Typical problems in junior high school physics

Typical problems in junior high school physics

Education information
1、 Teaching content:
Chapter 14 pressure and liquid pressure
pressure
Curriculum standard requirements
1. Knowledge and skills
(1) Understand what is pressure, what is the force area; understand the concept of pressure
(2) Understand the size of the pressure with which factors, understand the method of increasing or decreasing pressure
(3) Understand the name and unit of each physical quantity in the pressure formula
2. Process and method
Explore the effect of pressure with what factors? Through the exploration experiment, observe the experimental phenomenon, experimental data, to obtain a more in-depth understanding of pressure, preliminary master the use of control variable method
3. Emotional attitude and values
(1) Experience observation, experiment and inquiry learning activities to cultivate a scientific attitude of respecting objective facts and seeking truth from facts
(2) Through personal experience and perception, we can get the perceptual knowledge of pressure and lay a foundation for further understanding and application
Breakthrough of key and difficult points
（1） Key points
1. Concept of pressure
(1) Pressure is a physical quantity to describe pressure. In physics, the same force area (unit area) is defined, and the pressure on the same area is compared, so the concept of pressure is introduced, and the pressure on the unit area is used
It is convenient to compare the effect of pressure
(2) Pressure formula: in the formula, f is pressure, s is pressure area
(3) The unit of pressure: Pascal, 1 pa = 1 N / m2 (), which means that the pressure on an object per square meter is 1 n
Point: pressure is a physical quantity to describe the effect of pressure, so the greater the pressure on an object, the more obvious the effect of pressure
[example 1] a student with a mass of 50kg is standing on the horizontal ground. The contact area between each foot and the ground is
(1) How much pressure does he put on the ground?
(2) Can he stand on the muddy ground with the maximum pressure of?
(3) In order not to sink down, what is the minimum area of a board under your feet? (the quality of the board is not included)
Analysis: (1) the weight of the student. When a person stands on the horizontal ground, the pressure on the ground is f = g = 490N, and the force area is the contact area between two feet and the ground,
So when people stand on the ground pressure (2) because, so the mud can not bear
(3) According to, request
Therefore, in order not to sink down, the foot needs to pad a board with an area of at least
Answer: (1); (2) no; (3) 2. Ways to increase or decrease the pressure
It can be seen from the formula that if the pressure is to be reduced, it can be reduced or increased
If we want to increase the pressure, we can increase the pressure or reduce the stress area, but we should also pay attention to whether it is feasible from the actual life and production
Expansion: in real life and production, in order to increase the pressure (or reduce the pressure), the pressure is often increased (or decreased) at the same time, and the stress area is reduced (or increased)
[example 2] in order to reduce the pressure ()
A. The railway rails are laid on sleepers. B. the sewing needles are made very thin
C. The tip of the syringe is very sharp. D. the edge of the kitchen knife is very thin
Analysis: to reduce the pressure, method 1, the pressure is fixed, increase the stress area, method 2, when the stress area is fixed, reduce the pressure, method 3, at the same time, reduce the pressure, increase the stress area, the answer B, C, D in this question is to use the method of reducing the stress area to increase the pressure
Answer: a
（2） Difficulties
The determination of pressure F and stress area s in solving practical problems
The pressure in the formula is not gravity, and the difference between them is as follows:
Gravity pressure
Defines the force exerted perpendicular to the surface of an object due to the attraction of the object
The direction is down and in
Center of action
Pointing: when the object is placed on the horizontal plane and is still, and the object is only affected by gravity and supporting force, the pressure of the object on the horizontal plane is equal to gravity, and the force area is not necessarily equal to the surface area of the object. It should be considered in a specific problem
[example 3] as shown in the figure below, a cube a with a side length of 10 cm and a weight of 8 N is placed on a 1 m2 desktop, and the force is used to lift object a upward. The pressure of object a on the desktop is calculated
Analysis: to find the pressure of object a on the table, the pressure of object a on the table, and the stressed area of the table
Answer: a pressure on the table. Area of the table, pressure on the table
Development of thinking ability
Methods of increasing and decreasing pressure
[example 4] (2005 Shantou senior high school entrance examination · curriculum reform paper) Xiaohong's mother bought a lot of food in the supermarket. She carried it home in a plastic bag, and she didn't walk far away when she felt that her hand was strangled. Please give her a suggestion with your physics knowledge:
This is the use of change to ("increase" or "decrease") the pressure of the bag
Analysis: the hand was strangled pain, indicating that the pressure is too large, to try to reduce the pressure, this topic should obviously use the method of increasing the stress area, such as: with a paper towel or handkerchief pad on the hand, etc
Answer: open questions, such as: pad your hand with a tissue or handkerchief; increase the stressed area; reduce the stress
Commentary: in real life and production, what kind of method to increase (or decrease) pressure should be determined according to the specific situation
Tracking skill 1: a person walking on the frozen lake in the park accidentally falls into an ice hole. The staff need to get close to the ice hole to rescue him. Next to him are ladders and boards of similar length and width. In order to save people, they should ()
A. Run to the ice hole immediately
B. Lay the ladder on the ice and climb over it to save the drowning person
C. Climb over the ice to save the drowning man
D. Lay the board on the ice and climb over it to save the drowner
Answer: D
Tip: if people walk on the ice and break the ice, it means that people's pressure on the ice exceeds the pressure that the ice can bear. If the staff want to save them, they may also break the ice, so they should try to reduce the pressure. Under the existing conditions, they can only reduce the pressure by increasing the stress area. Because the surface area of the board is the largest, the pressure on the ice is the smallest when the board is laid, This is the safest, so choose D
Comprehensive exploration and innovation
Application of pressure formula
[example 5] (2005 Hubei high school entrance examination curriculum reform volume) as shown in the figure below, half of the square metal block with side length is placed on the horizontal table with area of. When the indication of the spring dynamometer is 10N, the pressure of the metal block on the table is 0
And find the density of the metal block
Analysis: the force area of the metal block: according to the topic meaning: get the density of the metal block as the method of extraction: when two objects contact, the force area refers to the area of two objects contact
Tracking skill 2: the maximum pressure that a river can bear on the ice is that there is a tank with a mass of 28T, and the contact area between one of its tracks and the ground is, can this tank pass through the ice?
