A block is a type 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 view of the block
Blocks are divided into movable and fixed.
The axis of a stationary block is fixed; when lifting or lowering a load, it does not rise or fall. The weight of the load that we lift will be denoted by P, the applied force will be denoted by F, and the fulcrum point will be denoted by O (Fig. 2).
Fig.2. Fixed block
The arm of force P will be the segment OA (arm of force l 1), force arm F segment OB (force arm l 2) (Fig. 3). These segments are the radii of the wheel, then the arms are equal to the radius. If the shoulders are equal, then the weight of the load and the force we apply to lift are numerically equal.
Fig.3. Fixed block
Such a block does not provide any gain in strength. From this we can conclude that it is advisable to use a stationary block for ease of lifting; it is easier to lift the load upward, using a force that is directed downward.
A device in which the axle can be raised and lowered with a load. The action is similar to the action of a lever (Fig. 4).
Rice. 4. Movable block
To operate this block, one end of the rope is fixed, a force F is applied to the other 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 every moment of movement the block rotates and point O serves as the fulcrum (Fig. 5).
Rice. 5. Movable block
The value of the force arm F is two radii.
The value of the force arm P is one radius.
The arms of the forces differ by a factor of two; according to the rule of lever equilibrium, the forces differ by a factor of two. The force required to lift a load of weight P will be half the weight of the load. The movable block gives the strength advantage twofold.
In practice, combinations of blocks are used to change the direction of action of the applied force for lifting and reduce it by half (Fig. 6).
Rice. 6. Combination of movable and fixed blocks
During the lesson, we got acquainted with the structure of a fixed and movable block, and learned that blocks are types of levers. To solve problems on this topic, it is necessary to remember the rule of lever equilibrium: the ratio of forces is inversely proportional to the ratio of the arms of these forces.
- Lukashik V.I., Ivanova E.V. Collection of problems in physics for grades 7-9 of general education institutions. - 17th ed. - M.: Education, 2004.
- Peryshkin A.V. Physics. 7th grade - 14th ed., stereotype. - M.: Bustard, 2010.
- Peryshkin A.V. Collection of problems in physics, grades 7-9: 5th ed., stereotype. - M: Publishing House “Exam”, 2010.
- Class-fizika.narod.ru ().
- School.xvatit.com ().
- scienceland.info().
Homework
- Find out for yourself what a chain hoist is and what power gains it gives.
- Where are fixed and movable blocks used in everyday life?
- What is easier to climb up: to climb on a rope or to climb using a stationary block?
Research assignment
Using the block system, you will gain 2,3,4 times the strength. What other winnings did you get? Provide block connection diagrams and photos .
Target: Using the block system, get a gain in strength of 2,3,4 times.
Plan:
Learn what blocks are and what they are needed for.
Conduct experiments with blocks, get a gain in strength of 2,3,4 times.
Apply for work.
Make a photo report.
Report:
We studied that a stationary block does not give a gain in strength, but a movable block gives a 2-fold gain in strength.
We put forward a hypothesis :
Experience No. 1. Getting a 2x win in power using a moving block .
Equipment: tripod, 2 couplings, 1 foot, rod, 1 movable block, 1 fixed block, 1 kg weight (weighing 10 N), dynamometer, rope.
Conducting the experiment:
1.Attach a stationary block or rod to a tripod, so that the plane of the stationary block and the end of the rod lie in the same plane.
2. Secure one end of the rope to the rod, throw the rope over the movable block and through the fixed block.
3. Hang a weight to the hook of the moving block, and attach a dynamometer to the free end of the rope.
5.Draw a conclusion.
Measurement results:
Conclusion: F= P/2, the gain in strength is 2 times.
Equipment. Installation for experiment No. 1.
Conducting experiment No. 1.
Experience No. 2. Get a 4x gain in power using 2 moving blocks.
