but a movable block provides a 2-fold gain in force. There is no mention of a cable on which a load hangs on a fixed block and a movable block with a load.
On the other hand, in the textbook by Gendenstein L.E., the proof of the gain in force is carried out on a cable on which a load or a movable block with a load hangs and the acting force is the tension force of the cable; when lifting a load, a stationary block can give a 2-fold increase in strength, but there is no mention in the text of the lever on the block wheel.
A search of literature describing the gain in force using a block and a cable led to the “Elementary Textbook of Physics”, edited by Academician G. S. Landsberg, in §84. Simple machines on pp. 168–175 are given descriptions of: “simple block, double block, gate, pulley and differential block.” Indeed, by its design, “a double block gives a gain in strength when lifting a load, due to the difference in the length of the radii of the blocks” with the help of which the load is lifted, and “a pulley block gives a gain in strength when lifting a load, due to the rope , on several parts of which a load hangs.” Thus, it was possible to find out why a block and a cable (rope) give a gain in strength when lifting a load, but it was not possible to find out how the block and cable interact with each other and transfer the weight of the load to each other, since the load can be suspended on a cable , and the cable is thrown over the block or the load can hang on the block, and the block hangs on the cable. It turned out that the tension force of the cable is constant and acts along the entire length of the cable, so the transfer of the weight of the load by the cable to the block will be at each point of contact between the cable and the block, as well as the transfer of the weight of the load suspended on the block to the cable. To clarify the interaction of the block with the cable, we will conduct experiments to obtain a gain in force with a moving block when lifting a load, using the equipment of a school physics classroom: dynamometers, laboratory blocks and a set of weights in 1H (102 g). Let's start the experiments with a moving block, because we have three different versions of obtaining a gain in strength with this block. The first version is “Fig.180. A moving block as a lever with unequal arms" - textbook by A. V. Peryshkin, the second "Fig. 24.5... two equal tension forces of the cable F" - according to the textbook by L. E. Gendenstein and finally the third "Fig. 145. Pull Block" . Lifting a load with a movable clip of a pulley on several parts of one rope - according to the textbook by G. S. Landsberg.
Experience No. 1. "Fig. 183"
To carry out experiment No. 1, obtaining a gain in strength on the movable block “with a lever with unequal shoulders OAB Fig. 180” according to the textbook by A. V. Peryshkin, on the movable block “Fig. 183” position 1, draw a lever with unequal shoulders OAB, as in “Fig. 180”, and begin lifting the load from position 1 to position 2. At the same instant, the block begins to rotate, counterclockwise, around its axis at point A, and point B, the end of the lever behind which the lift occurs, comes out beyond the semicircle along which the cable goes around the moving block from below. Point O - the fulcrum of the lever, which should be stationary, goes down, see “Fig. 183” - position 2, i.e. a lever with unequal shoulders OAB changes like a lever with equal shoulders (points O and B pass through the same paths).
Based on the data obtained in experiment No. 1 on changes in the position of the OAB lever on the moving block when lifting a load from position 1 to position 2, we can conclude that the representation of the moving block as a lever with unequal arms in “Fig. 180”, when lifting load, with rotation of the block around its axis, corresponds to a lever with equal arms, which does not provide a gain in strength when lifting the load.
We will begin experiment No. 2 by attaching dynamometers to the ends of the cable, on which we will hang a moving block with a load weighing 102 g, which corresponds to a gravity force of 1 N. We will fix one of the ends of the cable on a suspension, and using the other end of the cable we will lift the load on the moving block. Before the ascent, the readings of both dynamometers were 0.5 N each; at the beginning of the ascent, the readings of the dynamometer for which the ascent occurred changed to 0.6 N, and remained so during the ascent; at the end of the ascent, the readings returned to 0.5 N. The readings of the dynamometer, fixed for a fixed suspension did not change during the rise and remained equal to 0.5 N. Let us analyze the results of the experiment:
- Before lifting, when a load of 1 N (102 g) hangs on a movable block, the weight of the load is distributed over the entire wheel and transferred to the cable, which goes around the block from below, using the entire semicircle of the wheel.
