1.The stone lies at the bottom of the vessel, completely submerged in water (see picture). Will the pressure of the stone on the bottom change (and if so, how) if table salt is added to the water? Explain the answer.

Decision.

The pressure of the stone on the bottom can be calculated from following formula:

P=(F heavy– F a) / s,

whereF heavyis the force of gravity acting on the stone,F BUT- the force of Archimedes acting on the stone,S - the surface area of ​​the stone, which it rests on the bottom.

When salt is added to water, its density will increase, therefore, the Archimedes force acting on the stone will also increase, which is determined by the formula:

F a= ρ gV,

whereρ - density of water,V - the volume of the stone,g - acceleration of gravity. Since the volume of the stone and the acceleration of gravity will not change, and the density of water will increase, the Archimedes force acting on the stone will increase. The force of gravity acting on the stone will also not change, since the mass of the stone does not change. Thus, the difference between the force of gravity and the force of Archimedes will become smaller, and the pressure of the stone on the bottom will decrease.

Answer: the pressure of the stone on the bottom will decrease.

2.The tube, sealed at one end, is lowered with its open end into the water for half the length of the tube (see figure). What happens to the level of water entering the tube after Atmosphere pressure decrease? Explain the answer.

Answer: The water level in the tube will decrease.

Explanation: when atmospheric pressure decreases, according to Pascal's law, part of the water must leave the tube so that the total pressure of the air in the tube and the water column balances the reduced atmospheric pressure.

3. What burns the skin more: water or water vapor of the same mass at the same temperature? Explain the answer.

Answer: water vapor burns more.

Explanation: the surface of the skin during short-term contact with water receives energy only due to the cooling of a thin layer of water in the contact zone. If steam gets on the skin, then energy is released both when the steam condenses and when the water formed on the skin is cooled. And although the mass of water formed may be small, the condensation process is accompanied by the release a large number heat, which causes a more severe burn.

4. From which mug - metal or ceramic - is it easier to drink hot tea without burning your lips? Explain why.

Answer: ceramic.

Explanation: Since the thermal conductivity of metal is much greater than the thermal conductivity of ceramic, a mug made of ceramic will heat up much more slowly and will give off heat to the lips more slowly. It is easier to drink hot tea from it.

5. A boat with a rower is floating along the river, and next to it is a raft. Does it take the rower the same amount of time to overtake the raft by 10 meters, and in order to lag behind it by the same amount?

Answer: the same.

Explanation: relative to the raft, the speed of the boat is the same upstream and downstream and equal to the own speed of the boat.

6. The car can go down from the mountain to the plain along one of two roads: a short fairly straight road and a long winding one. Compare the work of gravity in these cases. Explain the answer.

Answer: The work done by gravity is the same.

Explanation: the work of gravity does not depend on the shape of the trajectory, but depends on the initial and final positions of the body.

7. Will the buoyancy force acting on a wooden block floating in kerosene change (and if so, how) if the block is moved from kerosene to water? Explain the answer.

Answer: The buoyant force will not change.

Explanation: The buoyant force acting on a body floating in a fluid balances the force of gravity. A wooden block floating in kerosene will not sink in water, since the density of water is greater than the density of kerosene. In water and kerosene, the buoyant forces balance the same gravity, but the volume of the immersed part of the bar changes.

8. The block floats when fully submerged in water. Will the buoyant force acting on the block change (and if so, how) if it is transferred to kerosene? Explain the answer.

Answer: The buoyant force will decrease.

Explanation: The buoyant force acting on the block in the water balances the force of gravity. A wooden block floating when completely submerged in water will sink in kerosene, since the density of kerosene less density water. In kerosene, the buoyant force will be less than the force of gravity, and therefore less than the buoyant force in water.

9. An uncharged conductive light ball hangs on a silk thread between a charged conductor of an electrophore machine and an uncharged conductive plate. What happens if the conductor of the electrophore machine is brought closer to the ball? Explain the answer.

Answer: the ball will begin to oscillate.

Explanation: when in contact with the conductor of the electrophore machine, the ball will receive electric charge, for example negative, and will push off from the conductor. Touching the conductive plate, the ball will transfer most of the negative charge to it and repel it. Then he will again touch the conductor of the electrophore machine, and the process will repeat.

10. Which car - truck or car - should have stronger brakes? Explain the answer.

Answer: the truck should have stronger brakes.

Justification: the mass of a truck is greater than the mass of a car, therefore, at the same speeds, a truck has more kinetic energy than a car, and a lot of work must be done to stop it. Accordingly, with the same braking distance, the braking force for a truck must be greater than for a car.

