Force and Pressure — AP Board Class 8 Physical Science
1. What is a Force?
'A force is a PUSH or a PULL that changes or tends to change the state of rest, motion, or shape of an object.' In our daily life, we apply force constantly — opening a door (push), drawing water from a well (pull), kneading dough (change of shape), or kicking a football (change of motion).
Effects of Force
| Effect | Example |
|---|---|
| Change in speed | A batsman hits a cricket ball — ball speeds away |
| Change in direction | A footballer kicks a moving ball towards the goal |
| Change in shape | Pressing a lump of clay — shape changes |
| Change in state of rest | Pushing a stationary table — it begins to move |
'Force is a VECTOR quantity — it has both MAGNITUDE and DIRECTION.' The SI unit of force is the newton (N).
2. Types of Forces
Contact Forces (require physical touch)
- Muscular Force: Force applied by our muscles. Example: Lifting a school bag, pushing a cart.
- Frictional Force: The force that OPPOSES motion when two surfaces are in contact. 'Without friction, we could not walk — our feet would simply slip.'
- Magnetic Force: Force exerted by a magnet. Can act at a distance (attraction/repulsion).
- Mechanical Force: Force applied through machines — a crane lifting a steel beam.
Non-Contact Forces (act without physical contact)
- Gravitational Force: The force of ATTRACTION between any two objects with mass. 'The Earth pulls everything towards its centre — this is why an apple falls DOWN, not UP.'
- Electrostatic Force: Force between two charged objects. A plastic comb rubbed on dry hair attracts small pieces of paper.
- Magnetic Force: Though listed as a contact force above, magnets also attract iron filings from a distance — acting as a non-contact force too.
'Non-contact forces are also called FIELD forces — they act through a force field that surrounds the object.'
3. Balanced and Unbalanced Forces
Balanced Forces
When two or more forces acting on an object CANCEL each other out, they are balanced. The net force is ZERO. The object does NOT change its state of motion.
Example: A book lying on a table. The GRAVITATIONAL force pulls it DOWN. The TABLE pushes it UP (normal force). Both forces are equal and opposite — the book stays at rest.
'Balanced forces do NOT produce motion. They only change the SHAPE of an object if applied in opposite directions — like two teams in a tug-of-war at a standstill.'
Unbalanced Forces
When the net force on an object is NOT zero, the forces are unbalanced. The object will:
- Start moving if it was at rest
- Stop moving if it was in motion
- Change speed or direction
Example: A football kicked by a player — only one force acts in one direction. The ball begins to move.
4. Pressure — Force Per Unit Area
Pressure is defined as the force acting PERPENDICULARLY (normal force) on a unit area of a surface.
[ \text{Pressure} = \frac{\text{Force}}{\text{Area}} \quad \text{or} \quad P = \frac{F}{A} ]
- SI unit of pressure: pascal (Pa)
- 1 Pa = 1 N/m²
Key Relationship
'Same force applied on a SMALLER area produces MORE pressure. Same force applied on a LARGER area produces LESS pressure.'
Applications of Pressure
| Situation | Area | Pressure | Why It Works |
|---|---|---|---|
| Sharp knife | Very small cutting edge | VERY HIGH | Cuts through vegetables easily |
| Nail with sharp tip | Tiny tip area | VERY HIGH | Penetrates the wall easily |
| Snowshoes | Large surface area | LOW | Prevents sinking into snow |
| School bus tyres (broad) | Large contact area | LOW | Does not sink into muddy roads |
| Camel's feet (broad) | Large area | LOW | Walks easily on desert sand |
| Drawing pin (pin side) | Tiny area | HIGH | Pin enters the board |
| Drawing pin (head side) | Larger area | LOW | Thumb does not get hurt |
Worked Example 1
Problem: A box weighing 200 N is placed on a table. The base of the box has an area of 0.5 m². Calculate the pressure exerted by the box on the table.
Solution: Given: Force (F = 200) N, Area (A = 0.5) m²
[ P = \frac{F}{A} = \frac{200}{0.5} = 400 \text{ Pa} ]
Answer: The pressure exerted is 400 Pa.
Worked Example 2
Problem: A girl of weight 400 N is wearing: (a) Shoes with heels of area 0.001 m² each (total area = 0.002 m²) (b) Flat chappals of area 0.02 m² each (total area = 0.04 m²)
Calculate the pressure exerted in each case.
Solution: (a) With heels: (P = \frac{400}{0.002} = 200,000) Pa = 200 kPa (b) With chappals: (P = \frac{400}{0.04} = 10,000) Pa = 10 kPa
'The pressure with heels is 20 TIMES more than with flat chappals. This is why high heels can dent or damage soft floors!'
5. Pressure Exerted by Liquids
'Liquids exert pressure on the walls of their container AND on any object immersed in them.' This is because the weight of the liquid above pushes downwards, and the liquid particles collide with the walls.
Properties of Liquid Pressure
- Pressure increases with depth: 'The deeper you go into a liquid, the greater the pressure.' This is why dams are thicker at the bottom.
- Pressure acts in ALL directions: A hole in the side of a water tank will cause water to SPRING OUT horizontally.
- Pressure depends on density: A denser liquid (like mercury) exerts more pressure at the same depth than a less dense liquid (like water).
- Pressure at the same depth is EQUAL in all directions.
AP Board Context
'At Visakhapatnam port, ships are designed to withstand the immense pressure of seawater at depth. The hulls are made thicker and stronger near the bottom.' The Bay of Bengal along the AP coastline also affects submarine operations — a topic of interest in applied physics.