Answer: the actual pressure on the ice is less than the maximum pressure on the ice, so it can pass through the ice
Tip: the total area of the two tracks, the tank's pressure on the ice. So it can pass
Misunderstanding and obstacle crossing
1. Calculation of pressure
The outward force perpendicular to the surface of the object is subtracted from the inward force perpendicular to the surface of the object. As shown in (a) (b) (c) (d) (E), if the gravity of the object is g and the external force is f, then the pressures on the surface of the object in the figure are F-G, F, G + F, G-F and g respectively. When the inward force minus the outward force is negative, it means that the surface is not affected by pressure, as shown in (a)
When the difference is negative, the surface is not stressed
2. Determination of the stressed area of the object
When a person walks, the stressed area of the ground is the contact area between one foot and the ground; when standing on the ground, the stressed area of the ground is the contact area between two feet and the ground, as shown in the figure below. The bottom area of object a is 100cm2, half of which is placed on a 1m2 desktop. At this time, the stressed area of the desk is the same as that of object a
, i.e
Pressure of liquid
Curriculum standard requirements
1. Knowledge and skills
(1) Understand the pressure inside the liquid and the calculation method of the pressure inside the liquid
(2) Understand the size of the liquid pressure with what factors
(3) Understand the practical application of liquid pressure
2. Process and method
Through the observation of the demonstration experiment, we can understand the fact that there is pressure inside the liquid, and know the magnitude and direction of pressure at different depths inside the liquid
3. Emotional attitude and values
(1) In the observation experiment, cultivate scientific attitude
(2) Close contact with practice, improve the application of science and technology in daily life and social awareness
Break through key difficulties
（1） Key points
1. The law of internal pressure of liquid
Like a solid, a liquid exerts pressure on the object supporting it due to the action of gravity. Because a liquid has no fixed shape and can flow, it also exerts pressure on the wall of the container which hinders it. Through experiments, it is found that there is pressure in the liquid
The internal pressure of liquid has the following four characteristics: (1) there is pressure inside the liquid; (2) the pressure in all directions is equal at the same depth; (3) the pressure of liquid increases with the increase of depth
(4) the pressure of a liquid is also related to that of a liquid. At the same depth, the higher the density of the liquid, the greater the pressure
Note: when applying the law of internal pressure of liquid to solve practical problems, the depth of liquid refers to the vertical height from the required position to the highest liquid level
[example 1] Xiaogang's dishwashing basin is blocked by vegetable dregs. He inserts a long and thin pipe into the hole of the basin, and then injects water into the pipe and sets it up. The basin is soon open. Why?
Analysis: the water in the pipe has a downward pressure on the bottom. When the pipe is vertical, the pressure generated by the water at the bottom of the pipe is very high, and the pool is soon connected
Answer: because the pressure inside the liquid increases with the increase of depth, he inserts a long and thin pipe into the pool hole, and then fills the pipe with water. When the water depth at the bottom of the pipe is deep, the pressure generated is high, and the pressure on the vegetable residue in the pool becomes great, so the pool can become unobstructed quickly
2. The application of the law of internal pressure of liquid
(1) Concept: a container with an open upper end and a connected lower end
(2) Working principle: at a certain depth inside the liquid, when the pressure is equal, the liquid is still, and when the pressure is not equal, the liquid flows
(3) Features: when the same liquid is filled in the connector, the liquid level will be increased; if different liquid is filled, the liquid level will flow
Thinking: if the connector is filled with different kinds of liquid, when the liquid does not flow, is the liquid level flat?
According to the law of internal pressure of liquid, when the liquid is still, the pressure at the same depth is equal, as shown in the figure below. Point a is subjected to the pressure of water to the right and kerosene to the left, because it is at the same depth, so, that is, because
.
In other words: if the connector is filled with different kinds of liquid, the liquid level is not flat when the liquid is still
[example 2] as shown in the figure below, close valve a, fill water in the left pipe and kerosene in the right pipe of the connector. When the two liquid surfaces are equal, open the valve and ask if the liquid is flowing
Analysis: valve A is subjected to both the pressure of water to the right in the left pipe and the pressure of kerosene to the left in the right pipe. Since the depth of water and kerosene to point a is equal, and the density of water is greater than that of kerosene, the pressure of water to the right is greater than that of kerosene to the left, that is, the pressure of water to the right at point a is greater than that of kerosene to the left, so the liquid will flow to the right after valve A is opened, When the left and right tubes of liquid produce equal pressure at point a
Answer: flow to the right
（2） Difficulties
Understanding of the formula of liquid internal pressure
The formula of liquid pressure is derived from the basic formula of pressure. The formula shows that liquid pressure only follows and
It is difficult to understand the conclusion that it has nothing to do with the total weight of liquid