1. When rowing in the lake, what is the driving force of the boat? )

A. the thrust of paddling B. the direct thrust of water on the ship C. the thrust of people on the ship D. the thrust of water on the paddle

2. What forces are acting on the football kicked into the air ()

A is subjected to the force and gravity of the foot? B is affected by gravity, and C is only affected by feet? D is not subject to any force.

3. When a car is moving at 6 m/s and 4 m/s respectively, what is its inertia? )

A. the same size; ? B. when the speed is 4 m/s, it is large; C. the speed is 6m/s; ? incomparable

4. How much force is exerted on the passengers standing in the car and driving at a constant speed? )

A. 1？ B.2？ C.3 d.4

5. Two students, A and B, pull the dynamometer in opposite directions, each with a force of 200 N, so the indicator of the dynamometer is ().

A, 100 cattle? B, 200 cows? C, 0 cattle? 400 cows

6. An object is acted by two forces, and the three elements of these two forces are exactly the same, then these two forces (? )

A must be a balancing force, B must not be a balancing force, C may be a balancing force, D? Unable to judge

7. On the physical education class, Xiaoming climbed the pole and Xiaogang climbed the rope at a constant speed. The following statement is true about the friction they are subjected to. )

A, Xiao Ming must be subjected to a lot of friction because of the great pressure of holding the pole when climbing it.

B, because the rope is rough, Xiaogang must have suffered a lot of friction.

C. The friction between Xiao Ming and Xiao Gang must be the same.

D. If Xiao Ming's weight is large, he must be subjected to great friction.

8. As shown in the figure, the object A is still on the vertical wall under the action of the horizontal force F. When the horizontal force is reduced to f/2, the object A just slides down from the vertical wall at a uniform speed. At this time, the friction force on object A is ().

A. Restore to the original1/2 b. Same as the original? C. Twice as much as before? D. unable to judge

9. Bungee jumping is a game in which one end of an elastic rope is tied to the body, and the other end is fixed at a high place, so you can jump from a high place. Point A is the position where the elastic rope naturally hangs down, and point C is the lowest point reached by the players. When the player leaves the platform to the lowest point, how does the speed of the object change? _______________

10. Two objects A and B are stacked on a horizontal desktop, as shown in three situations: ① Both objects in Figure A are at rest; (2) In Figure B, a horizontal constant force F acts on the object B, making A and B move in a straight line at a constant speed of 2m/s; (3) In Figure C, a horizontal constant force F acts on object B, making A and B move in a straight line at a constant speed of 20m/s. Comparing the stress of object A in the horizontal direction under the above three conditions, the following statement is correct. )

In three cases, A is not stressed in the horizontal direction. In three cases, A is subjected to force in the horizontal direction, and the force is the same.

C, ① A is not stressed in the horizontal direction, ②, ③ A is stressed in the horizontal direction, but the stress is different.

D, ① A is not stressed in the horizontal direction, ②, ③ A is stressed in the horizontal direction, but the stress is the same.

1 1. The bottled drinks produced by the beverage factory should be transported by conveyor belt on the automatic production line. As shown in the figure, a bottle of beverage moves horizontally and uniformly to the left with the conveyor belt regardless of air resistance. Please draw the stress diagram of the beverage bottle in the picture. (Point A in the figure represents the center of gravity)

Answers and analysis

1. may be wrong. A. Life experience, a boat can only go forward by paddling, and it won't go forward without paddling. So choose a.

The answer is correct: D B and C are impossible, which is very clear. This topic should start with the narrative analysis of the topic, and the phrase "the power to make the ship move forward" is the key to the topic. From the sentence, we should realize that paddles are stressed and paddles are stressed objects. It is precisely because the paddle is stressed that the ship will move forward. Option A: The force of paddling is exerted on the water surface. The water is a stressed object and should be moving. Therefore, option A D is not selected: when the paddle is paddling, because the forces are mutual, the water is also powerful to the paddle, and the force of water on the paddle acts on the paddle, which is equivalent to acting on the boat, so the boat can move forward. So choose d

2. possible mistakes: choose a? Influenced by life experience, but ignoring the important condition of "qi"? Correct answer: b

Because the ball is in the air, the feet in the air have no force, only gravity.

