Pressure

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Notes

Pressure & Forces

**Pressure** is defined as the **force per unit area**.
Unit: **pascal (Pa)** = 1 N/m².
Equation: **p = F / A**, where p = pressure (Pa), F = force (N), A = area (m²).
A given force over a **small area** produces a **large pressure**.
A given force over a **large area** produces a **small pressure**.
Example: **Tractors** have large tyres to reduce pressure and avoid sinking into mud.
Example: **Drawing pins** have a sharp point (small area) to concentrate force and push into walls.
Example: **High heels** have a small area, exerting high pressure and sinking into soft ground.

Pressure in a Liquid

Pressure in a liquid **increases with depth** due to the weight of liquid above.
Pressure also **increases with density** of the liquid.
At the same depth, pressure acts **equally in all directions**.
Liquid pressure is given by: **Δp = ρgΔh** (Extended tier only).
Δp = change in pressure (Pa), ρ = density (kg/m³), g = gravitational field strength (N/kg), Δh = change in depth (m).
**Total pressure** at a depth includes **atmospheric pressure** (≈ 100 kPa).
Example: A hole at the bottom of a water column produces a stronger jet than one at the top.

Calculating Pressure in Solids

Use **p = F / A** for solid objects.
Weight (force) = mass × gravitational field strength (W = mg).
Ensure area is in **m²**; convert cm² to m² by dividing by 10 000.
Worked example: A student of weight 550 N standing on two feet (total area 0.020 m²) exerts pressure = 550 / 0.020 = 27 500 Pa.

Calculating Pressure in Liquids (Extended)

Use **Δp = ρgΔh** to find pressure difference due to depth.
Example: At 5.0 m depth in water (ρ = 1000 kg/m³, g = 9.8 N/kg), Δp = 1000 × 9.8 × 5.0 = 49 000 Pa.
Total pressure = Δp + atmospheric pressure = 49 000 + 100 000 = 149 000 Pa.
Rearrange: Δh = Δp / (ρg) to find depth from pressure.

Applications of Pressure

**High pressure** applications: knives, nails, high heels – small area concentrates force.
**Low pressure** applications: snowshoes, skis, tractor tyres – large area spreads force.
Adding weight (e.g., roof rack) **increases** pressure on the ground.
Using narrower skis **increases** pressure on snow.

Gas Pressure (Extended)

Gas pressure is caused by **molecules colliding** with container walls.
Increasing temperature **increases** molecular speed and collision frequency, raising pressure.
Pressure in a gas also varies with depth and density (as a fluid).

Particle arrangement in solids, liquids, and gases – affects pressure transmission.

Practice questions

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1.Pressure is defined as the force per unit _____.
Easy
- Aarea
- Bvolume
- Cmass
- Dlength
2.Which of the following is the correct unit for pressure?
Easy
- Apascal (Pa)
- Bnewton (N)
- Cmetre squared (m²)
- Dkilogram (kg)
3.A tractor has large tyres to reduce the pressure on the ground. This is because pressure is inversely proportional to:
Easy
- Aforce
- Barea
- Cweight
- Dvolume
4.The pressure beneath the surface of a liquid increases with:
Easy
- Adepth and density
- Bdepth only
- Cdensity only
- Dsurface area
5.A drawing pin is pushed into a wall with a force of 50 N. The area of the point is 0.02 cm². What is the pressure exerted by the point on the wall?
Medium
- A2500 N/cm²
- B250 N/cm²
- C25 N/cm²
- D2.5 N/cm²
6.A student of weight 550 N stands on both feet. The total area of his feet in contact with the ground is 0.020 m². What is the pressure exerted on the ground?
Medium
- A27 500 Pa
- B2750 Pa
- C27.5 Pa
- D275 000 Pa
7.A swimmer dives to a depth of 2.0 m in a swimming pool. The density of water is 1000 kg/m³ and gravitational field strength is 10 N/kg. What is the pressure due to the water at that depth?
Medium
- A20 000 Pa
- B2000 Pa
- C200 Pa
- D20 Pa
8.A car of weight 20 000 N has four tyres, each with an area of 0.1 m² in contact with the road. What is the total pressure exerted by the car on the road?
Medium
- A50 000 Pa
- B200 000 Pa
- C5000 Pa
- D20 000 Pa

