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When an object moves on the surface of another object, there will be a force at the contact surface between the two objects called friction. For example, in daily life, cars move forward on the road by the friction between the tires and the ground. Friction is usually divided into sliding friction, rolling friction and static friction.

We know that the kicked football will stop slowly because of friction. Pushing a car hard, without pushing, is also due to friction. Brick-cutting performance, also by virtue of friction, can stick bricks together piece by piece until they stick in a big pile without falling off. Friction can create many wonderful scenes in the hands of acrobats. The traction of the locomotive on the carriage comes from the friction between the train wheels and the rails. When the wind blows over the sea surface, the friction of the wind on the sea surface and the pressure exerted by the wind on the windward side of the waves force the sea water to move forward, thus forming a wind current. In the friction layer, the wind walks on the rough and uneven ground surface, and the wind speed has to be reduced under the action of friction. Because the roughness of the surface is different and the friction is different, the degree of wind speed reduction is different. Generally speaking, the friction on the land surface is greater than that on the sea surface; On the other hand, the friction on the land surface is greater in mountains than in plains, and in forests than in grasslands. Friction will not only weaken the wind speed, but also interfere with the wind direction. Carpenters use sandpaper to smooth the wood board by the friction generated by sandpaper and wood board. Automobile engine transfers kinetic energy to generator to generate electricity by friction with belt; When people wash their hands, their hands rub to wash away the dust on their hands; The rotation of the washing machine causes friction between clothes and water when washing clothes; When eating, teeth rub against food; Wipe the floor with a mop; Wipe the table with a cloth; Wiping the blackboard with a board will produce friction. In our life, as long as objects touch each other, friction will be generated.

How to increase friction and reduce friction:

The sliding friction is related to the positive pressure between two objects and the surface conditions of the two objects. The greater the pressure, the rougher the surface and the greater the friction.

1. The coarser the joint surface of an object, the greater the friction. Such as the pattern of soles and tires. When the car is driving on the road, the tires contact with the rough asphalt road, so the friction can be increased. When a car runs on a snowy or watery road, the friction will be reduced. Therefore, we should pay attention to safety in rainy and snowy days.

2. reduce the roughness between contact surfaces; The fan shaft should be made smooth. When playing sand arc ball, if you want to make the ball slide fast, you should put more sand on the table. This reduces friction. If there is less sand and the friction surface is large, the ball will slide slowly. In ancient times, the direct friction between the wheel and the shaft was easy to be damaged. Now there is a bearing between the wheel and the shaft, which reduces the friction and is not easy to be damaged when the car is fast. Refueling the clock can reduce friction and make the time more accurate. On the skating rink, workers often clean the ice surface to make it smooth, which can reduce friction and speed up skating.

There are two factors that affect the friction force:

1. The friction force is related to the pressure between contact surfaces. When the roughness of contact surfaces is constant, the greater the pressure, the greater the friction force. In life, we have the common sense that it is more difficult to ride a bicycle when the tire is flat.

2. The magnitude of friction is related to the roughness of the contact surface. When the pressure is constant, the rougher the contact surface, the greater the friction.

what is the tug-of-war competition? Many people will say: of course, it is stronger than which team! In fact, this problem is not that simple.

By analyzing the forces on the two teams of tug-of-war, we can know that as long as the pulling force is less than the maximum static friction with the ground, it will not be pulled. Therefore, increasing the friction with the ground has become the key to success. First of all, wearing shoes with concave and convex soles can increase the friction coefficient and increase the friction force; In addition, the heavier the weight of the players, the greater the pressure on the ground and the greater the friction. When adults and children tug-of-war, adults can easily win. The key is that adults are heavier than children.

In addition, in tug-of-war, winning or losing depends largely on people's skills. For example, if you push your feet hard on the ground, you can put more pressure on the ground than your own weight in a short time. Another example is that people lean back and use the pull of the other side to increase the pressure on the ground, and so on. Its purpose is to maximize the friction against the soles of the feet in order to win the game.

Through the above study and observation, it is concluded that the magnitude of friction depends on the pressure of two objects and the roughness of the surface.

I wish you progress in your study! !

labor-saving examples:

1. pry things with iron bars (lever principle)

2. hoist things with moving pulleys (moving pulleys saves labor and saves crane energy)

3. If you carry cylindrical things on the bus, you can put an inclined plane to roll them up (inclined plane saves labor)

Lever principle formula: power × power arm = resistance.

Archimedes, an ancient Greek scientist, has such a famous saying: "If you give me a fulcrum, I can move the earth!" This sentence is not only an inspiring epigram, but also has a strict scientific basis.

Archimedes first put forward the lever principle in his book On the Balance of Plane Figures. He first regarded some empirical knowledge in the practical application of leverage as "self-evident axioms", and then from these axioms, he obtained the principle of leverage by using geometry and strict logical argumentation. These axioms are: (1) Hang equal weights at the two ends of a weightless rod at the same distance from the fulcrum, and they will be balanced; (2) Hang unequal weights at the same distance from the fulcrum at both ends of the weightless rod, and the heavy end will fall down; (3) Hang an equal weight at the two ends of the weightless rod with unequal distance from the fulcrum, and the far end will incline downwards; (4) The function of one weight can be replaced by the function of several evenly distributed weights, as long as the position of the center of gravity remains unchanged. On the contrary, several evenly distributed weights can be replaced by a weight hanging at their center of gravity; The center of gravity of a figure is distributed in a similar way ... It is from these axioms that Archimedes discovered the lever principle on the basis of the theory of "center of gravity", that is, "when two weights are in balance, their distance from the fulcrum is inversely proportional to their weight."

