Force and Pressure
1. What is Force?
A force is a PUSH or PULL that can change the state of motion or shape of an object.
Effects of force:
- Can start or STOP motion
- Can CHANGE the speed or direction of motion
- Can CHANGE the SHAPE of an object
2. Types of Forces
Contact Forces (Require PHYSICAL contact)
| Force | Description | Example |
|---|---|---|
| Muscular force | Force from MUSCLES | Pushing a cart, lifting a bag |
| Friction | Opposes relative motion | Brakes on a bicycle |
| Tension | In STRETCHED strings/ropes | Tug of war |
| Normal reaction | Perpendicular to SURFACE | Book on a table |
Non-Contact Forces (Act WITHOUT contact)
| Force | Description | Example |
|---|---|---|
| Gravitational force | Attraction between MASSES | Falling apple |
| Magnetic force | Between MAGNETS | Attracting iron pins |
| Electrostatic force | Between CHARGED objects | Balloon sticking to wall |
3. Turning Effect of Force (Moment)
Moment of force = Force × Perpendicular distance from pivot
M = F × d
Unit: Newton-metre (Nm)
Worked Example: A force of 20 N is applied at a distance of 0.5 m from the hinge of a door. Find the moment.
Moment = 20 × 0.5 = 10 Nm
'Clockwise moments and anticlockwise moments. When they are EQUAL, the object is in EQUILIBRIUM.'
4. Pressure
Pressure = Force / Area
P = F/A
Units: Pascal (Pa). 1 Pa = 1 N/m²
Worked Example: A box weighing 500 N rests on a surface. The area of contact is 2 m². Find the pressure.
P = 500/2 = 250 Pa
Applications of Pressure
- A SHARP knife cuts better (SMALL area → HIGH pressure)
- Snowshoes prevent sinking in snow (LARGE area → LOW pressure)
- School bags have WIDE straps (LARGE area → LESS pressure on shoulders)
- Nails have SHARP tips (SMALL area → HIGH pressure to penetrate)
5. Liquid Pressure
Factors affecting liquid pressure:
- Depth: Increases with depth (p ∝ h)
- Density: Increases with density (p ∝ ρ)
- Gravity: Increases with gravity (p ∝ g)
Formula: P = hρg
Properties:
- Pressure at the SAME depth is EQUAL in ALL directions
- Liquid pressure is INDEPENDENT of the shape of the container
- A dam is THICKER at the BOTTOM because pressure is GREATER at greater depth
6. Atmospheric Pressure
The EARTH's atmosphere exerts pressure in ALL directions.
At sea level: 1 atm = 1.013 × 10⁵ Pa = 760 mm of Hg
Characteristics:
- Decreases with ALTITUDE (mountains have LOWER pressure)
- Measured with a BAROMETER
- Is responsible for STRAW drinking, syringe action, vacuum cleaners
Worked Example: Why does a liquid rise in a straw?
When you suck, you REDUCE the air pressure inside the straw. The GREATER atmospheric pressure on the liquid surface PUSHES the liquid UP the straw.
7. Pascal's Law
'Pressure applied to an ENCLOSED fluid is transmitted EQUALLY to every part of the fluid.'
Applications:
- Hydraulic jack
- Hydraulic brakes
- Hydraulic press
Hydraulic lift: A small force applied on a SMALL piston produces a LARGE force on a LARGE piston.
F₂/F₁ = A₂/A₁
Worked Example: In a hydraulic press, the area of the small piston is 0.1 m² and the large piston is 2 m². If a force of 50 N is applied on the small piston, what force is produced on the large piston?
F₂/50 = 2/0.1 = 20 F₂ = 50 × 20 = 1000 N
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Pressure depends on the shape of container' | Pressure at a given depth depends ONLY on height and density, NOT on container shape |
| 'Force and pressure are the same' | Force is a PUSH/PULL. Pressure is FORCE PER UNIT AREA |
| 'Atmospheric pressure pushes DOWN only' | Atmospheric pressure acts in ALL directions |
| 'Greater area means greater pressure' | Greater area means LESS pressure (P = F/A). Pressure is INVERSELY proportional to area |
ICSE Exam Focus (6–8 marks)
- 2-mark questions: Define force and pressure, give SI units
- 3-mark questions: Differentiate between contact and non-contact forces
- 4-mark questions: Numericals on pressure (P = F/A)
- 5-mark questions: Liquid pressure or atmospheric pressure applications
- 6-mark questions: Pascal's law and hydraulic press numericals
Self-Test
Q1. A force of 100 N acts on an area of 2.5 m². Calculate the pressure. A1. P = 100/2.5 = 40 Pa.
Q2. Why are school bag straps made wide? A2. Wide straps increase the area of contact. For the SAME force (weight of bag), GREATER area means LESS pressure on the shoulders.
Q3. State Pascal's law. Give one application. A3. Pascal's law: Pressure applied to an enclosed fluid is transmitted EQUALLY to every part of the fluid. Application: Hydraulic lift.
Q4. A girl of weight 400 N wears shoes with a total area of 100 cm². Find the pressure. A4. Area = 100 cm² = 0.01 m². P = 400/0.01 = 40000 Pa = 40 kPa.
Q5. Why is a dam thicker at the bottom? A5. Liquid pressure INCREASES with depth. The bottom of the dam experiences the HIGHEST pressure, so it must be thicker to withstand the greater force.
Q6. In a hydraulic press, the area ratio A₂/A₁ = 50. Find the force on the large piston when 30 N is applied on the small piston. A6. F₂/F₁ = A₂/A₁ = 50. F₂ = 30 × 50 = 1500 N.