11. The word is written in red ink on white paper. Through what color glass is it impossible to read what is written? Explain the answer.

Answer: A word written in red ink cannot be read through a glass of red color (the same color as the color of the ink).

Explanation: Red ink absorbs light of all colors except red, and reflects red light. White paper reflects all colors, but red glass absorbs all light except red. The same rays from both ink and paper will fall into the eyes, so the word will be indistinguishable.

12. Where should ice be placed, with the help of which it is necessary to quickly cool a closed vessel completely filled with hot liquid - put it on top of the vessel or put the vessel on ice? Explain the answer.

Answer: you need to put ice on the lid of the vessel.

Rationale: if ice is placed on top, then the cooling of the vessel with the contents will go the fastest. The cooled upper layers of the liquid in the vessel will sink, being replaced by the warm liquid rising from below, until all the liquid in the vessel cools. On the other hand, the cooled air around the ice will also sink down and additionally cool the vessel.

13. When it became hot on the open volleyball court, the athletes switched to a cool gym. Will they have to pump up the ball or, on the contrary, release some of the air from the ball? Explain the answer.

Answer: the ball will have to pump up.

Explanation: when the ball in the hall is cooled, the thermal movement of air molecules will slow down, therefore, the pressure inside the ball will decrease. To restore the previous pressure, the ball will have to pump up.

14. What will happen to the draft of the ship when it passes from the sea with salt water to the river with fresh water?

Answer: The sediment will increase.

Rationale: when moving from the sea to the river, the buoyancy force acting on the ship does not change. The buoyancy force is directly proportional to the product of the liquid density and the volume of the submerged part of the body (ship). Since the density fresh water less, then the volume of the submerged part of the ship (draft) should become larger.

15. What material - steel or wood - should research vessels be built to study magnetic field Earth? Explain the answer.

Answer: wood.

Rationale: Magnetic field vessels should be built from non-magnetic materials. The steel parts of the vessel, being magnetized, can interfere with accurate measurements of the Earth's magnetic field with their magnetic field.

16. Is it possible to hear the roar of powerful processes occurring on the Sun? Explain the answer.

Answer: no, you can't.

Rationale: sound waves can only be transmitted in a medium (gases, liquids, solids) and are not transmitted through a vacuum. The space between the Earth and the Sun is filled with vacuum.

17. If you shoot a boiled egg with a small-caliber rifle, then a hole will form in the egg. What happens if you shoot a raw egg? Explain the answer.

Answer: The egg will break.

Explanation: in solid body(boiled egg) pressure is transferred in the direction of the force, so a hole is formed. In liquids, according to Pascal's law, pressure is transmitted in all directions.

18. Two identical brass balls fall from the same height. The first ball fell into the sand and stopped, and the second, hitting a stone, bounced off and was caught by the hand at a certain height. Which ball's internal energy has changed the most? Explain the answer.

Answer: the internal energy of the first ball has changed by a large amount.

Explanation: the first ball, having fallen on the sand, stopped; therefore, the change in its internal energy is equal to its initial mechanical energy, since all mechanical energy turned into the internal energy of the ball and sand. The second ball bounced off and rose to a certain height; therefore, the change in its internal energy is equal to the difference between its initial and final potential energy.

19. Two identical thermometers are exposed to the sun. The ball of one of them is smoked, and the other is not. Will the thermometers show the same temperature? Explain the answer.

Answer: thermometers will show different temperatures.

Explanation: a thermometer whose ball is smoked will show a higher temperature, since a smoked ball absorbs all the radiation of the Sun falling on it, and an unsmoky ball reflects most of the incident radiation.

20. A drop of oily liquid falls on the surface of the water and spreads, forming a thin film. Will this film necessarily cover the entire surface of the water? Explain the answer.

Answer: not required. The oil film may not cover the entire surface of the water.

Explanation: a thin film will spread over the surface of the water only up to certain limits, since the thickness of the film cannot be less than the diameter of the molecules of the oily liquid. If the surface area of ​​the water is greater than the maximum possible size of the oil spot, then the film will not cover the entire surface of the water, if less, it will.

21. The glass is half filled with boiling water. In which case will the water cool down the most?

1) if you wait 5 minutes, and then pour into a glass cold water;

2) if you immediately add cold water, and then wait 5 minutes?

Answer: in the first case, the water will cool to a greater extent.