More questions

9.A sealed bottle contains air. When the air inside is heated, what happens to the pressure and why?
Hard
- APressure increases because gas molecules move faster and collide more frequently with the walls.
- BPressure decreases because gas molecules move slower and collide less frequently.
- CPressure remains constant because the bottle is sealed.
- DPressure increases because the gas molecules expand.
10.Atmospheric pressure is approximately 100 kPa. A bubble of gas forms 5.0 m below the surface of a lake (density 1000 kg/m³, g = 10 N/kg). What is the total pressure at that depth?
Hard
- A150 kPa
- B50 kPa
- C100 kPa
- D200 kPa
11.A high-heeled shoe exerts a higher pressure on the ground than a flat shoe because:
Medium
- Athe heel has a smaller area, concentrating the force
- Bthe heel has a larger area, spreading the force
- Cthe heel is made of a denser material
- Dthe heel is taller, increasing the weight
12.A manometer measures gas pressure. The liquid in the manometer has density 800 kg/m³. The difference in height between the two columns is 0.15 m. What is the pressure difference? (g = 10 N/kg)
Hard
- A1200 Pa
- B120 Pa
- C12 000 Pa
- D1.2 Pa
13.A student measures the pressure at different depths in a beaker of water. Which graph shows the relationship between pressure and depth?
Medium
- AA straight line through the origin with positive slope
- BA horizontal line
- CA curve that flattens at greater depths
- DA straight line with negative slope
14.A manometer contains a liquid of density 800 kg/m³. The difference in height between the two columns is 0.15 m. What is the pressure difference measured by the manometer? (g = 10 N/kg)
Hard
- A1200 Pa
- B120 Pa
- C12 000 Pa
- D1.2 Pa
15.A hydraulic lift uses a liquid to transmit pressure. The input piston has an area of 0.01 m² and a force of 200 N is applied. What is the pressure transmitted through the liquid?
Medium
- A20 000 Pa
- B2000 Pa
- C20 Pa
- D2 Pa
16.A diver descends to a depth of 10 m in seawater (density 1025 kg/m³). What is the total pressure on the diver? (Atmospheric pressure = 100 kPa, g = 10 N/kg)
Medium
- A202.5 kPa
- B102.5 kPa
- C200 kPa
- D100 kPa
17.Which of the following is a correct unit for pressure?
Easy
- AN/m²
- BN m
- CN/m
- DN m²
18.A gas bubble at a depth of 5.0 m in a lake has a pressure of 150 kPa. The bubble rises to the surface where atmospheric pressure is 100 kPa. Assuming constant temperature and mass, what happens to the volume of the bubble?
Hard
- AIt increases by a factor of 1.5
- BIt decreases by a factor of 1.5
- CIt stays the same
- DIt doubles
19.A drawing pin is pushed into a wall. The force on the pin is 50 N. The area of the point is 0.02 cm². What is the pressure exerted by the point on the wall?
Easy
- A25 000 N/cm²
- B1 N/cm²
- C2500 N/cm²
- D0.0004 N/cm²
20.A student of weight 550 N stands on one foot. The area of one foot in contact with the ground is 0.010 m². What is the pressure exerted on the ground?
Medium
- A55 000 Pa
- B5500 Pa
- C55 Pa
- D5.5 Pa
21.In an experiment to investigate pressure in a liquid, a student uses a manometer. The manometer contains a liquid of density 800 kg/m³. The difference in height between the two columns is 0.15 m. What is the pressure difference measured by the manometer? (g = 10 N/kg)
Medium
- A1200 Pa
- B120 Pa
- C1.2 Pa
- D12000 Pa
22.A hydraulic lift uses a liquid to transmit pressure. The input piston has an area of 0.01 m² and a force of 200 N is applied. What is the pressure transmitted to the output piston?
Hard
- A20000 Pa
- B2000 Pa
- C2 Pa
- D200 Pa
23.A gas bubble at a depth of 5.0 m in a lake has a pressure of 150 kPa. The bubble rises to the surface where atmospheric pressure is 100 kPa. Assuming constant temperature, what is the ratio of the volume at the surface to the volume at depth?
Hard
- A1.5
- B0.67
- C1.0
- D2.0
24.In a manometer experiment, the liquid column height difference is 0.20 m. The liquid density is 900 kg/m³. If the atmospheric pressure is 100 kPa, what is the absolute pressure of the gas? (g = 10 N/kg)
Hard
- A101.8 kPa
- B100.18 kPa
- C118 kPa
- D98.2 kPa
25.A student sets up an experiment to compare the pressure exerted by a block on a soft surface. Which factor does NOT need to be controlled?
Medium
- AThe colour of the block
- BThe weight of the block
- CThe area of contact
- DThe material of the surface
26.A student investigates how the pressure in a liquid varies with depth using a pressure sensor. Which of the following should be kept constant?
Hard
- ADensity of the liquid
- BDepth of the sensor
- CVolume of the liquid
- DSurface area of the container
27.Which of the following is the correct definition of pressure?
Easy
- AForce per unit area
- BArea per unit force
- CForce times area
- DMass per unit area
28.What is the SI unit of pressure?
Easy
- APascal
- BNewton
- CJoule
- DWatt
29.A force of 10 N is applied over an area of 2 m². What is the pressure?
Easy
- A5 Pa
- B20 Pa
- C0.2 Pa
- D12 Pa
30.Tractors have large tyres to reduce pressure on the ground. This is because pressure is inversely proportional to:
Easy
- AArea
- BForce
- CWeight
- DDensity
31.Which of the following exerts the highest pressure on the ground?
Easy
- AA high-heeled shoe
- BA flat shoe
- CA tractor tyre
- DA snowshoe
32.A swimmer dives to a depth of 2.0 m in water of density 1000 kg/m³. What is the pressure due to the water? (g = 10 N/kg)
Medium
- A20 000 Pa
- B2000 Pa
- C200 Pa
- D20 Pa
33.A student investigates how the pressure in a liquid varies with depth. Which graph shows the correct relationship between pressure and depth for a liquid of constant density?
Medium
- AA straight line through the origin with positive slope
- BA horizontal line (constant pressure)
- CA curve that increases and then levels off
- DA straight line with negative slope
34.A manometer is used to measure the pressure of a gas supply. The liquid in the manometer has a density of 800 kg/m³. The difference in height between the two columns is 0.15 m. What is the pressure difference measured by the manometer? (g = 10 N/kg)
Medium
- A1200 Pa
- B120 Pa
- C12 000 Pa
- D1.2 Pa
35.A hydraulic lift uses a liquid to transmit pressure. The input piston has an area of 0.01 m² and a force of 200 N is applied. The output piston has an area of 0.5 m². What is the force exerted by the output piston?
Hard
- A10 000 N
- B10 N
- C1000 N
- D100 000 N
36.A diver descends to a depth of 10 m in seawater (density 1025 kg/m³). Atmospheric pressure is 100 kPa and g = 10 N/kg. What is the total pressure on the diver?
Hard
- A202.5 kPa
- B102.5 kPa
- C200 kPa
- D20.25 kPa
37.Which of the following correctly describes how the pressure in a liquid changes with depth and density?
Medium
- AIt increases with depth and increases with density
- BIt increases with depth and decreases with density
- CIt decreases with depth and increases with density
- DIt decreases with depth and decreases with density
38.In an experiment to investigate pressure in a liquid, a student uses a manometer. The manometer contains a liquid of density 800 kg/m³. The difference in height between the two columns is 0.20 m. What is the pressure difference? (g = 10 N/kg)
Medium
- A1600 Pa
- B160 Pa
- C16 000 Pa
- D1.6 Pa
39.In an experiment to investigate how the pressure in a liquid varies with depth, a student uses a pressure sensor attached to a probe. The probe is lowered into a tank of water. Which of the following should be kept constant during the experiment?
Medium
- AThe density of the liquid
- BThe depth of the probe
- CThe temperature of the water
- DThe size of the tank
40.A student uses a manometer to measure the pressure of a gas supply. The manometer contains a liquid of density 800 kg/m³. The difference in height between the two columns is 0.15 m. What is the pressure difference measured by the manometer? (g = 10 N/kg)
Medium
- A1200 Pa
- B120 Pa
- C12000 Pa
- D12 Pa