- Before lifting, the readings of both dynamometers are 0.5 N, which indicates the distribution of the weight of a load of 1 N (102 g) into two parts of the cable (before and after the block) or that the tension force of the cable is 0.5 N, and is the same along the entire length of the cable (the same at the beginning, the same at the end of the cable) - both of these statements are true.
Let's compare the analysis of experiment No. 2 with the textbook versions about obtaining a 2-fold gain in strength using a moving block. Let's start with the statement in the textbook by Gendenstein L.E. “... that three forces are applied to the block: the weight of the load P, directed downward, and two identical tension forces of the cable, directed upward (Fig. 24.5).” It would be more accurate to say that the weight of the load in “Fig. 14.5" was distributed into two parts of the cable, before and after the block, since the tension force of the cable is one. It remains to analyze the signature under “Fig. 181” from the textbook by A. V. Peryshkin “Combination of movable and fixed blocks - pulley block.” A description of the device and the gain in strength when lifting a load with a pulley is given in the Elementary Textbook of Physics, ed. Lansberg G.S. where it is said: “Each piece of rope between the blocks will act on a moving load with a force T, and all pieces of rope will act with a force nT, where n is the number of separate sections of rope connecting both parts of the block.” It turns out that if we apply to “Fig. 181” the gain in force with a “rope connecting both parts” of the pulley from the Elementary Textbook of Physics by G. S. Landsberg, then the description of the gain in force with a moving block in “Fig. 179” and, accordingly, Fig. 180" would be an error.
Having analyzed four physics textbooks, we can conclude that the existing description of how a simple block mechanism produces gains in force does not correspond to the real state of affairs and therefore requires a new description of the operation of a simple block mechanism.
Simple lifting mechanism consists of a block and a cable (rope or chain).
The blocks of this lifting mechanism are divided into:
by design into simple and complex;
according to the method of lifting loads into movable and stationary ones.
Let's start getting acquainted with the design of blocks with simple block, which is a wheel rotating around its axis, with a groove around the circumference for a cable (rope, chain) Fig. 1 and it can be considered as an equal-armed lever in which the arms of forces are equal to the radius of the wheel: OA=OB=r. Such a block does not provide a gain in strength, but allows you to change the direction of movement of the cable (rope, chain).
Double block consists of two blocks of different radii, rigidly fastened together and mounted on common axis Fig.2. The radii of the blocks r1 and r2 are different and, when lifting a load, they act like a lever with unequal shoulders, and the gain in force will be equal to the ratio of the lengths of the radii of the block of larger diameter to the block of smaller diameter F = Р·r1/r2.
Gate consists of a cylinder (drum) and a handle attached to it, which acts as a block large diameter, The gain in force given by the collar is determined by the ratio of the radius of the circle R described by the handle to the radius of the cylinder r on which the rope is wound F = Р·r/R.
Let's move on to the method of lifting a load with blocks. From the design description, all blocks have an axis around which they rotate. If the axis of the block is fixed and does not rise or fall when lifting loads, then such a block is called fixed block single block, double block, gate.
U moving block the axle rises and falls together with the load (Fig. 10) and it is intended mainly to eliminate the bending of the cable at the place where the load is suspended.
Let's get acquainted with the device and method of lifting a load; the second part of a simple lifting mechanism is a cable, rope or chain. The cable is made of steel wires, the rope is made of threads or strands, and the chain consists of links connected to each other.
Methods for hanging a load and gaining strength when lifting a load with a cable:
In Fig. 4, the load is fixed at one end of the cable, and if you lift the load by the other end of the cable, then to lift this load you will need a force slightly greater than the weight of the load, since a simple block of gain in strength does not give F = P.
In Fig. 5, the worker lifts the load by a cable that goes around a simple block from above; at one end of the first part of the cable there is a seat on which the worker sits, and by the second part of the cable the worker lifts himself with a force 2 times less than his weight, because the worker’s weight was distributed into two parts of the cable, the first - from the seat to the block, and the second - from the block to the worker’s hands F = P/2.