Explanation: the cooling rate decreases with decreasing temperature difference between the heated body and the surrounding air. Therefore, if you immediately pour cold water into hot water, further cooling will be slower.

22. An electrical circuit is fixed on a vertically located board (see figure), consisting of a current source, a lamp, an elastic steel plateAB . A weight was hung from one end of the plate, causing the plate to bend and open the circuit. What will be observed in electrical circuit when will the board start to fall freely? Explain the answer.

Answer: the circuit will close and the lamp will light up.

Explanation: when the board begins to fall freely, a state close to the state of weightlessness will come. The weight will practically become weightless and stop acting on the plate, the plate will gradually straighten up and close the circuit.

23. A boat floats in a small pool. How will the water level in the pool change if a large stone is carefully lowered from the boat into the pool? Explain the answer.

Answer: The water level will rise.

Explanation: a stone lying at the bottom of the pool displaces water in the volume of its body. For a stone floating in a boat, the weight of the displaced water is equal to the weight of the stone in the air. Considering that the density of the stone is greater than the density of water, we find that in this case the volume of displaced water will be more volume stone.

24. A straight rail is illuminated by the sun's rays. At the same time, her shadow is visible on the vertical wall. Can the linear size of the shadow be larger than the linear size of the rail? Explain your answer and illustrate with a picture.

Answer: maybe.

Rationale: Light rays propagate in a straight line. Therefore, the rail, the shadow on the wall and the rays going through the ends of the rail form a trapezoid. In order for the linear size of the shadow to be larger than the linear size of the rail, one side of this trapezoid (the shadow) must be larger than the other side (the rail). An example of the location of the rail, light beams and wall that meets this requirement is shown in the figure.

25. Can a biconvex glass lens under any conditions scatter a parallel light beam incident on it? Explain the answer.

Answer: maybe.

Rationale: if the refractive index of the medium in which the biconvex lens is located is greater than the refractive index of the lens material, then the lens will scatter the parallel light beam incident on it. Therefore, if a biconvex glass lens is immersed in a liquid with a refractive index greater than that of glass, then such a lens will be divergent.

26. An iron cube lying on a smooth horizontal surface is attracted to north pole a permanent bar magnet by sliding over this surface. How does the cube move: uniformly, uniformly accelerated, or with a constantly increasing modulo acceleration? Explain the answer.

27. An iron cube lying on a smooth horizontal surface is attracted to south pole a permanent bar magnet by sliding over this surface. How does the cube move: uniformly, uniformly accelerated, or with a constantly increasing modulo acceleration? Explain the answer.

Answer: the cube moves with a constantly increasing modulo acceleration.

Rationale: since the force of attraction from the pole of a permanent magnet increases as the iron cube approaches the pole, then according to Newton's second law, the acceleration of the cube will increase as it approaches this pole.

28. Two closed vessels of the same volume contain the same number of molecules of the same gas. Vessel 1 is placed in a warm room, vessel 2 - in a cold one. In which vessel is the pressure of the gas greater? Explain the answer.

Answer: in vessel 1.

Rationale: gas pressure depends on the number of collisions of gas molecules on the walls of the vessel per unit time. The number of collisions of molecules depends on the speed of chaotic motion of molecules, which, in turn, depends on temperature - with increasing temperature, the speed of chaotic (thermal) motion of gas molecules increases. Since the temperature of the gas in a warm room is greater than the temperature of the gas in a cold room, both the speed of the molecules and the pressure of the gas in the first vessel placed in the warm room will be greater.

29. Two bars of the same size have the same temperature of +300 °C. The specific heat capacities of the bars and their densities are also the same. Bar 1 has a higher thermal conductivity than bar 2. Which of these bars will cool faster in air whose temperature is +20 °C? Explain the answer.

Answer: bar 1.

Rationale: since the masses, specific heat capacities and initial temperatures of the bars are the same, the same amount of heat is stored in them. The thermal conductivity of bar 1 is greater than the thermal conductivity of bar 2. This means that bar 1 transfers energy from the inside to the air faster. Therefore, it will cool down faster.

30. Dima examines red roses through green glass. What color will roses appear to him? Explain the observed phenomenon.

Answer: roses will appear black.

Explanation: their color depends on the light that enters Dima's eyes. Red roses absorb all colors except red and reflect red. Green glass absorbs all light except green. But the green color is not in the light that the roses reflect - they have absorbed it. No light from red roses will get into Dima's eyes through the green glass - they will seem black.

Burns are injuries to the skin and tissues, which most often occur due to the action of high temperature: leaning against hot, spilling boiling water on the skin.