Equipment: tripod, 2 movable blocks, 2 fixed blocks, 2 weights weighing 1 kg (weighing 10 N) each, dynamometer, rope.
Conducting the experiment:
1.On a tripod, using 3 couplings and 2 legs, secure 2 fixed blocks and a rod so that the planes of the blocks and the end of the rod lie in the same plane.
2. Secure one end of the rope to the rod, throw the rope sequentially through the 1st movable block, 1st fixed block, 2nd movable block, 2nd fixed block.
3. Hang a weight on the hook of each moving block, and attach a dynamometer to the free end of the rope.
4. Measure the traction force (of the arm) with a dynamometer and compare it with the weight of the weights.
5.Draw a conclusion.
Installation for experiment No. 2.
Measurement results:
Conclusion:F= P/4, the gain in strength is 4 times.
Experience No. 3. Obtaining a 3-fold increase in strength using the 1st moving block.
To get a 3-fold gain in strength, you need to use 1.5 moving blocks. Since it is impossible to separate half of the moving block, you should use the rope twice: once throw the rope completely over it, the second time attach the end of the rope to its half, i.e. to the center.
Equipment: tripod, 1 movable block with two hooks, 1 fixed block, 1 weight of 1 kg (weighing 10 N), dynamometer, rope.
Conducting the experiment:
1.Fix 1 fixed block on the tripod using a coupling.
2. Attach one end of the rope to the upper hook of the movable block, attach a weight to the lower hook of the movable block.
3. Throw the rope sequentially from the upper hook of the movable block through the fixed block, again around the movable block and again through the fixed block, and hook the dynamometer to the free end of the rope. There should be 3 ropes on which the movable block rests - 2 at the edges (full block) and one towards its center (half block). So we use 1.5 moving block.
4. Measure the traction force (of the arm) with a dynamometer and compare it with the weight of the weight.
5.Draw a conclusion.
Installation for experiment No. 3. Conducting experiment No. 3.
Measurement results:
Conclusion:F= P/3, the gain in strength is 3 times.
Conclusion:
Having carried out experiments Nos. 1-3, we tested the hypothesis put forward before the study. She was confirmed. Based on the results of the experiments, we found out the following facts:
to get a 2-fold increase in strength, you need to use 1 movable block;
to win 4 times in strength, you need to use 2 moving blocks;
to win 3 times, you need to use 1.5 moving blocks.
We also noticed that the gain in strength is equal to the number of ropes on which the movable blocks rest:
in experiment No. 1: 1 movable block rests on2 ropes - gain in strength in2 times;
in experiment No. 2: 2 movable blocks rest on4 ropes - gain in strength in4 times;
in experiment No. 3, the movable block rests on3 ropes - gain in strength in3 times.
This pattern can be applied to obtain any winning number in power. For example, to get an 8-fold win, you need to use 4 moving blocks so that they rest on 8 ropes.
Application:
Block diagrams for experiments No. 1-3.
See next page.
The term "block" means some mechanical device, which is a roller that is mounted on a perpendicular axis. This roller can either move freely, or, on the contrary, 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 shows the appearance of the block.
If the roller is fixed, for example, on the ceiling, it is a stationary block. If the roller moves with the load, it is a moving block. In a general sense, this is the only difference.
The point of using a moving block is to gain strength when lifting or moving loads and physical bodies. A fixed block does not provide any benefit, but it often greatly simplifies the movement of the body and is used in systems together with a movable block.
Application of movable and fixed blocks
The block system is found everywhere. These include cranes, various devices for moving goods in a garage, and even drive belts in a modern car. Often a block is used even without a clear understanding that this is the same mechanism.
Surely at construction sites you have come across movable wheels attached to the upper floors of a house under construction. A rope or chain is thrown over such a wheel and the worker, securing the bucket on the first floor, lifts it to the upper floor, moving the rope. This is a simple example of using a fixed block. If you add another wheel to the bucket, you get a system of blocks - movable and stationary.
