Physics 8 Lesson 7: Pressure
1. Theoretical summary
1.1. Pressure
Pressure is the force perpendicular to the face being pressed.
– The effect of pressure is greater when the magnitude of the pressure is larger and the area of the face being pressed is smaller.
1.2. Pressure
– Pressure is the magnitude of the pressure per unit area pressed.
Pressure formula: \(p = \frac{F}{S} \)
In which:
Outside unit N/m^{2,} The unit of pressure is also in Pa (paxcan)
1 Pa = 1 N/m^{2}
– With the same pressure, the smaller the area being pressed, the greater the pressure
1.3. Solution method
a) How to recognize pressure
– Not any force is called pressure. To determine whether a force is a pressure or not, we must determine which face is pressed to know if the direction of that force is perpendicular to the area of the pressed surface or not.

When an object is placed on a horizontal plane, gravity is called pressure.

When an object is placed on an inclined plane, gravity is not called pressure because then gravity has a direction that is not perpendicular to the area of the face being pressed.
b) Calculate the pressure, the pressed surface area
– Based on the pressure formula: we get:
Note:
– Units of quantities in the formula have been agreed or not.
– If the pressed surface area is:

If the square is square, then S = a^{2} (a is the length of each side of the square).

For a rectangle, S = ab (a and b are the length and width of the rectangle, respectively).

If it’s a circle, then S = \(\pi \)r^{2 }(where r is the radius of the circle).
2. Illustrated exercise
2.1. Form 1: Determine the pressure of an object
An object of mass 0.84 kg has the shape of a rectangular box, dimensions 5cm x 6cm x 7cm. Place the three sides of this object in turn on the horizontal floor. Calculate the pressure the object acts on the floor in each case and comment on the calculated results.
Solution guide
Pressure in all three cases:
P = 0.84.10 = 8.4 N
\(\begin{array}{l} {P_1} = \frac{{0.84.10}}{{0.06.0.07}} = 2000\,N/{m^2}\\ {P_1} = \ frac{{0.84.10}}{{0,05.0,07}} = 2400\,N/{m^2}\\ {P_1} = \frac{{0.84.10}}{{0,05.0.06 }} = 2800\,N/{m^2} \end{array} \)
Comment: The pressure exerted by the object on the floor in all three cases is the same but the pressure in the different cases.
2.2. Form 2: Determine the weight and mass of the person
A person exerts a pressure of 1.7.104 N/m . on the floor^{2}. The area of the foot in contact with the table is 0.03m^{2}. Ask the person’s weight and mass?
Solution guide
Weight of person : P = pS = 17000.0.03 = 510 N
Weight of person: m = 51 kg
3. Practice
3.1. Essay exercises
Question 1: A person exerts a pressure of 1.7.10 . on the floor^{4 }N/m^{2}. The area of the foot in contact with the table is 0.03 m^{2 }. Ask the person’s weight and mass?
Verse 2: Place a 60 kg bag of rice on a fourlegged stool of mass 4 kg. The area of contact with the ground of each leg of the chair is 8 cm^{2}. Calculate the pressure the legs exert on the ground.
Question 3: Compare pressure and pressure?
Question 4: Why is it that when it rains and the dirt road is soft and muddy, people often use a board on the road for people or cars to go?
3.2. Multiple choice exercises
Question 1: A pressure of 600 N exerts a pressure of 3000 N/m^{2} to a large compressed area.
A. 2000 cm^{2 } B. 200 cm^{2} C.20 cm^{2}. D. 0.2 cm^{2}
Verse 2: Two people have masses of m . respectively_{first }and m_{2}. The first person stands on a plank of area S_{first}, the second person stands on a plank of area S_{2}. If m_{2 }= 1.2m_{first }and WILL_{first }= 1.2S_{2}, then when comparing the pressure exerted on the ground by two people, we have:
A. p_{first }= p_{2}. B. p_{first }= 1.2p_{2} C. p_{2 }= 1.44p_{first} D. p_{2 }= 1.2p_{first}.
Question 3: When the motorcycle is moving in a straight line on a horizontal road, the pressure exerted by the vehicle on the ground is equal to
A. the weight of the vehicle and the rider. B. the traction of the motorcycle engine.
C. the force of road surface resistance acting on the vehicle. D. no.
Question 4: The force exerted on this plane by a stationary body on an inclined plane is of magnitude
A. equals the weight of the object. B. is less than the weight of the object.
C. greater than the weight of the object. D. equals the force of friction between the object and the inclined plane.
4. Conclusion
Through this lesson, students will be familiar with the knowledge related to Pressure along with related exercises at many levels from easy to difficult…, they need to understand:

Defined statements about pressure and pressure.

Can write the formula for pressure and name, the units of the quantities included in the formula and apply the formula to solve simple problems

State how to increase and decrease pressure and use it to explain some simple phenomena in life
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