Chapter Overview & Weightage
Force and Pressure is a foundational Class 8 Science chapter that introduces concepts directly used in Class 9 (Newton’s Laws) and Class 11 (Physics). The chapter defines force, types of forces, pressure, atmospheric pressure, and practical applications.
In CBSE Class 8 annual exams, Force and Pressure typically carries 8–10 marks. Common questions: define force and give examples (2 marks), calculate pressure (2 marks), explain atmospheric pressure and its effects (3 marks), difference between contact and non-contact forces (2 marks). Numerical problems on pressure are a reliable scoring opportunity.
Key Concepts You Must Know
1. Force: A push or pull that can change the state of motion, shape, or direction of an object.
Effects of force:
- Move a stationary object
- Stop a moving object
- Change the speed of a moving object
- Change the direction of motion
- Change the shape of an object
2. Types of Forces:
Contact forces (require physical contact):
- Muscular force (a person pushing a cart)
- Friction force (brakes slowing a bicycle)
- Spring force (compressed spring pushing back)
- Normal force (floor supporting weight)
Non-contact forces (act at a distance, without touching):
- Gravitational force (Earth pulling objects down)
- Magnetic force (magnet attracting iron)
- Electrostatic force (charged comb attracting paper bits)
3. Resultant Force: When multiple forces act on an object, they combine into a single resultant force. If forces act in the same direction, they add. If in opposite directions, the smaller is subtracted from the larger.
4. Pressure:
Unit: Pascal (Pa) = N/m². Also N/cm² in Class 8 problems.
Key insight: Same force, smaller area → higher pressure. This is why:
- Sharp knife cuts better than blunt knife
- Camel’s wide feet sink less in sand
- Nails are pointed at the tip
5. Atmospheric Pressure: The Earth’s atmosphere exerts pressure on all surfaces due to the weight of the air above. At sea level, atmospheric pressure ≈ 101,325 Pa (approximately 10⁵ Pa).
Why atmospheric pressure decreases with altitude: Less air above means less weight pushing down.
Evidence of atmospheric pressure:
- Sucking liquid through a straw (you lower air pressure inside; atmospheric pressure outside pushes liquid up)
- Suction cups sticking to walls
- Collapsed bottle when cooled (air inside contracts, outside atmospheric pressure crushes the bottle)
6. Pressure in Liquids:
- Increases with depth ( — though the formula is introduced later, Class 8 introduces the concept)
- Acts in all directions at a given depth
- Why dams are thicker at the base — more water pressure at greater depth
Important Formulas
= Pressure (Pa or N/m²) = Force (N) = Area (m² or cm²)
To increase pressure: increase force OR decrease area To decrease pressure: decrease force OR increase area
Solved Previous Year Questions
PYQ 1 — Numerical (3 marks)
A force of 200 N acts on a surface of area 0.04 m². Calculate the pressure.
Solution:
Pressure = 5000 Pa = 5 kPa
PYQ 2 — Why are nails pointed, not flat? (2 marks)
Answer: Nails have pointed tips so they can be driven into wood easily. The pointed tip has a very small surface area. When you strike the nail head with a hammer (applying force), the same force is concentrated over the tiny pointed area, creating very high pressure at the tip. This high pressure easily pierces the wood.
If the tip were flat (large area), the same force would create much less pressure — insufficient to pierce the wood.
PYQ 3 — Why does a sharp knife cut better than a blunt knife? (2 marks)
Answer: A sharp knife has an extremely thin cutting edge (very small area of contact). When you press a sharp knife onto a material, the entire force is concentrated on this tiny area, creating very high pressure. This high pressure easily cuts through the material.
A blunt knife has a wider edge (larger area). The same pressing force is spread over more area, creating less pressure — insufficient to cut effectively.
PYQ 4 — How does a straw work? (3 marks)
Answer: When you suck through a straw:
- You lower the air pressure inside the straw and your mouth by sucking (removing some air)
- The atmospheric pressure outside (acting on the surface of the liquid in the glass) is now higher than the pressure inside the straw
- This pressure difference pushes the liquid up the straw and into your mouth
You don’t “pull” the liquid up — atmospheric pressure pushes it up. This only works when there’s atmospheric pressure outside to push. In a vacuum (like outer space), a straw would not work.
Difficulty Distribution
| Difficulty | Topics | Approximate % |
|---|---|---|
| Easy | Types of forces with examples, effects of force | 35% |
| Medium | Pressure formula and numericals, sharp vs blunt tools | 40% |
| Hard | Atmospheric pressure mechanisms, combining forces | 25% |
Expert Strategy
Connect pressure to everyday objects. For every pressure example, link it to the formula: small area = high pressure (knife, nail, needle), large area = low pressure (camel’s feet, army boots on snow, snowshoes).
For numericals: Always write the formula first, then substitute with units, then solve. Show all steps — CBSE gives marks for each step.
Questions about “why is X designed this way” (broad tyres on tractors, pointed heels, wide straps on heavy bags) are all answered with the same logic: “to increase/decrease pressure by increasing/decreasing the contact area.” Identify whether you want high or low pressure, then explain which way area is changed.
Common Traps
Trap 1: Confusing force and pressure. They are different quantities. Force is measured in Newtons; pressure in Pascals (N/m²). You can have a large force creating low pressure (if area is large) or a small force creating high pressure (if area is tiny). Always ask: what is the area over which force acts?
Trap 2: Saying atmospheric pressure acts only downward. Atmospheric pressure acts in all directions — upward, downward, and sideways. This is why inflated tyres don’t collapse on one side, and why suction cups work (atmospheric pressure pushes the cup against the wall from all sides).
Trap 3: Categorising gravity as a contact force. Gravity is a non-contact force — it acts between masses without them touching. Earth pulls you down without physically touching your body (at the fundamental level). Similarly, magnetic force and electrostatic force are non-contact.