Slides

What is Pressure?

Pressure is defined as the force per unit area.
Pressure = force ÷ area (p = F / A).
Measured in pascals (Pa) where 1 Pa = 1 N/m².

Which equation correctly defines pressure?

p = F × A
p = F / A
p = A / F
p = F + A

Pressure and Area

A large force spread over a large area gives a small pressure.
A small force concentrated on a small area gives a large pressure.
Example: Tractors have wide tyres to reduce pressure and avoid sinking into mud.

Which of these are true about pressure?

A drawing pin has a sharp point to increase pressure.
High heels exert less pressure than flat shoes.
Pressure is measured in newtons (N).

Pressure in Liquids

Pressure in a liquid increases with depth.
It also increases with the density of the liquid.
At the same depth, pressure acts equally in all directions.

Pressure in a liquid is the ____ of the liquid × gravitational field strength × change in ____.

Calculating Liquid Pressure (Extended)

Equation: Δp = ρgΔh
Δp = change in pressure (Pa), ρ = density (kg/m³), g = 9.8 N/kg, Δh = depth (m).
Example: At 2 m depth in water (ρ=1000 kg/m³), Δp = 1000 × 9.8 × 2 = 19600 Pa.

A diver is 5 m deep in water (density 1000 kg/m³, g=9.8 N/kg). What is the pressure due to the water?

49000 Pa
4900 Pa
5000 Pa
98000 Pa

Total Pressure in a Liquid

Total pressure on an object includes atmospheric pressure (≈100 kPa).
Total pressure = atmospheric pressure + ρgΔh.
Example: At 5 m depth, total pressure = 100000 + 49000 = 149000 Pa.

At a depth of 10 m in water, the pressure due to the water alone is ____ Pa. (ρ=1000 kg/m³, g=9.8 N/kg)

Applications of Pressure

Drawing pins: small area → high pressure → easy to push into wall.
High heels: small area → high pressure → sink into soft ground.
Skis: large area → low pressure → stay on top of snow.

Which of these are true?

Increasing the area decreases pressure for the same force.
Pressure in a liquid is the same at all depths.
Atmospheric pressure is about 100 kPa.

Worked Example: Force from Pressure

A pump creates pressure 5.28×10⁵ Pa on a piston of area 2.73×10⁻² m².
Force = pressure × area = 5.28×10⁵ × 2.73×10⁻² = 14400 N (14.4 kN).

A force of 100 N is applied over an area of 0.5 m². What is the pressure?

50 Pa
200 Pa
500 Pa
20 Pa

Key Points Summary

Pressure = force / area (p = F/A).
In liquids: Δp = ρgΔh.
Pressure increases with depth and density.
Total pressure includes atmospheric pressure.

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