Urgent for 40 physical calculation problems in the last semester of the second day of junior high school

Physics and mechanics in junior high schools occupy a high score, and the investigation methods are often mainly physical calculation and physical experiment questions. The main knowledge points of physical mechanics in junior middle school are: the combination of motion and force, reference object, mechanical motion, the function of force, inertia and inertia law, the balance of two forces, pressure (liquid pressure, atmospheric pressure), buoyancy, mechanical efficiency, kinetic energy and potential energy, mechanical energy and its transformation. This paper is attached with classic physical mechanics calculation questions and experimental questions for students to learn.

First, junior high school physical mechanics knowledge points induction

→ Reference object

1. Definition: An object assumed to be stationary for studying its motion is called a reference object. 2. Any object can be used as a reference.

3. Choosing different reference objects to observe the same object may lead to different conclusions. Whether the same object is moving or static depends on the selected reference object, which is the relativity of motion and static.

→ mechanical movement

1, definition: In physics, the change of object position is called mechanical motion.

2. Features: Mechanical motion is the most common phenomenon in the universe.

3. The method of comparing the speed of objects: (1) The same time, long distance, fast movement; (2) at the same time, short distance, fast movement; (3) Compare the distance traveled per unit time.

Classification: (according to the motion route) (1) Curve motion (2) Linear motion.

ⅰ uniform linear motion:

A. definition: the speed is constant, and the motion along a straight line is called uniform linear motion.

Definition: In uniform linear motion, the speed is equal to the distance traveled by the moving object in unit time.

Physical meaning: Speed is a physical quantity representing the speed of an object.

Calculation formula:

B speed unit: in the international system of units, m/s unit is greater than km/h unit in m/s transportation.

Conversion: 1m/s = 3.6km/h

Ⅱ variable speed movement:

Definition: Motion with variable speed is called variable speed motion.

Average speed = total distance/total time

Physical meaning: indicates the average speed of variable speed movement.

→ the influence of force

1, the concept of force: force is the action of objects on objects.

2. Nature of force: The forces between objects are mutual (mutual forces are equal in size and opposite in direction in any case, and act on different objects). When two objects interact, the force applying object is also a force applying object, and vice versa.

3. The function of force: Force can change the motion state of an object. Force can change the shape of an object.

4. unit of force: in the international system of units, the unit of force is Newton, abbreviated as cow, and represented by n. ..

Perceptual knowledge of force: the force used to take two eggs is about1n.

5. Force measurement:

(1) dynamometer: a tool for measuring the magnitude of force.

(2) Spring dynamometer:

6. The three elements of force: the size, direction and action point of force. 7. Schematic diagram of force → inertia and inertia law

1, Newton's first law: all objects are always at rest or moving in a straight line at a constant speed without force.

2. inertia:

⑴ Definition: The property that an object keeps its state of motion unchanged is called inertia.

⑵ Description: Inertia is an attribute of an object. Any object has inertia under any circumstances.

→ Balance of two forces

1, definition: When an object is acted by two forces, it is said that the two forces are in equilibrium if it can maintain a static state or a uniform linear motion. 2. Equilibrium conditions of two forces: two forces act on the same object with equal magnitude and opposite directions, and the two forces are in a straight line.

3. The relationship between force and motion state: the condition of the object's force and the motion state of the object show that force is not the cause of (maintaining) motion, and the resultant force of unbalanced force is not zero, which is the cause of changing the motion state of the object.

1, pressure:

① Definition: The force perpendicular to the surface of an object is called pressure.

② Pressure is not always caused by gravity. Usually, an object is placed on a table. If the object is not subjected to other forces, the pressure F = the gravity g of the object.

(3) Experimental study on the factors affecting the pressure effect;

Textbooks A and B show that the greater the pressure, the more obvious the pressure effect under the same stress area. B and C show that the smaller the stress area is, the more obvious the pressure effect is when the pressure is the same. The conclusion of these two experiments is that the effect of pressure is related to pressure and stress area.

2. Pressure:

① Definition: The pressure per unit area of an object is called pressure.

② Physical meaning: Pressure is a physical quantity indicating the action of pressure.

③ Formula p=F/ S, where the unit of each quantity is: p: Pascal (Pa); F: Newton (N)S: meter 2(m2).

④ Understanding of pressure unit Pa: When a newspaper is laid flat, the pressure on the table is about 0.5Pa, and when an adult stands, the pressure on the ground is about 1.5× 104Pa.

⑤ Method of increasing or decreasing pressure: change pressure, stress area and the first two at the same time.

→ Liquid pressure

1. The reason why there is pressure inside the liquid: the liquid is gravity and has fluidity. 2. The law of liquid pressure:

(1) There is pressure inside the liquid in all directions;

(2) At the same depth, the pressures in all directions are equal;

(3) With the increase of depth, the pressure of liquid increases;

(4) The pressure of liquid is also related to the density of liquid. At the same depth, the greater the density of the liquid, the greater the pressure.

3. Liquid pressure formula: p=ρgh(→ click to enter the summary of mechanical formula-pressure formula).

The applicable condition of (1) formula is: liquid.

