A simple machine that can rotate around the central axis consists of a grooved disc that can rotate around the central axis and a flexible cable (rope, adhesive tape, steel cable, chain, etc.). ) across the disk. Pulley is the deformation of lever, which belongs to lever simple machinery. In China, as early as the Warring States Period, there was a record about pulleys in the book Mo Jing. The pulley with the fixed central shaft is called the crown block, which is a deformed equal arm lever, and can change the direction of force without any effort. The pulley whose central axis moves with the weight is called the movable pulley, which is a deformed unequal arm lever, and can save half the force without changing the direction of the force. In practice, a certain number of moving pulleys and crown block are often combined into various pulley blocks. The pulley block can save labor and change the direction of force.
The differential pulley (commonly known as chain block) commonly used in factories is also a kind of pulley block. Pulley blocks are widely used in cranes, winches, elevators and other machinery.
There are two kinds of pulleys: crown block and moving pulley.
(1) The crown block is essentially an equal arm lever, which is labor-saving and effortless, but it can change the direction of the acting force.
Characteristics of crown block
It is not labor-saving to pull the hook code through the crown block. Whether there is a crown block or not, the reading of the spring scale is the same. It can be seen that the direction of force can be changed easily by using the crown block. Many times, changing the direction of force will bring convenience to work.
Crown block principle
The crown block is essentially an equal arm lever, and the power L 1 and the resistance L2 arm are equal to the pulley radius. According to the lever balance condition, it can also be concluded that the crown block is not labor-saving.
(2) The moving pulley is essentially a lever with twice the power arm as the resistance arm, which saves the force of 1/2 and spends the distance of 1 times more.
Characteristics of moving pulley
Using a moving pulley can save half the force and waste the distance. This is because when using the moving pulley, the hook code is hung by two ropes, and each rope only bears half the weight of the hook code. Although the use of moving pulley saves power, the distance of power moving is greater than the distance of hook code rising, that is, the distance is wasted.
Moving pulley principle
The moving pulley is essentially a lever, and its arm of force (L 1) is twice as big as the arm of resistance (L2).
(3) Pulley block: A pulley block consisting of a crown block following pulleys, which can save labor and change the direction of force.
The pulley block uses several lengths of rope to lift the object, and the force used to lift the object is a fraction of the total weight. The free end of the rope bypasses the moving pulley, but it is not.
The pulley block is labor-saving, but it consumes distance, and the distance of power movement is greater than that of heavy objects.
Purpose of pulley block:
In order to save energy and change the direction of power, the crown block and the moving pulley can be combined into a pulley block.
Labor-saving size
When using the pulley block, the pulley block uses several pieces of rope to lift the object, and the force used to lift the object is a fraction of the weight of the object.
Characteristics of pulley block
It is easy to see that the pulley block is labor-saving, but it consumes distance-the distance of power moving is greater than the distance of goods rising.
A simple machine that can rotate around the central axis consists of a grooved disc that can rotate around the central axis and a flexible cable (rope, adhesive tape, steel cable, chain, etc.). ) across the disk. Pulley is the deformation of lever, which belongs to lever simple machinery. In China, as early as the Warring States Period, there was a record about pulleys in the book Mo Jing. The pulley with the fixed central shaft is called the crown block, which is a deformed equal arm lever, and can change the direction of force without any effort. The pulley whose central axis moves with the weight is called the movable pulley, which is a deformed unequal arm lever, and can save half the force without changing the direction of the force. In practice, a certain number of moving pulleys and crown block are often combined into various pulley blocks. The pulley block can save labor and change the direction of force.
The differential pulley (commonly known as chain block) commonly used in factories is also a kind of pulley block. Pulley blocks are widely used in cranes, winches, elevators and other machinery.
There are two kinds of pulleys: crown block and moving pulley.
(1) The crown block is essentially an equal arm lever, which is labor-saving and effortless, but it can change the direction of the acting force.
Characteristics of crown block
It is not labor-saving to pull the hook code through the crown block. Whether there is a crown block or not, the reading of the spring scale is the same. It can be seen that the direction of force can be changed easily by using the crown block. Many times, changing the direction of force will bring convenience to work.
