"Pressure" PPT courseware 7

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"Pressure" PPT courseware 7

┃Review the textbooks and analyze knowledge┃

1. The difference between pressure and gravity: Pressure refers to ______ the force acting on the surface of an object. Usually when an object is placed on the table, if the object is not subject to other forces, the pressure F______ the gravity G of the object. The pressure is not all caused by gravity. When an object with a weight of G is stationary on the water, the pressure is caused by gravity. The magnitude of the pressure in various situations in Figure 8-1 is:

2. Solution to solid pressure and liquid pressure: Pressure is a physical quantity that represents _______, and the calculation formula is _______. The general method for calculating the pressure of liquid on the bottom of the container and pressure issues: ① First determine the pressure p = _______; ② Secondly determine the pressure F = _______. In special circumstances, to find the pressure at the bottom of a liquid in a straight cylindrical container, you can first find F, F = _______, and then use p = _______ to find the pressure.

For a straight cylinder (such as cylinder, cube, cuboid, prism, etc.) placed on the table, the pressure on the table can be calculated using p = _______. The pressure of a liquid is only related to the _______ of the liquid and the _______ of the liquid, but has nothing to do with the mass, volume, gravity of the liquid, the bottom area of ​​the container, or the shape of the container.

3． Characteristics and applications of atmospheric pressure: Atmospheric pressure is caused by the air being affected by _______ and having _______ properties. The cup-covering experiment, the egg-swallowing-in-a-bottle experiment, and the famous _______ experiment in history prove the existence of atmospheric pressure; piston pumps and centrifugal water pumps utilize atmospheric pressure in production and life. Within an altitude of 3,000 meters, the atmospheric pressure decreases approximately _______Pa for every 10 meters of elevation. This feature can be used to measure the height of the mountain. Since the boiling point of all liquids is _______ when the air pressure decreases, and _______ when the air pressure increases, so _______ is used for cooking in plateau areas. In the sugar industry, the characteristics of atmospheric pressure are used to remove moisture from sugar juice.

Typical example 1: Calculation of pressure and pressure

Example 1 As shown in Figure 8-2, the finger presses the tip of the pencil with a force of 4 N. The pressure of the pencil on the horizontal table is ________N (the gravity on the pencil is ignored). The contact area between the finger and the tip of the pencil is 0.5 mm2, the contact area between the tail end of the pen and the tabletop is 0.4 cm2, the pressure on the finger is ________Pa, and the pressure on the tabletop is ________Pa.

Method instructions:

This question examines the simple calculation of pressure and the determination of the stressed area. Note that when calculating pressure, the pressure should correspond to the contact area, and the units should be unified during the calculation process. When it comes to the contact area, pay attention to the size of the area: for example, an elephant stands on four legs, a person stands on two legs, one foot touches the ground when walking, etc.

Example 2 As shown in Figure 8-3, Xiaohua skates quickly on the skating rink, gliding 80 m in 10 seconds. The total mass of her and the skates is 40 kg. It is known that the contact area between each skate and the ice surface is 15 cm2. (G is taken as 10 N/kg) Find:

(1) How much pressure does Xiaohua put on the ice?

(2) Xiaohua’s average sliding speed.

(3) How much pressure does Xiaohua exert on the ice when skating on one foot? When Xiaohua stands on both feet, how does the pressure on the ice surface change? Briefly explain the reason.

[ Answer The pressure on the ice surface remains unchanged, but the contact area becomes larger, so the pressure decreases.

Method instructions:

When calculating using formulas, special attention should be paid to changes in the force-bearing area. In a question, there may be multiple stressed areas or changes in the stressed area. In this case, we need to correctly select the stressed area and its corresponding pressure.

┃Become a master through practical exercises┃

1. In the example shown in Figure 8-6, what reduces the pressure is ()

2. As shown in Figure 8-7, when a brick is placed flat, vertical, or sideways, its pressure on the ground ()

A. Maximum when laid flat

B． Maximum when standing

C. Maximum when placed on its side

D． The same size when placed flat, vertically or sideways

3． As shown in Figure 8-8, when the test tube is placed from tilted to vertical, the pressure of water on the bottom of the test tube ()

A. Get bigger B. constant

C. become smaller D. unconfirmed

4. A student used the device shown in Figure 8-9 to explore "Characteristics of Liquid Pressure." Which of the following analysis of experimental phenomena is incorrect ()

A. When only the plugs of holes a and c are removed, water flows out of both holes, indicating that the water has pressure in all directions.

B． When only the plugs of holes B and C are unplugged, it is observed that the range of water in the two holes is the same, indicating that the pressure of water is equal at the same depth.

C. When only the plugs of holes a and c are unplugged, it is observed that the range of water in hole c is farther than that of hole a, indicating that the pressure of water increases with depth.

D． When only the hole plug of D is removed, water is observed to flow out, indicating that the water has pressure on the bottom of the container.

Question 5 of the first monthly exam is about training

1. Regarding the inertia of an object, which of the following statements is correct ()

A. Long jumpers have to run before taking off in order to increase their inertia.

B． A racing car can start and brake quickly because it has no inertia

C. Athletes cannot stop immediately when they reach the finish line because they have inertia

D． The football has no inertia when it is at rest, but it has inertia after it is kicked.

2. Through investigation, the students found that some drivers around them did not like to wear seat belts when driving. Which of the following reasons do you think is correct for requiring the wearing of seat belts in traffic regulations ()

A. Wearing a seat belt reduces the inertia of the car

B． Wearing seat belts is to reduce the inertia of people in the car

C. Wearing seat belts can reduce injuries caused by inertia to people in the car

D． Wearing a seat belt is for appearance

3． As shown in Figure 8-23, use a spring dynamometer to pull the object on the horizontal table horizontally so that it slides in a straight line at a constant speed. Among the groups of forces described below, which one belongs to a pair of balanced forces is ()

A. The pulling force of the spring dynamometer on the object and the gravity on the object

B． The pressure of the object on the table and the gravity of the object

C. The pressure of objects on the table and the support of objects on the table

D． The pulling force of the spring dynamometer on the object and the friction force of the table on the object

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Update Time: 2024-11-24

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