But a burn can be earned in other ways. Dangerous:

• Radiation. Therefore, we either suffer because of an overdose of the solarium.
• Chemical substances. Therefore, household chemicals must be removed to a safe place and work with them in protective clothing: gloves, an apron and goggles.
• Friction. Therefore, you must carefully descend the rope.
• Electricity. Therefore, electrical injuries are so dangerous: they affect tissues with deep burns.

What to do if you get burned

Whatever the burn, it is necessary to act in the same way.

1. Stop burn

In first aid textbooks, this is called "terminate the effect of the damaging factor." This means that you need to get a person out of a stream of boiling water or an acid pool, for example, as soon as possible. Seems obvious, but anything can happen in a moment of panic.

If you are helping someone, first check that you are safe. That is, make sure that you yourself do not fall under boiling water and do not stand in a puddle of acid.

2. Call a doctor if needed

Be sure to call 911 or an ambulance if:

• The injury was due to electric shock.
• Chemical burn.
• A burn of the third degree or more, that is, when the skin becomes covered with blisters, when they merge into one big one, when the skin in the burn area is brown or black, dry and insensitive.
• Burns of any degree greater than 10% of the body surface. To roughly determine how much it is, focus on the size of the palm of the victim. One palm is approximately 1% of the body area.
• A child or a person over 70 years of age has been burned.

If the burn is mild, but more than five centimeters in diameter, you can not call an ambulance. And yet you still need to get to the emergency room. And be sure to show the doctor burns if they are on the face or genitals.

3. Cool burn

Send the affected area under cool running water for about 15-20 minutes. The water should not be ice cold.

4. Put a dry clean bandage on the burn

It is better if the bandage is sterile. The size is needed so that the bandage or gauze completely covers the burn. The bandage should not be too tight.

5. Give pain medication

Any tablet based on paracetamol, ibuprofen or nimesulide will help the victim.

6. Give a drink

The victim needs to drink as much as possible, because burns, even small ones, reduce the volume of circulating blood. You need to drink something warm and sweet: tea, compote.

What Not to Do

It is definitely impossible to apply ointments to the affected area.

Moreover, you can’t use eggs, butter, sour cream and all other means: they will only slow down the healing and interfere with the doctors who will treat the wound. In addition, bacteria can get into the wound with sour cream or butter.

Even if these methods worked for ten generations of your grandparents, don't do this. Leave the wound clean.

Can't wait to treat - treat with chlorhexidine if the burn is shallow.

Also, do not apply ice to the surface of the wound, so as not to injure it with cold when the skin is already damaged.

When you can do without a doctor

Minor household injuries can be treated on your own. Small ones are when there is only redness or a few blisters from the burn, and the affected area is not more than five centimeters in diameter.

In this case, you need to take painkillers, drink a lot and treat the burn with a special spray with dexpanthenol. This substance helps to heal quickly, and in the form of a spray it is convenient to apply it to a painful burn.

1. 
   Where will cream on milk settle faster: in a warm room or in a refrigerator? Why?
2. 
   What burns the skin more: water or water vapor at the same temperature and mass? Explain the answer.
3. 
   What kind of spot (dark or light) at night on an unlit road does a puddle appear to a pedestrian in the headlights of an approaching car? Explain the answer.
4. 
   Two lenses were cut out from a thin plane-parallel plate: convex and concave (see figure). Compare the optical powers of the lenses modulo. Explain the answer.

5. The boat floats in a small pool. How will the water level in the pool change if a large stone is carefully lowered from the boat into the pool? Explain the answer.

6. How does skating on ice change with increasing frost? Explain the answer.

7. Two students simultaneously measured atmospheric pressure with a barometer: one in the open-air school yard, the other in the physics room on the fifth floor. Will the barometer readings be the same? If not, which barometer will show the higher barometric pressure? Explain the answer.

8. A drop of an oily liquid falls on the surface of the water and spreads, forming a thin film. Will this film necessarily cover the entire surface of the water? Explain the answer.

9. Can cinema screens be mirrored? Explain the answer.

10. A piece of ice floats in a glass of water. How will the water level in the glass change as the ice melts? Explain the answer.

11. The stone lies at the bottom of the vessel, completely submerged in water. How will the pressure of the stone on the bottom change if salt is added to the water? Explain the answer.