Crown block principle
The crown block is essentially an equal arm lever, and the power L 1 and the resistance L2 arm are equal to the pulley radius. According to the lever balance condition, it can also be concluded that the crown block is not labor-saving.
(2) The essence of moving pulley is power arm is resistance ... >; & gt
Question 2: What are the benefits of roller skating? 1. Adjusting the amount of exercise can get various exercise effects, such as losing weight.
-Burning 570 to 1 150 calories per hour is as good as running.
Generally speaking, the maximum oxygen consumption of roller skating (the benchmark for measuring exercise intensity) is 90% of that of running, which is very effective for maintaining the optimal intensity of aerobic exercise.
According to research, the heartbeat measured when skating at a speed of 23km/h is 74% of the maximum heartbeat. It is a typical aerobic exercise, which can strengthen the cardiovascular system and burn fat.
-When the speed is increased to 28km/hr, the heart rate will reach 85% of the highest heart rate and exceed the anaerobic threshold, which can improve muscle endurance.
You can adjust your body to the best condition by skating constantly.
2. The risk of injury is very small.
-Because roller skating is different from running, it will cause the wheels to turn when stepping, and the elasticity of the wheels made of polyurethane has little influence on the joints, so the elderly are also suitable for this sport. -Wear a helmet and protective gear, and there is little risk of injury after falling.
This is a very good exercise for growing children.
-Improve the sense of balance, exercise, physical strength and endurance, and help children grow and develop during their growth.
-Do more outdoor sports with others to make yourself cheerful.
-The effect of burning fat can prevent the symptoms of childhood obesity.
4. Exercise with your family
-From growing children to the elderly, men, women and children can enjoy sports together.
-Easy to learn exercise
Women are easy to learn. In fact, the ratio of male and female skaters is 50: 50.
5. Economic sports
-If you buy good equipment when you are a beginner, you hardly need to spend any other money.
-Unlike swimming, tennis and other sports, roller skating doesn't need a specific place, and it doesn't cost money like tickets.
-you can use it as a means of transportation according to your own strength.
You can make more friends by roller skating.
-Get to know more people and make many friends by participating in clubs and other activities.
-You can skate alone, and more people are interested in skating together.
7. It's not easy to get bored
-Roller skating can do leisure, speed skating, street brushing, flat flower, roller skating, pattern and other diversified sports, and will not get tired easily.
Look at the development of your roller skating skills, and you will have a great sense of accomplishment.
When you reach a certain technical level, you will get extra points in the college entrance examination.
-candidates who won the title of national second-class athlete or above in the same year and were tested and approved by the provincial admissions Committee. On the basis of the total score of candidates' unified examination results, the score can be appropriately improved.
Question 3: What is the difference between a wheel and a pulley in life? Wheels are used to put cars underground. Rolling friction replaces sliding friction, which is more labor-saving Pulleys are used to lift goods and are divided into moving pulleys and crown blocks. The crown block is used to change the stretching direction of the rope, which has no labor-saving effect. The moving pulley does not have the function of changing the stretching direction of the rope, but it saves half the effort.
Question 4: What are the skills of roller skating? Roller skates are divided into single row and double row. When the roller skates are viewed from the front, we should pay attention to sliding in figure 8. Just stand up and work hard.
Double row is different. It seems that double row is better than single row. Actually, it's not like that. Double row takes one foot as the axis and the other foot as the force, which can be interchanged and changed direction!
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Roller skating skills
Jump (high jump)
Bend your knees before taking off and lower your center of gravity. When jumping, the knees are instantly straightened and the body is pulled up. When your feet leave the ground, raise your knees as high as possible and get as close to your chest as possible. When you fall, your feet touch the ground in tandem and your center of gravity is stable!
Down the stairs:
There is a difference between before and after sliding down the stairs. Don't think it's a staircase, it just slides past like a flat ground. Feet apart, center of gravity in the middle! If you slide forward, you will usually hit the stairs in three or four laps, with the center of gravity at the heel, but in the middle of your feet, remember to squat down and open your feet back and forth! !