(2) The unit of physical quantity in the formula is: p: pa; G: Newton/kg; Male: Male.

⑶ As can be seen from the formula, the pressure of the liquid is only related to the density and depth of the liquid, but has nothing to do with the mass, volume, gravity, bottom area and shape of the container. The famous Pascal's broken barrel experiment fully illustrates this point.

4. Communicator: (1) Definition: a container with an open top and a communicated bottom.

⑵ Principle: When the communication device is filled with liquid and the liquid does not flow, the liquid level of each container remains flat.

⑶ Application: teapot, boiler water level gauge, automatic water feeder for cows, ship lock, etc. All the work is based on the principle of communication equipment.

→ Atmospheric pressure

1, determination of atmospheric pressure-Torricelli experiment (key experiment).

⑴ Experimental process: Fill a glass tube with a length of about 1m and a closed end with mercury, plug the nozzle, then insert it backwards in the mercury tank and let go of the finger blocking the nozzle. When the mercury level in the tube drops, it will not drop. At this time, the height difference of mercury column inside and outside the tube is about 760 mm.

⑵ Principle analysis: Take a liquid film in the pipe at the level of the liquid level outside the pipe. Because the liquid does not move, the liquid film is balanced by the upper and lower pressures. That is, upward atmospheric pressure = pressure generated by mercury column.

⑶ Conclusion: Atmospheric pressure P0 = 760mmhg =1900pxhg =1.01×105pa (its value varies with the change of external atmospheric pressure).

(4) Description:

A. The purpose of filling the glass tube with mercury before the experiment is: after the glass tube is inverted, there is vacuum above the mercury; If it is not filled, the measurement result is very small.

B In this experiment, if mercury is replaced by water, the length of the glass tube needs to be10.3 m..

C gently lift or press the glass tube, and the height difference between the inside and outside of the tube remains unchanged. Tilt the glass tube, the height remains the same and the length becomes longer.

2. Standard atmospheric pressure-the atmospheric pressure supporting 1900px mercury column is called standard atmospheric pressure. 1 standard atmospheric pressure = 760mmhg =1900pxhg =1.013x105pa, which can support the water column height of about10.3m..

3, the change of atmospheric pressure

With the increase of altitude, the atmospheric pressure decreases. Within 2000m above sea level, it can be approximately considered that the atmospheric pressure decreases by about 1mm for every elevation increase of 12m. The air pressure varies unevenly with altitude, and decreases rapidly at low altitude and slowly at high altitude. The air pressure value is related to the change of location, weather and season. Generally speaking, the air pressure in sunny days is higher than that in cloudy days, and the air pressure in winter is higher than that in summer.

4, measuring tools:

⑴ Definition: The instrument for measuring atmospheric pressure is called barometer.

⑵ Classification: Mercury barometer and aneroid barometer.

5. Uses: piston pump and centrifugal pump. → Relationship between fluid pressure and flow rate

1. Relationship between gas pressure and flow rate: In gas and liquid, the greater the flow rate, the smaller the pressure. 2, the lift of the aircraft → buoyancy 1, the size of buoyancy.

The buoyancy of an object immersed in a liquid is equal to the gravity of the liquid it displaces, which is the famous Archimedes principle (also applicable to gases).

2. Formula: F float = G row = ρ liquid V row g (→ click to enter the summary of mechanical formula-buoyancy formula).

It can be seen from the formula that the buoyancy of liquid to an object is related to the density of liquid and the volume of liquid displaced by the object, but has nothing to do with the mass, volume, gravity, shape and immersion depth of the object.

→ Application of buoyancy

1, the ups and downs of objects:

When buoyancy is greater than gravity, objects submerged in liquid will float; When buoyancy is less than gravity, the object sinks; When its buoyancy is equal to gravity, it is suspended in the liquid or floats on the liquid surface.

2. Application of buoyancy

Ship: hollow method is adopted to increase the displacement.

Submarines: change themselves and realize floating and sinking.

Balloon and airship: change buoyancy to achieve ascending and descending.

→ work

1, engaged in mechanical work

① Significance of doing work: If a force acts on an object and the object moves a certain distance along the direction of this force, it is said that this force has done work in mechanics.

The work mentioned in mechanics includes two necessary factors: one is the force acting on the object; The second is the distance the object moves in the direction of this force.

③ Three cases of no work: strong distance, weak distance, vertical force and distance.

2. Calculation of work: (→ Click to enter mechanical formula-work formula summary)

In physics, the product of the force and the distance moving in the direction of the force is called work.

② formula: W=FS③ work unit: joule (j),1j =1n m.

④ Note: ① Distinguish which force really acts on the object, and F is the force when calculating; (2) in the formula, s must be the distance traveled by the direction of force, emphasizing correspondence. (3) The unit of work "Coke" (N·m = Coke) should not be confused with the product of force and arm (n m, which cannot be written as "Coke").

→ Mechanical efficiency

1, useful work and extra work

1 definition of useful work: useful work for people, and useful work is necessary work.

Example: weightlifting w useful =Gh

② Extra work:

Definition of extra work: work we don't need but must do.

Example: Hoist the heavy object with pulley block w = G and move h(G movement: indicates the weight of the moving pulley).