12. An electrical circuit (see figure) is fixed on a vertically located board, consisting of a current source, a lamp, an elastic steel plate AB. A weight was hung from one end of the plate, causing the plate to bend and open the circuit. What will be observed in the electrical circuit when the board begins to fall freely? Explain the answer.

thirteen . Two identical brass balls fall from the same height. The first ball fell into the sand and stopped, and the second, hitting a stone, bounced off and was caught by the hand at a certain height. Which ball's internal energy has changed the most? Explain the answer.

14. Which car - truck or car - should have stronger brakes? Explain the answer.

15. In what case will less fuel be required: when launching an artificial satellite from the Earth or from the Moon? Explain the answer.

Solving quality problems

EXERCISE 1 . 1. Faster cream will settle in the refrigerator.

2. Milk is a mixture of tiny droplets of fat and water. Fat droplets have a density less than water and tend to float to the surface. This process is hindered by the thermal motion of water molecules. At low temperatures, the influence of the thermal movement of molecules is less, and the cream settles faster.

TASK 2 . 1. Water vapor burns more.

2. The surface of the skin during short-term contact with water receives energy only due to the cooling of a thin layer of water in the contact zone. If steam gets on the skin, then energy is released both when the steam condenses and when the water formed on the skin is cooled. And although the mass of water formed may be small, the condensation process is accompanied by the release of a large amount of heat, which causes a more severe burn.

TASK 3 .1. A puddle appears as a bright spot against a darker road.

2. Both the puddle and the road are illuminated only by the headlights of the oncoming car. From the smooth surface of the water, the light is reflected specularly, that is, forward, and enters the eyes of the pedestrian. Therefore, the puddle will appear as a bright spot. The rough road surface scatters the light and hits the pedestrian's eyes to a lesser extent.

TASK 4 . 1. The optical powers of the lenses are equal in absolute value.

2. The optical power of a plane-parallel plate is zero (parallel beams remain parallel after passing through the plate). And since the optical power of thin lenses put together is equal to algebraic sum optical powers of individual lenses, it turns out that the optical powers of the lenses are equal in absolute value and opposite in sign.

TASK 5 . 1. The water level will drop.

2. A stone lying at the bottom of the pool displaces water in the volume of its body. For a stone floating in a boat, the weight of the displaced water is equal to the weight of the stone in the air. Given that the density of the stone is greater than the density of water, we find that in this case the volume of displaced water will be greater than the volume of the stone.

TASK 6 . 1. In severe frosts, slip deteriorates.

2. As the blade rubs against the ice, heat is released and the thin layer of ice adjacent to the blade melts. The resulting water acts as a lubricant. In severe frosts, this lubricant is not formed, since the amount of heat released during friction is not enough to heat and melt ice.

TASK 7 . 1. Different. The barometer located in the school yard will show a higher value.

2. The value of atmospheric pressure depends on the height relative to the surface of the earth. Therefore, the atmospheric pressure in the classroom on the fifth floor is less than in the school yard.

TASK 8 . 1. Not necessarily.

2. A thin film will spread over the water surface only up to certain limits, since the thickness of the film cannot be less than the diameter of the molecules of the oily liquid. If the surface area of ​​the water is greater than the maximum size of the oil spot, then the film will not cover the entire surface of the water, if less, it will.

TASK 9 . 1. You can't.

2. A beam of light from a film projector in a mirror will be reflected in accordance with the law of reflection, that is, only viewers sitting in the places where the reflected beam passes will see the image. In order for the image to be seen by all spectators in the cinema hall, it is necessary to make a screen from a material that scatters light.

TASK 10 . 1. The water level will not change.

2. The mass of water displaced by floating ice is exactly equal to the mass of ice. When ice melts, it turns into water of the same mass and occupies the same volume as the initially displaced water.

TASK 11. 1. The pressure force will decrease.

2. The modulus of the pressure force is equal to the difference between the force of gravity acting on the stone and the Archimedean force. When dissolved table salt the average density of the liquid will increase, therefore, the Archimedes force will increase and the pressure force of the stone on the bottom of the vessel will decrease.

TASK 12. 1. The circuit will close and the lamp will light up.

2. When the board begins to fall freely, a state close to the state of weightlessness will come. The weight will practically become weightless and stop acting on the plate, the plate will gradually straighten up and close the circuit.

TASK 13 . 1. The internal energy of the first ball has changed by a large amount.

2. The first ball, having fallen into the sand, stopped, therefore, the change in its internal energy is equal to its initial mechanical energy, since all mechanical energy has turned into the internal energy of the ball and sand. The second ball bounced off and rose to a certain height, therefore, the change in its internal energy is equal to the difference between its initial and final potential energy.

