🌟 Class 8 Science Notes 🌟
Chapter 3: Force and Pressure

20 Most Important Words (with Hindi Meaning)

Important Notes

• Force is a push or pull which can change the position, direction, speed, or shape of an object. Its SI unit is Newton (N).
• There are two main types of forces: Contact forces (muscular, frictional) and Non-contact forces (gravitational, magnetic, electrostatic).
• Friction is the opposing force between two surfaces moving against each other.
• Pressure is the force applied per unit area: \( \text{Pressure} = \frac{\text{Force}}{\text{Area}} \)
• Its SI unit is Pascal (Pa; 1 Pa = 1 N/m²).
• On increasing area, pressure decreases if force is constant.
• Buoyant Force is the upward force exerted by a fluid; its value equals the weight of the fluid displaced (Archimedes' Principle).
• Density = \( \frac{\text{Mass}}{\text{Volume}} \); SI unit = kg/m ³.
• Relative density = \( \frac{\text{Density of substance}}{\text{Density of water}} \) (no unit).
• Atmospheric pressure at sea level is about \( 1.01 \times 10^5 \) Pa (1 atmosphere = 101,000 Pa = 1 bar).
• Balanced forces do not change the state of motion; unbalanced forces cause change.
• Inertia: tendency of an object to remain in its previous state (rest or motion).

20 One-Word Answer Type Q&A

1. SI unit of force? Newton
2. SI unit of pressure? Pascal
3. Instrument to measure density? Hydrometer
4. Instrument for milk purity? Lactometer
5. Force resisting motion? Friction
6. Non-contact pulling force by Earth? Gravity
7. Relative density has which unit? No unit
8. Force acts in which way? Vector
9. Liquid upward force? Buoyancy
10. Density formula? Density = Mass/Volume
11. Unit of area (SI)? m²
12. What’s 1 bar? \(10^5\) Pa
13. ‘Pressure’ word derived from? Press
14. Force can change what? Shape/state
15. Gas that exerts pressure? Air
16. Fluid? Any liquid or gas
17. Specific gravity synonym? Relative density
18. Upward force in fluid? Buoyant force
19. Sharp knife gives more? Pressure
20. Contact force example? Muscular force

20 Very Short Answer Type Q&A

1. What is force? A push or pull on an object.
2. Two types of forces? Contact, Non-contact
3. Example of non-contact force? Gravity
4. Unit of pressure? Pascal
5. Formula for pressure? Pressure = Force/Area
6. What is inertia? Tendency to resist change in motion.
7. What is friction? Resistance when two surfaces move over each other.
8. SI unit of density? kg/m³
9. Floating object is due to? Buoyant force
10. Example of pressure in daily life? Bag straps are wide to reduce pressure.
11. Why do camels not sink? Wide feet decrease pressure on sand.
12. Most important factor for liquid pressure? Depth
13. Why do we not feel air pressure? Balanced by body fluids’ pressure.
14. Why is dam’s base broad? To withstand higher pressure at bottom.
15. Buoyant force depends on? Volume and density of liquid.
16. Unit of force? Newton
17. What happens when force is removed on moving object? It moves with constant speed.
18. Which tool measures gas pressure? Manometer
19. Which property decides floating? Density
20. Example of inertia of motion? Fan blades moving after switch off.

