【Review】
In the effect of pressure, when the pressure is the same, the smaller the force area, the more obvious the effect of pressure.
If you think about it carefully, in the case of B and C, the pressure on each square centimeter of the sponge is different, which is the reason for the different indentation depths.
[Raise your hand to ask a question]
1. What is pressure?
2. How to calculate pressure?
3. What are the ways to increase or decrease the pressure?
【New Lesson Learning】
1. What is pressure
● The concept of pressure
In the experiment that affects the effect of pressure, if the pressure of the small table on the sponge is 12N, the area of the four legs is 3cm², and the pressure per cm² is 12N/3cm²=4N/cm², that is, the pressure is 4N per square centimeter.
As shown in the image:
If the pressure of the small table on the sponge is 12N, and the area of the table top is 30 cm², the pressure per cm² is
12N/30cm²=0.4N/cm².
That is, a pressure of 0.4 N per square centimeter of area.
As shown in the image:
How to compare the magnitude of the pressure of the same force area?
In physics, the ratio of the pressure to the area of force experienced by an object is called pressure.
That is, pressure is the pressure exerted on a unit area of an object.
Pressure indicates the effect of pressure.
●Pressure formula:
According to the concept of pressure: pressure = pressure/area of force
It is symbolized as:
p=F/S 变式：F=pS S=F/p
● The meaning and unit of the symbol
p——压强——帕斯卡（Pa）
F – Pressure – Newton (N)
S - Stress area - square meters (m²)
● The meaning of the pressure unit
Because:P=F/S So, the unit of pressure is N/m²,
Since the unit of pressure has a special name called Pascal, referred to as Pascal.
1Pa=1N/m²
So: 1N/m² is called 1 Pascal, also called 1 Pa.
For example, if the pressure on an object is 100Pa, it means that the object is subjected to a pressure of 100N per square meter of the stressed area.
The unit of pressure expressed by Pascal is in honor of the French scientist Pascal.
pascal
2. Calculation of pressure
Examples:
Compare the pressure of a ballet dancer and an elephant on the ground.
(1) The ballet dancer weighs 475N and has a ground contact area of 9.5cm², and the pressure of the ballet dancer on the ground is calculated.
Solution: The pressure of a ballet dancer on the ground
Because the ballet dancer stands on a level ground, so: F₁=G₁=475N,
Ground contact area S₁ = 9.5 cm² = 9.5×10-4 m²
According to the pressure formula:
P₁=F₁/S₁=475N/9.5×10-4m²=5.0x105Pa
(2) The elephant weighs 60,000N, and the area of each foot is 600cm², and the pressure of the elephant on the ground is calculated.
Solution: The pressure of an elephant on the ground
Because elephants stand on level ground, so:
F₂=G₂=6x104N,
Ground contact area S₂=600cm²×4=0.24m².
According to the pressure formula:
P₂=F₂/S₂=6x104N/0.24m²=2.5×105Pa.
Because P₁>P₂
So, the ballet dancer presses more on the ground than the elephant presses on the ground.
3. Methods for increasing and decreasing pressure
According to the experimental conclusion of "influencing the effect of pressure" and the pressure formula P=F/S, it can be known that:
There are two ways to increase or decrease the pressure:
☛ Increase or decrease the pressure
☛ Increase or decrease the force area
In general applications, it is often when the pressure is constant, by increasing or decreasing the force area to increase or decrease the pressure.
For example:
★ The bulldozer has wide tracks and sharp shovels
(1) The wide track increases the area with the ground and reduces the pressure on the ground.
(2) The sharp soil shovel reduces the stress area of the shovel on the soil and increases the pressure on the soil.
★ The bag is flattened by bandwidth
By increasing the force area to reduce the pressure on the person's shoulder, the backpack feels more comfortable.
★ Woodpecker's beak
The woodpecker's hard, slender beak increases the pressure on the trunk and makes it easy to obtain food.
★ "Ship of the Desert" camel
The camel weighs less than twice as much as the horse, but the soles of the feet are more than three times the hooves, and the wide feet reduce the pressure on the sand so that the camel does not sink too deep.
☛ There is a certain limit to the amount of pressure that any object can withstand, beyond which the object will be damaged.
★ The frozen river surface exceeds the pressure limit
【Synchronous training of past questions of the high school entrance examination】
1.（2018牡丹江6题）
The following examples of pressure reduction are ( )
2.(2018兰州10题3分)
As shown in the figure, the solid cuboids A and B with the same height and material are placed on a horizontal tabletop. The pressure of B on the table is FA and FB, respectively, and the pressure is PA and PB, respectively, and the following relationship between their sizes is correct ( )
A. FA<FB, PA<PB
B. FA>FB, PA=PB
C. FA>FB, PA<PB
D.FA>FB, PA>PB
3. (2018 Hangzhou 18 questions)
As shown in the figure, the rectangular weight A B with uniform mass distribution, the density is ρA and ρB, the bottom area is SA and SB respectively, and SA >SB, they are placed on the horizontal ground, their pressure on the ground is equal, now the upper part is cut horizontally (as shown by the dotted line in the figure), the pressure of the remaining part on the ground is PA and PB respectively, and the pressure on the ground is FA and FB respectively, and the relationship between the following physical quantities is correct ( )
A. FA<FB B. pA>PB C. ρA>ρB
D. The pressure of A and B on the ground before and after cutting is unchanged
4. (2021 Zigong)
As shown in the figure, there are three solid cylinders A, B and C, placed on the horizontal ground, wherein A and B are the same height, the bottom area of B and C is the same, and the pressure of the three on the ground is equal, and the following judgment is correct ( )
A.ρ甲=ρ乙>ρ丙
B.ρ甲=ρ乙=ρ丙
C. mA = m B = m C
D.m甲>m乙=m丙
5. (2021 Yangzhou)
As shown in the figure, the cube wooden block with a side length of 0.1m is placed in the center of the horizontal desktop, and the density of the wooden block is known to be 0.5×10³kg/m³, the mass of the wooden block is kg, and its pressure on the desktop is Pa.
The frictional force on which the block moves in a straight line from point A to point B under the action of a horizontal to the right of a size of 1.5 N is, and in the process, its pressure on the table top (optionally "bigger", "smaller" or "unchanged"). (g takes 10N/kg)
6. ( Yancheng 22 questions)
Two-wheel electric self-balancing scooter is becoming more and more popular among people, as shown in the figure, Xiaohong with a mass of 40 kg drives the self-balancing scooter at a constant speed on a straight road surface, and the distance passed in 5 min is 900 m.
Find: (1) the speed of the balance car.
(2) The gravity of the balance car.
(3) When Xiaohong drives a balance bike, the pressure of the car on the ground.
7. (2021 Yongzhou)
As shown in the figure, a cube block is stationary on the horizontal ground, and the side length of the known block is L=0.1m, the density ρ=2.7×10³kg/m³, the density of water ρ water = 1.0×10³kg/m³, and g takes 10N/kg
(1) Find the mass of the block;
(2) find the pressure of the block on the horizontal ground;
(3) Put the block into enough water to find the amount of buoyancy the block will experience when it is stationary
three
test
answer
table
at
under
side
look for
哦
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Refer to the answer
1.D 2.B 3.C 4.A
5. 0.5 500 1.5N unchanged
6.（1）3m/s （2）100N
(3)2×105Pa.
7.