3.D 4。 B 5。 The correct answer B analysis: Not C, the spring is said to have been pulled, there must be instructions. It is said that the two opposite forces on the spring can only be subtracted but not added, so it can't be D. "200N each" can't display 100N, so it can't be A. Note that the spring is said to be static when in use. It must be a couple.

6.B 7。 C 8。 B

9. See the article for the analysis of first increase and then decrease.

10.A 1 1。 The stress is shown in the figure.

100 classic case of junior high school physics-object movement

1. When Xiaoming measures the length of an object, the scene is as shown in the figure, and the length of the object _ _ _ _ _ _ _ _ _ cm.

2. A classmate measured the length of an object four times. The data are as follows: 18.39 cm, 18.40 cm, 18.4 1 cm, 18.52 cm. His measurement result is _ _ _ _ cm.

3. A classmate measured the diameter of the steel ball with a scale, and the four results were 1.82cm, 1.87cm, 1.68cm, 1.8 1cm, so the diameter of the ball should be () a.

4. A student measures that the thickness of a physics book 150 is 5.4mm, so what should the thickness of each paper be? millimetre

5. Measure the length of the object with an extended ruler, and the result is ()

Answer? Too big? b？ Same as C? Small? d？ Unable to determine

6. When an object moves in a straight line, the distance in the second second is 4m, the distance in the third second is 6m and the distance in the fourth second is 8m. This object moves in _ _ _ _.

7. The object moves in a straight line at a uniform speed, and the formula v=s/t shows that ()

A.v is directly proportional to s, B. v is inversely proportional to t, c? S is proportional to t? None of the above statements are correct.

Analyze and answer

1. Error 1:3.50cm, it is not clear that the starting point of calibration is 1cm, not 0.

Error 2:2.5cm, next error 3:3.5cm, error 1 and error 2 are not estimated.

Correct answer; 2.50 cm

2. Error 1: 18.45cm, and the error data 18.52cm was not deleted before calculation.

Mistake 2:? 18.4cm, after removing the error, the calculated results do not retain the same accuracy as the original data.

Correct answer: 18.40cm?

3. Correct answer A. The analysis is the same as question 2.

4. Error: 0.0036 mm, 5.4 mm ÷ 150 = 0.0036 mm, I didn't realize that a piece of paper had pages.

Correct case: 0.00 18mm? 5.4 mm ÷75 = 0.0036 mm

5. error a

When the scale is elongated, each division value of the scale increases correspondingly, and the same length measured with the increased division value decreases, so C is selected.

6. Error: Uniform linear motion. The reason is that the second, third and fourth second time is understood as 2 seconds, 3 seconds and 4 seconds.

Correct answer: variable speed linear motion

7. Correct answer C

100 Cases of Classical Physics Errors in Junior High School and Their Analysis-Density Part

1. The density of oxygen in oxygen cylinder is 6kg/m3, which is required for workers to use oxygen for welding 1/3. What is the density of oxygen in the bottle? )A？ 6 kg/m3，B 12kg/m3，C.4 kg/m3，D？ Unable to determine

2. There are three small balls with the same mass and volume. One is a copper ball, one is an iron ball and the other is an aluminum ball. _ _ _ _ _ _ must be a hollow ball. _ _ _ _ _ can be a hollow ball.

3. If a copper ball is burned on an alcohol lamp for a while, the quality and density of the copper ball will decrease. )

One hasn't changed, bigger? B, unchanged, smaller C, unchanged? d？ Smaller and unchanged.

4. If a bottle can hold 1kg of salt water, it must hold 1kg of salt water. ) a mercury b water c kerosene? alcohol

Xiao Xin and Xiao Yang designed an experimental scheme respectively. Please fill in the blanks in the scheme:

Scheme 1: (1) Measure the mass of the empty beaker with the adjusted balance M 1; (2) Pour some edible oil into a beaker and measure their total mass m2. What is the quality of these edible oils? ; (3) pouring the edible oil in the beaker into a measuring cylinder, and measuring the volume v of the edible oil; (4) Calculate the density ρ of edible oil.