③ Total workload:

Definition of total work: The sum of useful work and extra work is called total work. That is, the work done by power.

Formula: w total =W useful +W amount, w total =FS.

2. Mechanical efficiency

① Definition: The ratio of useful work to total work.

② Formula: η=W Useful /W Total

③ Methods to improve mechanical efficiency: reduce the dead weight of machinery and reduce the friction between parts.

④ Description: Mechanical efficiency is usually expressed as a percentage, and always less than 1① Physical meaning: Power is a physical quantity representing working speed. ② Definition: The work done per unit time is called power ③ Formula: P=W/t④ Unit: watt (w), kilowatt (kw)1w =1j/s1kw =10w.

→ kinetic energy and potential energy

1, kinetic energy

① Energy: An object can do external work (but not necessarily work), indicating that the object has energy, referred to as energy.

(2) Kinetic energy: The energy possessed by an object due to motion is called kinetic energy.

(3) The greater the speed of an object with the same mass, the greater its kinetic energy; The greater the mass of an object moving at the same speed, the greater its kinetic energy.

2. Potential energy

① Gravitational potential energy: The energy that an object has because it is lifted is called gravitational potential energy.

The higher an object is lifted, the greater its mass and the greater its gravitational potential energy.

(2) Elastic potential energy: The energy possessed by an object due to elastic deformation is called elastic potential energy.

The greater the elastic deformation of an object, the greater its elastic potential energy.

③ Potential energy: Gravity potential energy and elastic potential energy are collectively referred to as potential energy. (→ Click to enter the mechanical formula-summary of kinetic energy and potential energy formulas)

→ mechanical energy and its transformation

1, mechanical energy: kinetic energy and potential energy are collectively called mechanical energy.

If only kinetic energy and potential energy are transformed into each other, the sum of mechanical energy remains unchanged, or mechanical energy is conserved.

2, the conversion law between kinetic energy and gravitational potential energy:

(1) When an object with a certain mass descends at an accelerated speed, the kinetic energy increases, the gravitational potential energy decreases, and the gravitational potential energy is converted into kinetic energy;

(2) When an object with a certain mass decelerates and rises, the kinetic energy will decrease, the gravitational potential energy will increase, and the kinetic energy will be converted into gravitational potential energy;

3. Conversion law between kinetic energy and elastic potential energy;

① If the kinetic energy of one object decreases and the elastic potential energy of another object increases, the kinetic energy is converted into elastic potential energy;

② If the kinetic energy of one object increases and the elastic potential energy of another object decreases, the elastic potential energy is converted into kinetic energy.

Second, junior high school physical mechanics calculation problems:

1. Kettle 1 kg water. The bottom area of the pot is about 0.003 m2, and the water depth is 0.1m. Find the pressure of water on the bottom of the pot.

Answer: p = ρ GH =1000kg/m3× 9.8n/kg× 0.1m = 980pa.

(Formula 1 min, substituting 2 points, the result is 1 min)

2. On the horizontal desktop, thin-walled cylindrical containers A and B are respectively filled with 1.8kg alcohol and 2kg water. The bottom area of A is 0.0 16m2, and that of B is 0.0 1m2. (Known alcohol density =0.8g/cm? ). Q:

(1) the volume of water;

(2) the pressure p of water at the bottom of the container;

(3) If the same volume of alcohol and water is extracted from container A and container B, is it possible to make

Alcohol and water have equal pressures at the bottom of their respective containers. If possible, please calculate

Extract the volume of Δ v; If not, please explain why.

Answer:

Third, junior high school physical mechanics experiment:

1. In order to find out what factors are related to the pressure inside the liquid, Xiaohua tied a rubber film to one end of a glass tube with openings at both ends and immersed it in a beaker filled with water to continuously increase the depth of the glass tube immersed in water. The experimental operation process and experimental phenomena are shown in Figure 7(a), (b) and (c). He continued his experiment by injecting alcohol and salt water into the glass tube to make it flush with the water in the beaker. The process and phenomena of the experiment are shown in figs. 7(d) and 7 (e). (Known ρ brine >: ρ water >; ρ alcohol)

(1) Observing and comparing Figures (a), (b) and (c), the preliminary conclusion can be summarized as: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.

(2) Observing and comparing Figure 7(d) and Figure 7 (e), the preliminary conclusion can be summarized as: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _.

2. Fill in the relevant items in the following experiments respectively:

(1) in the experiment of "measuring the density of substances", the mass of aluminum blocks should be measured with a balance, and the aluminum blocks should be placed in _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

(2) In order to explore whether the internal pressure of liquid is related to the type of liquid, two large cylinders should be filled with _ _ _ _ _ kinds of liquid respectively. During the experiment, the metal box of the pressure gauge should be placed at the depth of _ _ _ _ in the liquid of two measuring cylinders. (Select "Same" or "Different" for both)

Answers to physical mechanics experiment questions in junior middle school:

The pressure in the same liquid 1 and (1) increases with the increase of depth.

(2) Different liquids at the same depth, the higher the liquid density, the higher the liquid pressure.

2.( 1) Left, Different and Same (2) Different and Same