Pressure on a Surface

What you’ll learn

Relate perpendicular force to contact area.

ភ្ជាប់កម្លាំងកែងទៅនឹងផ្ទៃប៉ះ។

Meaning and use

Average pressure on a surface is normal force divided by contact area. The same force produces higher pressure on a smaller area. Use the component perpendicular to the surface, not an unrelated horizontal force. One pascal is one newton per square metre.

សម្ពាធមធ្យមស្មើកម្លាំងកែងចែកផ្ទៃប៉ះ។ កម្លាំងដូចគ្នាលើផ្ទៃតូចបង្កសម្ពាធខ្ពស់ជាង។ 1 Pa = 1 N/m²។

Formulas

Average surface pressure

\[p=\frac{F_\perp}{A}\]

Rearrangement

\[F_\perp=pA\]

Symbols and units

Symbol	Meaning	Unit or restriction
p	Average pressure	pascal (Pa)
F⊥	Force perpendicular to the surface	N
A	Contact area	m² for pressure in Pa

Conditions and limits

• The contact area must be positive.
• This formula gives average pressure if the force distribution is not uniform.
• ផ្ទៃក្នុង cm² ត្រូវបម្លែងទៅ m² មុនឆ្លើយជា Pa។

Worked examples

Pressure from force

A normal force of 100 N acts on 0.02 m².

1. Use \(p=F/A\).
2. Calculate \(100/0.02=5000\) Pa.

5000 Pa

ផ្ទៃតូចធ្វើឱ្យសម្ពាធកើនក្រោមកម្លាំងដដែល។

An area conversion

A 20 N force acts on 10 cm².

1. Convert \(10\,\mathrm{cm^2}=0.001\,\mathrm{m^2}\).
2. Calculate \(p=20/0.001=20000\) Pa.

20000 Pa

ការបម្លែងផ្ទៃគឺចែក 10000 មិនមែន 100។

Common mistakes

Instead of	Use	Why
Pressure = force × area.	Pressure = force / area.	A smaller area gives higher pressure for fixed force.
10 cm² = 0.1 m².	10 cm² = 0.001 m².	Square-unit conversion uses 10000.

Practice

Solve these before revealing the worked solutions. Include units where relevant.

សាកល្បងដោះស្រាយមុនបើកមើលចម្លើយ។ សូមបង្ហាញជំហាន និងឯកតាដែលពាក់ព័ន្ធ។

1. Find pressure for 60 N over 0.03 m².
2. Find force for pressure 500 Pa and area 0.2 m².
3. If force stays fixed and area doubles, what happens to pressure?

Answers and steps

Show solutions / បង្ហាញដំណោះស្រាយ

1. 2000 Pa

   60/0.03 = 2000.

2. 100 N

   500 × 0.2 = 100.

3. It halves.

   Pressure is inversely proportional to area in this model.

Quick recap

• Use perpendicular force.
• Convert area to square metres for Pa.
• Smaller contact area means higher average pressure for fixed force.