Option 2: (1) After placing the balance on the horizontal platform, immediately adjust the balance nut to balance the beam; (2) measuring the total mass m1of a beaker containing a proper amount of edible oil with a balance; (3) part of the edible oil in the beaker is poured into a measuring cylinder, and the volume v of the edible oil in the measuring cylinder is recorded; (4) measuring the total mass m2 of the beaker and the remaining edible oil; (5) Calculate the density of edible oil ρ =? .

Evaluation and communication (1) Please find out the shortcomings of the two schemes respectively:

Scheme:; Scheme:? ; (2) Are you going to choose this scheme? When doing experiments, in order to successfully complete the experiment, the shortcomings in the scheme should be changed to:

Answers and analysis

1. Wrong answer: A. Because gas has fluidity and no fixed volume, the volume of gas depends on the volume of the container. The volume of the gas is equal to the volume of the container. Because it is a steel cylinder, the volume of the container remains unchanged, so the volume of the remaining oxygen remains unchanged, but the mass is only 2/3 of the original, so the density is 2/3 of the original correct answer: C.

2. Incorrect answer: copper ball and aluminum ball? Because ρ copper >: ρ iron > ρ aluminum density, copper and iron have the same mass, V copper and V empty aluminum. If aluminum is solid, because v is empty and copper V >；; Empty iron >: v empty aluminum. V empty aluminum =0, then V empty copper >; 0, v empty iron > 0, so copper is hollow. Iron must also be hollow. If aluminum is hollow, because V is hollow, copper V >；; Empty iron >: v hollow aluminum. V hollow aluminum > 0, then V is empty copper > 0, and V is empty iron > 0, so copper is hollow. Iron must also be hollow. Correct answer: copper balls and iron balls must be hollow, and aluminum balls may be hollow.

3. Pay attention to the law of thermal expansion and cold contraction. 4.A

5. Disadvantages of Option 1:

In the third step, the liquid in the beaker will not be emptied, but will remain, resulting in inaccurate measurement quality and low measurement density.

What are the disadvantages of Option 2? Before adjusting the balance of the balance, the wandering code was not adjusted to 0 scale.

Option 1, add a step: measure the mass of beaker and residual liquid.

Option 2, when adjusting the balance of the balance, first adjust the travel code to 0 scale.

Analysis of Classical Errors in Junior Middle School Physics 100 —— Pressure and Liquid Pressure

1. The following statement is correct.

A. The greater the gravity of an object, the greater the pressure; B. The smaller the stress area, the greater the pressure; ? C. Pressure is directly proportional to the gravity of the object and inversely proportional to the stress area; ? D under the same pressure, the larger the stress area, the smaller the pressure.

2. There are three cylinders with the same material and the same height, and their mass ratio is m 1:m2:m3=2:3:5.

If they are placed vertically on a horizontal plane, the ratio of horizontal pressure is ()

A.2:3:5? b . 5:3:2 c . 1: 1: 1？ D. 15: 10:6

3. A cubic iron block with a mass of 7.9kg is placed on a horizontal desktop with an area of 0.5m 2, opposite to the horizontal desktop.

The pressure generated is _ _ _ _ _ _ _ _

4. Put a uniform cube with a weight of 100N and a side length of 20cm in the center of a horizontal desktop with a side length of 10cm. It will be a square.

The pressure of the body on the desktop is _ _ _ _ _

5. Place the flat glass filled with 3N water on a horizontal table top, and the contact area between the glass and the table top is 20cm 2.

When the height of the inner water surface is 10cm, the pressure of water on the bottom of the cup is _ _ _ _.

6. As shown in the figure, two identical test tubes A and B are respectively filled with equal mass of liquid. Tube a is placed vertically.

Test tube B is placed obliquely, and the liquid levels of the two test tubes are even. Let the pressure of liquid at the bottom of two test tubes be P A and P A respectively. P B, then ()

Lapd. B. P a =？ Fpa >? P, B, D. The conditions are insufficient to judge.