Another rarer example of using a fixed block. When a person pulls a car out of the mud by wrapping the tow rope around a 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 barrel. 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.
There are many examples of the use of these simple mechanisms around us.
The most famous device that works on the principle of blocks is the chain hoist. It is actively used in lifting mechanisms. The block system reduces force and overall work is reduced by 4-8 times.
Solving problems with moving and fixed blocks
In physics problems, it is often necessary to determine what the total gain in strength will be obtained by using blocks. The student is presented with a complex circuit in which several blocks of different types are connected in a row.
Key to the solution Such tasks lie in the ability to understand the interaction of these devices. Each block is calculated separately and then added to the overall formula. The calculation formula for the entire problem is drawn up according to the diagram that the student drew while reading the condition.
To better understand such problems, it should be remembered that a block is a kind of lever. The strength 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 resistance 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 stationary block. As you can see, the relationship is direct and there is no coefficient.
For moving block we have a double gain in power. The calculation formula is F=0.5fmg, where the letter designations are similar to the formula just above. Accordingly, when using a movable block, such a box with mass m will be lifted twice as easily with the block than using only your own back.
note that drag coefficient- this is the resistance that arises in the block when the rope moves along it. Usually these values are specified in the problem statement or are tabular values. Sometimes in school problems 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 of calculating the triangle of forces. If the problem says that a person pulls a load on a rope that is at 30 degrees to the horizon, then this should certainly be taken into account and indicated on the calculation diagram.
Device Description
A block is a simple mechanism, which is a wheel with a groove around its circumference for a rope or chain, capable of freely rotating around its axis. However, a rope thrown over a tree branch is also a block to some extent.
Why are blocks needed?
Depending on their design, pulleys can allow you to change the direction of the applied force (for example, in order to lift a certain load suspended on a rope thrown over a tree branch, you need to pull the other end of the rope down... or to the side). At the same time, this block will not give a gain in strength. Such blocks are called motionless, since the axis of rotation of the block is rigidly fixed (of course, if the branch does not break). Such blocks are used for convenience. For example, when lifting a load to a height, it is much easier to pull a rope with a load thrown over a block down , putting your body weight on it, rather than standing at the top and pulling a load with a rope towards you.
In addition, there are blocks that allow you not only to change the direction of the applied force, but also provide a gain in strength. This block is called mobile and it works exactly the opposite of the moving block.
In order to gain strength, you need to firmly secure one end of the rope (for example, tie it to a branch). Next, a wheel with a groove is installed on the rope, from which the load is suspended (this must be done in such a way that the wheel with the load can move freely along our rope).Now, by pulling the free end of the rope up, we will see that the block with the load also began to rise.
The effort that we will need to expend to lift the load in this way will be approximately 2 times less than the weight of the load together with the block. Unfortunately, this type of block does not allow changing the direction of force over a wide range, so it is often used in conjunction with a stationary (rigidly fixed) block.
Description of the experience
First, the video demonstrates the principle of operation of a fixed block: loads of equal mass are suspended from a rigidly fixed block, while the block is in equilibrium. But as soon as you hang one extra weight, the advantage immediately begins to increase.
Next, using a system of movable and fixed blocks, we try to achieve a state of equilibrium by selecting the optimal number of weights suspended on both sides. As a result, the block is balanced when the number of weights suspended from the movable block becomes twice as large as the weights suspended from the free end of the thread.
Thus we can conclude that the movable block gives a double gain in strength.
This is interesting
Did you know that moving and fixed blocks are widely used in transmission mechanisms of cars? In addition, blocks are used by builders to lift large and small loads (or themselves. For example, when repairing the external facades of buildings, builders often work in a cradle, which can move between floors. Upon completion of work on the floor, workers can quickly move the cradle one floor higher, using only your own strength). The blocks have become so widespread because of the ease of their assembly and the ease of working with them.