20 Short Answer Type Questions (2–3 lines)

1. Explain balanced and unbalanced forces. Balanced forces are equal, opposite, and do not change an object’s state of motion. Unbalanced forces are unequal and cause movement or change in direction.
2. Why does a sharp knife cut better than a blunt knife? A sharp knife has a smaller surface area at the edge, so the same force produces more pressure, making cutting easier.
3. State Archimedes’ Principle. When an object is partially or fully immersed in a fluid, it experiences an upward buoyant force equal to the weight of the displaced fluid.
4. Why do pens sometimes leak in airplanes? Atmospheric pressure outside decreases at altitude, but inside the ink pressure is higher, causing ink to flow out.
5. How does friction help in walking? Friction between our feet and the ground prevents slipping and allows us to move forward while walking.
6. Why do we use wide tyres for heavy vehicles? Wide tyres increase surface area, reducing pressure on roads and preventing tyre damage.
7. What is meant by vector quantity? Give an example. Quantities with both magnitude and direction are vectors; e.g., force, velocity.
8. Why do we slip on banana peel? Banana peel reduces friction, making the surface slippery and causing us to slip.
9. Why do ships float but nails sink in water? Ships have large volume and displace more water, getting enough buoyant force, while nails have high density and small volume, so they sink.
10. On what basis is relative density determined? Relative density is the ratio of substance’s density to water’s density; it has no unit.
11. How is atmospheric pressure measured? It’s measured using a barometer and is about 1 atmosphere (1.01 x 10⁵ Pa) at sea level.
12. Why is pressure more at the bottom of the sea? Because pressure increases with depth; greater column of water means greater force acting downwards.
13. What is inertia of rest? Give an example. It is the tendency of an object to remain at rest unless acted upon by an external force. E.g., dusting a carpet shakes dust free.
14. Name and define SI unit of pressure. SI unit is Pascal (Pa), defined as 1 Newton per square metre.
15. How does area affect the pressure produced by a force? Pressure is inversely related to area: increasing area reduces pressure if force is constant.
16. What are contact and non-contact forces? Give one example each. Contact forces act via physical touch (e.g., friction). Non-contact forces act at a distance (e.g., gravity).
17. State two applications of Archimedes’ Principle. Used in the design of ships/submarines and devices like hydrometers and lactometers.
18. A solid object floats partially in salty water but sinks in freshwater. Why? Saltwater is denser, gives more buoyant force, thus holds the object up better than freshwater.
19. Why do mountain climbers carry oxygen cylinders? Atmospheric pressure and oxygen decrease with altitude, making it difficult to breathe.
20. What happens if area is doubled, force unchanged? Pressure will become half as compared to initial value.

All Textbook Exercise Questions and Answers

• The SI unit of force is ______ (Dyne, Newton, Joule) Newton
• The air pressure on our body is equal to ______ pressure (Atmospheric, sea bottom, space) Atmospheric pressure
• For a given object, buoyant force in liquids of different ______ is ______ (the same, density, different, area) Density, different
• The SI unit of pressure is ______ (N/m3 , N/m2, kg/m2, Pa/m2) N/m² (Pascal)
• Match the pairs:
  Group A: Fluid – e. Same pressure in all directions
  Blunt knife – d. Lower pressure
  Sharp needle – a. Higher pressure
  Relative density – c. Specific gravity
  Hectopascal – b. Atmospheric pressure
• A plastic cube is released in water. Will it sink or come to surface? It will come to the surface (density is less than water).
• Why do heavy vehicles have large number of wheels? So that load is distributed, reducing pressure and preventing tyre burst.
• How much pressure do we carry on our heads? Why don’t we feel it? About 1 atmosphere or 101,000 Pa. Our body fluids balance it, so we don't feel it.
• Why does a ship dip more in freshwater than marine water? Freshwater is less dense, so buoyant force is less; ship goes deeper.
• Why can fruits be cut easily with a sharp knife? Sharp knife has less area, creates more pressure, so cuts easily.
• Why is dam wall broad at base? Higher water pressure at the bottom, so it needs a stronger, broader base.
• Why do passengers go backward when bus starts suddenly? Due to inertia, the body resists change in motion.

• Table:

  Mass (Kg)	Volume (m3)	Density (kg/m3)
  350	175	2
  760	190	4

• Table:
  Density of Metal (kg/m³) – 5 × 10³
  Density of Water (kg/m³) – 10³
  Relative Density – 5
• Table:
  Weight (N) – 800 | Area (m²) – 0.04 | Pressure (N/m²) – 20,000

• A metal’s density is 10.8 × 10³ kg/m³. Find its relative density. Relative density = 10.8
• An object has volume 20 cm³ and mass 50g. Density of water is 1g/cm³. Will it float? It will sink (density higher than water).
• Volume of sealed box: 350cm³, mass 500g. Will it float on water? Mass of water displaced? It will sink (density higher). It will displace 350g water.

📚 End of Notes for Force & Pressure – Class 8 📚