7. As shown in figure 1, put a hard plastic bottle with a sealed cover filled with water on a level ground. What is the pressure of water on the bottom of the bottle? P 1, and the pressure of the bottom of the bottle on the desktop is p1'; After the bottle is inverted, as shown in Figure 2, the pressure of water on the bottle cap is P2, and the bottle cap is right. The pressure on the desktop is P2, so ()

A.p 1 > P2P 1′> P2 B . p 1 = P2？ p 1′< P2′

C.p 1 < P2P 1′< P2 D . p 1 = P2 p 1′= P2

8. As shown in the figure, container A and container B are filled with the same liquid, and the pressure at point A is less than that at point B. When A and B

Which of the following statements is true when the valve between is open? )

A. liquid flows from a to B.

C. the liquid is still. Liquid flows back and forth

Answers and analysis

1. Possible error: a? Correct answer: D.

2. Possible mistakes A. Only the pressure generated by different gravity is different, and there is no difference in the area of the virtual bottom.

Correct answer c? Because it is a cylinder, v = rhoghs. Because it is placed on a horizontal platform, g = f.

It can be seen from p = f/s = g/s = rhoghs/s = rhoghs that the pressure of a cylinder placed on a horizontal platform has nothing to do with the mass, but only with the density.

Education, height, so? Correct answer c

3. Possible error: 158Pa. p = mg/s =(7.9kg× 10N/kg)/0.5m ^ 2 = 158 pa？ Correct answer: 7900 Pa

V iron = m/ρ = 7.9 kg/7.9×103 kg/m3 = 0.001m3, so the side length of iron is 0. 1m, and the bottom area is 0.0 1m2.

Because the area of the table is larger than the bottom area of the iron block and the stress area is smaller, the contact area is iron.

The bottom area of the block. Then s = 0.01m2 = mg/s = (7.9 kg×10n/kg)/0.01m2 = 7900pa.

4. Possible error: 250Pa

Correct answer: 1000Pa. The two contact areas are small (similar to question 3).

5. Possible error: 3N. The reason for the error is that the gravity of water is equal to the pressure of water on the container.

Correct answer: 2N

If the cup is cylindrical, g water = rhoghs =/kloc-0 /×103kg/m3×10n/kg× 0.1m× 20×10-4m2 = 2n.

The actual gravity of the water in the cup is 3N,

So the cup is a special-shaped container with a big mouth and a small bottom.

For special-shaped containers, the pressure of liquid should be calculated first and then.

P=ρgh= 1000Pa，F = PS = 1000 pa×20× 10-4 m2 = 2N

6. Possible mistake: B didn't notice the difference in liquid density between the two containers.

Correct answer: C.

7. correct answer: b

8. Possible error: A is affected by the topic condition "The pressure at point A of liquid level is less than the pressure at point B"

Correct answer: C.

Whether the liquid flows after opening the valve does not depend on the pressure at point A and point B, but on the pressure at both sides of the valve.

Because the density and depth of the liquid on both sides of the valve are the same, and the pressure is the same, the liquid will not flow after opening the valve.

An Analysis of Errors in Physics (Optics) in Grade Three.

1. When doing pinhole imaging experiment with candles in darkroom, Xiao Ming poked four tiny holes in different positions of the cardboard, round, square, regular triangle and pentagon, which may be () on the wall.

A has one real image of candle flame, and B has four real images of candle flame.

C has four clear points with the same shape as the holes, and D has four virtual images of candle flames.

2. At night, spread a piece of white paper on the desktop, press a small flat mirror on the white paper, and let the light of the flashlight shine on the flat mirror and the white paper. What people see when they look at flat mirrors and white paper is (? )

A white paper is brighter, B flat mirror is brighter, C may be white paper is brighter, D may be flat mirror is brighter. None of the above statements is true.

The night after the rain, it just cleared up, although the ground was dry, there was still a lot of water. In order not to step on the water on the ground, the following statement is correct ()

A. Facing the moonlight, there is water in the bright place on the ground, and there is water in the dark place on the ground with moonlight on the back.

B. Walking against the moonlight is water in the dark, and walking behind the moonlight is water in the light.

C. Go against the moonlight or walk with the moonlight on your back. There is water on the ground where it shines.

D, walking against the moon or carrying the moonlight, people standing on the ground in the dark are upright.

4. A beam of light from the air into the water, a certain change is _ _ _ _ _ _.

Chopsticks in water seem to bend upward when viewed from above, because _ _ _ happened when light entered _ _ _ _.