Beginners should start with 1.2. The wider the ladder, the better (don't exaggerate). Practice at 30 degrees, 45 degrees, 60 degrees and 75 degrees. Pay attention to the speed, or you will get stuck.
A) Slide forward: first accelerate to a suitable speed, and when you get to the stairs, you will be in a high kneeling position and your body will be straight. Don't lean forward or backward, slide with the trend. Squat your knees slightly, separate your feet, tilt your toes to your heels at an angle of about 45 degrees, keep a certain speed, relax your knees, and keep the distance between your feet at a certain level, starting from the first step, then the second, third and fourth steps. If you have the opportunity to correct the angle, 60 degrees ... 75 degrees is uncertain, don't try 90 degrees easily, and pay attention to the softness of your knees so as not to cause sports injuries. ..
B) Backward slide: practice first and then slide. Generally speaking, backward sliding is more stable than forward sliding. Remember to wear protective gear. Accelerate to the right speed, turn around when you get to the stairs, sprint with your feet in tandem and slide down. Focus on your hind feet, and the most important thing is to overcome your fears. The key point is to support your feet and don't deform as soon as you reach the foot of the stairs. Don't put your feet together no matter what happens. Don't go forward or backward.
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1, stand
1)t-stand method: the left heel stands against the inside of the right foot in a T-stand posture, and the upper body leans forward slightly.
2) Legislation of parallel standing: 2 feet stand parallel to shoulder width, and the upper body leans forward slightly.
Note: When standing, don't fold inward or spread out the corners, keep the knife rest vertical to the ground, and exercise at ordinary times to develop a good standing posture.
3) Squatting posture: the upper body leans forward, the shoulders are slightly higher than the hips, the hands are clenched or naturally swung, the legs are bent, the upper body and the ground form an angle of 15-20 degrees, the knee joint forms an angle of 90-1/0, and the ankle joint forms an angle of 50-70 degrees.
Pay attention to the squat posture practice of each movement during practice.
2, moving the center of gravity practice
1) Stand still and step on it.
Wear roller skates, stand with your feet shoulder-width apart, your legs slightly bent, your upper body slightly leaned forward, your arms naturally drooped, your center of gravity shifted to your left leg, and your right leg was slightly lifted and put down; Move the center of gravity to the right leg, lift the left leg slightly and then put it down, and so on, gradually speed up.
2) One-leg support balance
On the basis of mastering standing still, keep the original posture, hold the railing or companion, shift the center of gravity to one leg, and then stretch the other leg to the side and retract to the starting posture. Change your feet and repeat the above actions.
3) "figure-eight" walking practice
Stand with your feet open, move your center of gravity to your left foot, move your right foot forward, then move your center of gravity to your left leg, then move your center of gravity to your right leg, and so on.
3) Walking across steps
Stand in the same place, first move the center of gravity to the left leg, retract the right leg, take a step to the front and outside of the left leg to form a cross posture, then move the center of gravity to the right leg, and then retract the left side of the left leg to form a starting posture, and practice repeatedly; At the same time, practice walking towards the skeleton.
Step 3 slide in a straight line
1) One-pedal double-foot sliding exercise
Push the inner edge of the right foot to the ground and push the center of gravity to the left leg that slides forward. After the right foot hits the ground, it will quickly slide into two feet with the left foot, and then do the action of touching the ground with the left foot, and practice sliding forward alternately and repeatedly.
2) Sliding on one leg
The upper body leans forward, the arms droop naturally, the feet are differentiated, and it is heavy to stand in an outer figure of eight ... >>
Question 5: What are the advantages of playing with pulleys? I've been playing for several years, and I don't think it's very good, but it's quite fun to play in my spare time. Playing roller skating when I am in a bad mood can also make me happy. Personally, I think roller skating is a challenging sport. I want to make friends with you!
Question 6: What's the difference between axle, bearing and pulley? The axle is a round wheel with a central rotating shaft in the middle.
A bearing is an object that is inserted into an object with a hole in the middle and can rotate freely.
There are two kinds of pulleys: moving pulley and static pulley, that is, the whole system in which a rope bypasses a round wheel is called pulley.