Force and Pressure

What you'll learn

• What force is; contact vs non-contact forces.
• Pressure — formula, unit, factors affecting it.
• Fluid pressure and Pascal's Law.
• Atmospheric pressure and how it is measured.

Key concepts

Force

• Force: a push or pull that can change the shape, speed, or direction of an object.
• Unit: Newton (N).
• Force is a vector (has direction).
• Effects of force: change in speed, change in direction, change in shape.

Types of forces

Contact forces (require physical contact):

Non-contact forces (act without touching):

Friction

• Friction: force that opposes the relative motion between two surfaces in contact.
• Always acts opposite to the direction of motion.
• Depends on: nature of surfaces (rough vs smooth) and normal force (weight pressing surfaces together).
• Types:
  • Static friction: prevents an object from starting to move (strongest type).
  • Sliding (kinetic) friction: opposes motion while object slides.
  • Rolling friction: least friction; opposes rolling motion.
• Friction is useful: walking, writing, braking.
• Friction is harmful: wears out surfaces, wastes energy as heat.
• Reduce friction: lubricants (oil, grease), ball bearings, smooth surfaces, streamlining.

Pressure

Pressure = Force ÷ Area P = F / A

• Unit: Pascal (Pa) = N/m²
• Also: bar, atm, mmHg.
• Greater force on smaller area = greater pressure.
• Same force on larger area = less pressure.

Examples of pressure in daily life:

Calculations:

• A 500 N person stands on one foot (area = 0.02 m²): P = 500/0.02 = 25,000 Pa.
• Same person on two feet (area = 0.04 m²): P = 500/0.04 = 12,500 Pa (half the pressure).

Pressure in fluids (liquids and gases)

• Fluids (liquids and gases) exert pressure in all directions (unlike solids which only push down).
• Fluid pressure increases with depth:

P = ρ × g × h

(ρ = fluid density, g = gravity, h = depth)

• Deeper in a liquid → higher pressure (why dams are thicker at base; why deep-sea fish have strong bodies).
• At the same depth in a connected liquid: pressure is equal in all directions (Pascal's principle).

Pascal's Law

Pressure applied to an enclosed fluid is transmitted equally in all directions throughout the fluid.

Applications:

• Hydraulic lift (car jack): small force on small piston → same pressure transmitted → large force on large piston. F₁/A₁ = F₂/A₂.
  • Example: 100 N on a 0.01 m² piston → pressure = 10,000 Pa → force on 0.5 m² piston = 5000 N (50× amplification).
• Hydraulic brakes: foot pedal applies small force → hydraulic fluid transmits → large force on brake pads at all four wheels.
• Hydraulic press: used to shape metal, compress bales.

Atmospheric pressure

• Atmosphere: layer of air around Earth; has mass; exerts pressure.
• Atmospheric pressure at sea level: ~101,325 Pa ≈ 1 atm ≈ 101 kPa.
• Air pressure decreases with altitude (less air above).
• Torricelli (1643) invented the barometer to measure atmospheric pressure.

Simple barometer:

• Take a long glass tube (1 m+); fill with mercury; invert into a trough of mercury.
• Mercury column supported by atmospheric pressure.
• At sea level: height of mercury column = 76 cm (760 mmHg).

Why atmospheric pressure is important:

• Straws work: atmospheric pressure pushes liquid up when you create a partial vacuum by sucking.
• Suction cups: pressing removes air → atmospheric pressure holds cup to surface.
• Altitude sickness: at high altitude, low air pressure → less O₂ per breath → headache, fatigue.
• Weather: high pressure → clear weather; low pressure → clouds and rain.

Quick check

• What is pressure? Write the formula and state its SI unit.
• Why does a knife have a sharp edge? Explain using pressure.
• State Pascal's Law. How does a hydraulic lift use it?
• What is atmospheric pressure? How did Torricelli measure it?
• A force of 200 N acts on an area of 0.5 m². What is the pressure? If the area is halved, what happens to the pressure?

Open the Practice tab for graded questions on Force and Pressure.