6. As shown in the figure, a beam of light in the same direction is obliquely incident to the water surface from the right, at which time the reflection angle is β and the refraction angle is γ. If the left end of the tank is slightly raised, and the water surface is calm again, the reflection angle is β' and the refraction angle is γ', then ().

a)β' & lt； βγ' & lt； γB)β' =βγ' =γC)β'γD)β' & gt； βγ′& gt； γ

7. After a beam of light passes through a convex lens ()

A must be parallel light, B must converge at one point, C must be divergent light, and D must converge more than the incident light.

8. An object becomes a narrow and clear image on the screen through a convex lens, and now the object distance is halved, so the following judgment is correct ().

A must be a reduced image on the screen and b must be an enlarged image on the screen.

C must be an image of the same size on the screen. D There must be an image on the screen, but the size is uncertain.

9. As shown in the figure, a beam of light will converge at the S point of the main optical axis after entering the optical instrument at the O point, and will converge at the S' point after removing the optical element, so the mirror must be ().

A convex mirror b concave mirror c convex lens d concave lens

10. A classmate accidentally blocked half of the convex lens with a book while doing the convex lens imaging experiment, and got a perfect image on the screen.

1 1. When light is emitted from air into water, the following statement is correct.

A. refracted light must deviate from the normal. Light must be refracted.

C. the refraction angle must be less than the incident angle d. The refraction angle is equal to or less than the incident angle.

12. When the sun shines through the gap in the leaves, a circular spot will appear on the ground, which is ().

A shadow of the sun b real image of the sun c shadow of the leaves d real image of the leaves

The physical meaning of 1 B pinhole means that only one beam of light passes through 2.c3.a4. The propagation speed of light is 5. When water enters the air, light is refracted. B 7。 D 8。 D 9。 B 10。 The energy is11.d12.b.

An analysis of easy-to-mistake problems in physics of Grade Three (Hot)

1. Physical concepts (physical quantities): specific heat (c), heat (q), combustion value (q), internal energy and temperature (t).

Second, experimental instruments: thermometers, thermometers.

3. Physical laws: the law of straight-line propagation of light in uniform medium, the law of light reflection, the law of plane mirror imaging, the law of light refraction, the law of convex lens imaging, the law of state change, the law of internal energy change, the calculation formula of heat: Q=cmDt and the calculation of combustion value Q=qm, and the theory of molecular motion.

The first category: exercises about physical quantities;

Example 1: Pour half a cup of alcohol, and the rest of the alcohol ()

A. the specific heat is constant, and the combustion value is half of the original. B. the specific heat and combustion value are unchanged.

C. the specific heat is half of the original, and the combustion value remains unchanged. D. the specific heat and combustion value are both half of the original.

[Analysis]: Specific heat is a characteristic of matter. Has nothing to do with the mass of the object; Independent of the temperature of the object; It has nothing to do with whether the object absorbs heat or emits heat. Once this substance is determined, its specific heat is determined. The specific heat of alcohol is 2.4× 103 coke /(kg? C) A bottle of wine and a barrel of wine. The specific heat of alcohol at 0℃ and 20℃ is the same. Combustion value is a characteristic of fuel. Refers to the heat released by the complete combustion of a certain combustion per unit mass. The burning value of alcohol is 3.0× 107 coke/kg, which does not change with the quality of alcohol. The quality of alcohol releases more heat when it is completely burned, but the combustion value of alcohol has not changed. So the correct answer to this question should be B.

Example 2: Two ice cubes, A and B, have the same mass and the temperature is 0℃. A ice cube is still on the ground, and B ice cube is stopped at the height of 10 meter. These two ice cubes ().

A. The mechanical energy is as big as B. The internal energy of mechanical energy of B is as big as that of D.. ..

[Analysis]: Mechanical energy includes kinetic energy and potential energy. The two pieces of ice are of the same mass. Their kinetic energy and potential energy can be judged by their speed and position height. The basis for judging the internal energy of an object is temperature and state. According to the meaning of the question, both ice cubes are static, their kinetic energy is zero, and the two ice cubes have the same mass. The position of ice cube B is higher than that of ice cube A, and the gravitational potential of ice cube B is large. The conclusion is that the mechanical energy of B ice is large. Both ice cubes are 0℃, with the same mass, the same material state and the same temperature, so their internal energy is also the same. Options b and c are correct.

The second category: exercises about thermometers;