In Fig. 6, the load is lifted by two workers using two cables and the weight of the load will be distributed equally between the cables and therefore each worker will lift the load with a force of half the weight of the load F = P/2.
In Fig. 7, workers are lifting a load that hangs on two parts of one cable and the weight of the load will be distributed equally between the parts of this cable (as between two cables) and each worker will lift the load with a force equal to half the weight of the load F = P/2.
In Fig. 8, the end of the cable, by which one of the workers was lifting the load, was secured on a stationary suspension, and the weight of the load was distributed into two parts of the cable, and when the worker lifted the load by the second end of the cable, the force with which the worker would lift the load was doubled less than the weight of the load F = P/2 and lifting the load will be 2 times slower.
In Fig. 9, the load hangs on 3 parts of one cable, one end of which is fixed and the gain in force when lifting the load will be equal to 3, since the weight of the load will be distributed over three parts of the cable F = P/3.
To eliminate the bend and reduce the friction force, a simple block is installed in the place where the load is suspended and the force required to lift the load has not changed, since a simple block does not provide a gain in strength (Fig. 10 and Fig. 11), and the block itself will be called moving block, since the axis of this block rises and falls along with the load.
Theoretically, a load can be suspended on an unlimited number of parts of one cable, but in practice they are limited to six parts and such a lifting mechanism is called chain hoist, which consists of a fixed and movable clip with simple blocks, which are alternately encircled by a cable, one end fixed to the fixed clip, and the load is lifted using the other end of the cable. The gain in strength depends on the number of parts of the cable between the fixed and movable cages; as a rule, it is 6 parts of the cable and the gain in strength is 6 times.
The article examines the real-life interactions between the blocks and the cable when lifting a load. The existing practice in determining that “a fixed block does not give a gain in strength, but a movable block gives a gain in force by 2 times” erroneously interpreted the interaction of the cable and the block in lifting mechanism and did not reflect the full diversity of block designs, which led to the development of one-sided erroneous ideas about the block. Compared to the existing volumes of material for studying a simple block mechanism, the volume of the article has increased by 2 times, but this made it possible to clearly and intelligibly explain the processes occurring in a simple lifting mechanism not only to students, but also to teachers.
Literature:
- Pyryshkin, A.V. Physics, 7th grade: textbook / A.V. Pyryshkin. - 3rd ed., additional - M.: Bustard, 2014, - 224 p.,: ill. ISBN 978–5-358–14436–1. § 61. Application of the lever equilibrium rule to the block, pp. 181–183.
- Gendenstein, L. E. Physics. 7th grade. In 2 hours. Part 1. Textbook for educational institutions / L. E. Gendenshten, A. B. Kaidalov, V. B. Kozhevnikov; edited by V. A. Orlova, I. I. Roizen. - 2nd ed., revised. - M.: Mnemosyne, 2010.-254 p.: ill. ISBN 978–5-346–01453–9. § 24. Simple mechanisms, pp. 188–196.
- Elementary textbook of physics, edited by academician G. S. Landsberg Volume 1. Mechanics. Heat. Molecular physics. - 10th ed. - M.: Nauka, 1985. § 84. Simple machines, pp. 168–175.
- Gromov, S. V. Physics: Textbook. for 7th grade. general education institutions / S. V. Gromov, N. A. Rodina. - 3rd ed. - M.: Education, 2001.-158 p.,: ill. ISBN-5–09–010349–6. §22. Block, pp.55 -57.
Key words: block, double block, fixed block, movable block, pulley block..
Annotation: Physics textbooks for the 7th grade, when studying a simple block mechanism, interpret in different ways the gain in force when lifting a load using this mechanism, for example: in the textbook by A. V. Peryshkin, the gain in force is achieved using the wheel of the block, on which the forces of the lever act, and in the textbook by Gendenstein L.E. the same gain is obtained using a cable, which is acted upon by the tension force of the cable. Different textbooks, different objects and different forces - to obtain a gain in strength when lifting a load. Therefore, the purpose of this article is to search for objects and forces with the help of which a gain in strength is obtained when lifting a load with a simple block mechanism.