TASK 14 . 1. Stronger brakes should have a truck.

2. The mass of a truck is greater than the mass of a car, therefore, at the same speeds, a truck has more kinetic energy than a car, and a lot of work must be done to stop it. Accordingly, with the same braking distance, the braking force for a truck must be greater than for a car.

TASK 15 . 1. When launching from the moon, less fuel will be required.

2. To make the rocket artificial satellite Earth, she needs to be told first. cosmic speed, equal to approximately 8 km/s. To launch a rocket from the Moon, a lower speed is needed: after all, the force of gravity on the Moon is about 6 times less than on Earth.

Boiling

Every day we observe how water and its vapor pass into each other. Puddles on the pavement dry up after rain, and the water vapor in the air in the morning often turns into tiny droplets of fog.

What happens if a container with a certain volume of liquid is closed with a lid? Every second, the fastest molecules will still leave the surface of the liquid, its mass will decrease, and the concentration of vapor molecules will increase. At the same time, part of the vapor molecules will return to the liquid from the vapor, and the greater the vapor concentration, the more intense the condensation process. Finally, a state will come when the number of molecules returning to the liquid per unit time will, on average, become equal to the number of molecules leaving it during this time. Such a state is called dynamic balance, and the corresponding pair saturated steam.

Pressure saturated steam depends on the type of liquid and temperature. The harder it is to tear the molecules of a liquid apart, the lower will be the pressure of its saturated vapor. The dependence of saturated water vapor pressure on temperature is shown in the figure.

Dependence of pressure of saturated water vapor on temperature

Boiling called the process of formation a large number vapor bubbles occurring throughout the volume of the liquid and on its surface when heated. In fact, these bubbles are always present in the liquid, but their size grows, and they become noticeable only when boiling. The bubbles expand and, under the action of the buoyant force of Archimedes, break away from the bottom, float up and burst on the surface.

Boiling begins at the temperature when the gas bubbles have the opportunity to expand, and this happens if the saturated vapor pressure rises to atmospheric pressure. Thus, the boiling point is the temperature at which the saturated vapor pressure of a given liquid is equal to atmospheric pressure (the pressure above the surface of the liquid).

The pressure cooker has a safety valve that opens at a pressure of 1.4∙10 5 Pa. Boiling point of water in a pressure cooker

Answer:

2?

When completing task 22 with a detailed answer, use a separate sheet. Write down the number of the task first, and then the answer to it. A complete answer should include not only the answer to the question, but also its detailed, logically connected justification. Write your answer clearly and legibly.

Is it possible to observe the process of nucleate boiling of water on space station in weightlessness? Explain the answer.

You can, because body weight does not affect the boiling of water. It is affected by atmospheric pressure. On a space station, the boiling process is much slower than on Earth, but it does happen.

Part 2

Use a separate sheet for answers to tasks 23-26. First write down the number of the task (23, 24, etc.), and then the answer to it. Write your answers clearly and legibly.

Using a tripod with a clutch and foot, a weight with a thread attached to it, a meter ruler and a stopwatch, assemble an experimental setup for studying the free oscillations of a thread pendulum. Determine the time of 30 complete oscillations and calculate the oscillation frequency for the case when the length of the thread is 50 cm.

On the answer sheet:

1) draw a drawing of the experimental setup;

2) write down the formula for calculating the oscillation frequency;

3) indicate the results of direct measurements of the number of oscillations and the time of oscillations;

4) write down numerical value frequency of pendulum oscillations.

Task 24 is a question that requires a written answer. A complete answer should contain not only the answer to the question, but also its detailed, logically connected justification.

What burns the skin more: water or water vapor of the same mass at the same temperature? Explain the answer.

Water vapor burns the skin more strongly, because the internal energy of steam is greater than the internal energy of water, therefore, when the steam comes into contact with the skin, more energy will be released than hot water, a burn occurs.

For tasks 25–26, you must write complete solution, which includes the entry short term problem (Given), writing formulas, the use of which is necessary and sufficient to solve the problem, as well as mathematical transformations and calculations leading to a numerical answer.

The potential energy of a bullet fired from the barrel vertically upwards, 4 s after the start of movement, is 40 J. What is the mass of the bullet if its initial velocity is 40? The potential energy of the bullet is counted from the start level.

An electric heater brings 2.2 kg of water to a boil in 20 minutes, the initial temperature of which is 10 °C. The current in the heater is 7 A, the efficiency of the heater is 45%. What is the voltage in the electrical network?