Archimedes' research on lever not only stays in theory, but also makes a series of inventions according to this principle. It is said that he used to make the mast ship parked on the beach launch smoothly with the help of lever and pulley block. In the battle to defend Syracuse from the attack of Roman navy, Archimedes used the lever principle to make long-range and short-range catapults, and used them to shoot various missiles and boulders to attack the enemy, which once kept the Romans out of Syracuse for three years.

Incidentally, there are also records about leverage in the history of our country. Mohist school in the Warring States period once summarized the laws in this respect, and there are two special records on the principle of leverage in the Book of Mohism. These two articles are very comprehensive about the balance of leverage. There are equal arms and unequal arms in it; Some change the weight of both ends to make it biased, and some change the length of both arms to make it biased. Such a record is also very valuable in the history of world physics.

Recoil movement means that when an object shoots (or throws) a part of it in one direction, the rest of the object will move in the opposite direction.

Recoil phenomenon

Objective: To understand the recoil phenomenon.

material: syringe, rubber hose, glass elbow, rubber hose pad.

Method:

(1) Connect the syringe and the glass elbow to the two ends of the hose respectively. Put a piece of paper next to the glass elbow (ask the students for help) and push down the syringe piston. It can be seen that the paper is blown, indicating that gas is ejected from the elbow. The direction of blowing air can be known from the direction of paper bending.

(2) Hold the syringe vertically, so that the hose will naturally droop.

(3) Push the syringe up and down repeatedly, and it can be seen that the glass elbow is pushed to the opposite direction of the air injection direction under the recoil. Push the piston hard, the exhaust speed is accelerated, the recoil force is increased, and the action range of the elbow is also increased.

Note:

(1) Rubber tube or plastic tube pad must be placed in the syringe to prevent the syringe from being damaged due to excessive thrust. After the experiment, the rubber tube pad was taken out.

(2) If the glass elbow is installed in the airplane model, it can explain the principle that the jet generates recoil to make the airplane move forward; Putting it into the rocket model can also explain the rocket launching principle.

the rocket is propelled by the rocket engine. After the rocket engine is ignited, the propellant (liquid or solid combustion agent plus oxidant) burns in the combustion chamber of the engine, producing a large amount of high-pressure gas; High-pressure gas is ejected from the engine nozzle at high speed, and the reaction force to the combustion chamber (that is, to the rocket) makes the rocket advance in the opposite direction of gas injection. The rocket propulsion principle is based on Newton's third law: the acting force and the reaction force are equal in magnitude and opposite in direction. Once a tight balloon filled with air is released, the air will be ejected from the balloon and the balloon will fly out in the opposite direction.

solid propellant burns rapidly from bottom to top or from inner layer to outer layer.

liquid propellant, which pressurizes the combustion agent and oxidant tank with high-pressure gas, and then transports the combustion agent and oxidant into the combustion chamber with a turbo pump.

the energy of propellant is converted into the kinetic energy of gas in the engine, which forms a high-speed air jet and generates thrust.

Thrust is one of the main parameters representing the performance of rocket engine, which is the reaction force generated by high-temperature gas produced by propellant combustion in thrust chamber and high-speed injection through nozzle. Thrust is the resultant force of forces directly acting on the inner and outer surfaces of thrust.

specific impulse is another important parameter indicating the performance of rocket engine. It means that the thrust value per unit mass of propellant produced by rocket engine in stable working state, the magnitude of specific impulse and the ratio of nozzle outlet area to thrust chamber throat area (area ratio) are related. The larger the area ratio, the higher the specific impulse. The shape of the nozzle directly affects the specific impulse (the speed at which the gas is ejected from the nozzle).

(1) On the battlefield, the artillery will suddenly retreat after firing the shells.

On the battlefield, the gun body will go backwards while firing shells, and commanders and fighters need the gun to return to its original position and aim again. Modern guns are equipped with devices that automatically and quickly reset the gun after firing.

(2) impact turbine

When water flows out of these bends, the blades of the generator start to rotate. Use this to generate electricity.

(3) automatic sprinkler

when water is sprayed from the nozzle of the elbow, the elbow will automatically rotate under the push of the reaction force of water, thus increasing the spraying area.

(4) Squid escapes

In the water, the squid sucks the water into the gill cavity through the hole on its side, and then forcibly squeezes the water out, and the squid swims in the opposite direction.

demonstration experiment: water is injected into the needle tube, and the needle tube is pressed down hard, and the rubber tube tilts backward while the water is sprayed out.

We find that these phenomena or applications have the following characteristics from demonstration experiments and introduced pictures:

When an object shoots (or throws) a part of it in one direction, the rest of the object will move in the opposite direction.

we call this kind of movement recoil.

definition: the recoil movement means that when an object shoots (or throws) a part of it in a certain direction, the rest of the object will move in the opposite direction.

Recoil movement is very common in our daily life. For example, when a new soldier is first exposed to shooting, he often lies in the wrong posture of the gun. After the bullet is fired, the handle of the gun will hit his shoulder heavily because of recoil. We also used the recoil principle to make jets and rockets.