Worked Example 3
Problem: A water tank has a height of 5 m. Calculate the pressure at the bottom of the tank due to water. (Density of water = 1000 kg/m³, (g = 10) m/s²)
Solution: Pressure due to liquid column: (P = h\rho g)
[ P = 5 \times 1000 \times 10 = 50,000 \text{ Pa} = 50 \text{ kPa} ]
6. Pressure Exerted by Gases
Gases also exert pressure. 'Air is a mixture of gases, and its weight exerts pressure on everything on Earth — this is ATMOSPHERIC PRESSURE.'
Characteristics of Gas Pressure
- Gases fill their container COMPLETELY and exert pressure on ALL walls of the container.
- Gas pressure INCREASES when the gas is compressed (volume decreases).
- Gas pressure DECREASES when the gas expands (volume increases).
Everyday Examples
- Inflating a balloon: Air pushed inside exerts pressure on the inner walls — the balloon expands.
- Syringe: Pulling the plunger reduces pressure inside — liquid is sucked in.
- Drinking straw: Sucking reduces air pressure inside the straw. The higher atmospheric pressure outside pushes the liquid UP into your mouth.
7. Atmospheric Pressure
'The blanket of air surrounding the Earth exerts a pressure of about (1.013 \times 10^5) Pa (101.3 kPa) at sea level.' We do NOT feel this pressure because our body exerts an equal pressure from the inside.
Proofs of Atmospheric Pressure
- Magdeburg Hemispheres Experiment: Two hollow copper hemispheres are joined and the air inside is removed. Teams of horses cannot pull them apart — because the outside atmospheric pressure holds them together.
- Crushing Can Experiment: Heat water in a can, seal it, and let it cool. The steam inside condenses — internal pressure drops. The atmospheric pressure from outside CRUSHES the can.
- Suction pump: Atmospheric pressure pushes water up when we create a vacuum inside the pump.
Variation with Altitude
'As we go UP a mountain, atmospheric pressure DECREASES because the column of air above us becomes shorter.' At the top of Mount Everest, atmospheric pressure is only about 33.7 kPa — roughly one-third of sea-level pressure.
AP Board Context
The Eastern Ghats region of Andhra Pradesh, including the Araku Valley and Horsley Hills, shows noticeable changes in atmospheric pressure with altitude. Students visiting these areas may experience ear-popping sensations — a direct effect of changing air pressure.
8. Applications in Daily Life
| Application | Principle |
|---|---|
| Straw for drinking | Atmospheric pressure pushes liquid up |
| Syringe/medicine dropper | Reduced internal pressure draws liquid in |
| Vacuum cleaner | Creates low pressure — air rushes in carrying dust |
| Nail with pointed tip | Small area → high pressure → penetrates easily |
| Broad foundation of buildings | Large area → low pressure → prevents sinking |
| Hydraulic brakes (in vehicles) | Liquid pressure transmits force (Pascal's law) |
| Dams (thicker at bottom) | Must withstand higher pressure at greater depth |
9. Common Mistakes and Fixes
| Mistake | Why It's Wrong | Correction |
|---|---|---|
| "Force and pressure are the same thing" | Force causes pressure; pressure is force per unit area | Use (P = F/A) to relate them |
| "Pressure has direction" | Pressure in fluids acts in ALL directions | Pressure is a SCALAR quantity |
| "Atmospheric pressure only pushes downwards" | It pushes in ALL directions | Air particles collide from every direction |
| "Balanced forces cause motion" | Balanced forces produce ZERO net force — no change in motion | Only unbalanced forces change motion |
| "Weight and mass are the same" | Weight = mass × gravity; it changes with location | Mass stays constant; weight varies |
10. AP Exam Focus and Marks Distribution
| Topic | Likely Marks | Question Type |
|---|---|---|
| Definition and effects of force | 2 | Very short answer |
| Types of forces (contact vs non-contact) | 2 | Short answer |
| Balanced vs unbalanced forces | 2 | Short answer / Diagram |
| Pressure formula (P = F/A) and numericals | 4 | Problem solving |
| Applications of pressure in daily life | 2 | Short answer |
| Liquid pressure and its properties | 2 | Short answer |
| Atmospheric pressure and experiments | 2 | Very short answer |
| Total | 16 marks |
11. Quick Self-Test
Q1: A force of 100 N is applied on an area of 2 m². Calculate the pressure produced.
Q2: Why does a camel have broad feet while a horse has hooves?
Q3: Give ONE example each of a contact force and a non-contact force.
Q4: Why are dam walls made thicker at the bottom?
Q5: What is the SI unit of pressure?
Q6: Two teams are playing tug-of-war and neither team is winning. Are the forces balanced or unbalanced? Why?
Q7: Explain why drinking through a straw works, using the concept of atmospheric pressure.
Q8: A sharp knife cuts better than a blunt one. Which principle of pressure does this illustrate?
Answers
A1: (P = 100/2 = 50) Pa
A2: Camels have broad feet to increase the area in contact with desert sand, thereby REDUCING pressure and preventing sinking. Horses on hard ground need smaller hooves for better grip.
A3: Contact force → muscular force (pushing a cart). Non-contact force → gravitational force (apple falling from tree).
A4: Pressure in a liquid INCREASES with depth. The bottom of a dam experiences the HIGHEST pressure, so it must be built thicker to withstand it without cracking.
A5: The SI unit of pressure is the pascal (Pa). 1 Pa = 1 N/m².
A6: BALANCED forces. Neither team is winning, so the net force is ZERO — the rope does not move.
A7: When you suck on a straw, you REDUCE the air pressure inside the straw. The atmospheric pressure OUTSIDE (which is now higher) pushes the liquid UP into the straw.
A8: A sharp knife has a very SMALL cutting area. The same force applied produces VERY HIGH pressure (since (P = F/A)), allowing it to cut easily.
