Pressure of solids and gases

Содержание и основные понятия

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Alterna · physics · 3.1
Pressure of solids and gases
Pressure, Pascal Act, hydraulic press
Why don't skis fall into the snow and heels fail?
GEF · Grade 7
PIF · Baseline
01
Pressure, pressure units
Why don't skis fall into the snow and shoes fail?
Pressure
Pressure
is a physical value equal to the ratio of the force operating perpendicular to the surface to that surface area.
p = F / S
Pressure unit:
Pascal (Pa)
1 Pa = 1 N/m2.
Other units: 1 kPa = 1000 Pa, 1 gPa = 100 Pa,1 MPa = 106 Pa.
How to Change Pressure
Increase
pressure: decrease the area (finger, needle, blade) or increase the force
Reduce
Pressure: increase the area (skis, tractor caterpillars, wide tyres)
💜
Hard body pressure — p = F/S (animation)
IV-022 · Pulsing force vectors + surface deformation + pressure indicator
Mass m:
5
kg
Area S:
02
Gas pressure
Why is the gas pressing on the walls of the vessel?
Reason for gas pressure
The gas consists of a huge number of molecules that
continuous and chaotic
The molecules give them the impulse, that's what they create.
pressure
.
The gas pressure depends on:
Temperatures
- The higher the T, the faster the molecules, the higher the pressure
Interpretation
— when the gas is compressed (reduction V) the pressure increases
Number of molecule
- more molecules ~ more impacts ~ greater pressure
💨
Gas pressure - molecular model
IV-023 · molecules in container, slides T and V
Temperature T:
300
K
Grouping V:
100
%
03
Pascal Act
The pressure produced on the liquid or gas shall be applied in all directions equally.
Pascal Act
The pressure produced on the liquid or gas shall be passed to each point of the liquid and gas without changing in all directions.
This is only true for liquids and gases because their molecules are mobile. In solid bodies, the pressure is applied only in the direction of force.
It's curious: Hydraulic machines
Pascal's law is based on
hydraulic press
: Small force on small piston creates greater force on large pistons.
🔨
Pascal Law - Hydraulic Press (Animation)
IV-024 · F1/S1 = F2/S2 · Animated pistons + liquid particles + pressure transfer
Force F1:
10
H
S₁:
5
cm2
S₂:
50
cm2
04
Pressure in the liquid, Pascal Law
Why is the water pressing deep?
Liquid pressure
The pressure inside the liquid is created
two
factors:
Force of gravity
The deeper the pressure, the higher the pressure.
External pressure
- pressure is passed by Pascal law.
Liquid pressure at depth h:
p = ρ · g · h
where the density of the liquid, g = 9.8 N/kg, h is the depth.
It's curious: Pressure at the bottom of the seas and oceans
At the bottom of the Mariana Fall (deep ~11 km), the water pressure is about 110 MPa (1100 atmosphere), which would crush most of the objects. Special deep-sea vehicles withstand such pressure with a strong structure.
C
What needs to be understood
Outcome 1
Explain the physical meaning of the topic "The pressure of solids and gases" through the model, not just through the formula.
Outcome 2
Diluting and correctly using terms: pressure, pressure, area.
Outcome 3
Indicate the conditions for the applicability of the law, verify units and limit cases.
T
Terms of the topic
The definition should indicate the physical meaning, not a substitute for a domestic synonym.
pressure
The ratio of normal force to the surface area on which it operates.
Pressure force
Equivalent normal forces with which the environment or body operates on the selected surface.
area
Physico-geometric value of surface size.
Pascal Law
The pressure produced on the liquid or gas shall be passed without change to all points of the environment.
F
Origin and conclusion of the formulas
The fundamental law cannot be "proven" by algebras: for each record, it is stated whether it is a definition, an experimental law, or a model effect.
p
=
F
S
p=\frac{F}{S}
p
=
S
F
​
definition
Pressure.
The following is an indication of the definition, experience or law from which this entry arises and where it is applicable.
F
⊥
=
нормальная сила
F_{\perp}=\text{нормальная сила}
F
⊥
​
=
normal
force
For pressure, the force component perpendicular to the surface shall be taken into account.
F
:
=
F
⊥
,
p
=
F
S
F:=F_{\perp},\qquad p=\frac{F}{S}
F
:=
F
⊥
​
,
p
=
S
F
​
In this formula F, it is the normal force of the pressure; the average pressure is equal to this force divided by the area.
Conditions:
F is a module of normal pressure that is evenly distributed over the area S > 0; in unequal distribution, the formula gives an average pressure.
Monitoring:
The dimensions of both parts shall coincide; the result shall be checked for sign, order and limit case.
!
Model boundaries and typical errors
Error: Forget model condition
The key condition of this topic is to increase the pressure: reduce the area (leg, needle, blade) or increase the force.
How to Check the Mind
Reduce the pressure: increase the area (skis, tractor caterpillars, wide tyres) then check the units, vector direction and behaviour of the result at zero or very large parameter.
Σ
Conclusion on the topic
Pressure of solids and gases
Why don't skis fall into the snow and heels fail?
Increase pressure: reduce area (leg, needle, blade) or increase force
Reduce pressure: increase the area (skis, tractor caterpillars, wide tyres)
Any numerical result shall be accompanied by a unit, a sign and an explanation of its physical meaning.
The content is consistent with the FRP on Physics of the OS/OSS 2025 and the subject matter of the current lines of the PNP: I. M. Peryškin and co-authors (7–9), G. J. Mäkishev and coauthors (10–11), and with the lines "Engineers of the Future" and V. A. Kasyanov. The original texts are not reproduced, the text of the textbooks is not reproduced.