Question Answers For All Chapters – General Science Class 8
Exercise
1. Write proper word in the blank space.
a) The SI unit of force is Newton (N).
b) The air pressure on our body is equal to atmospheric pressure.
c) For a given object, the buoyant force in liquids of different density is different.
d) The SI unit of pressure is N/m² or Pascal (Pa).
2. Match the columns.
| Column A | Column B |
|---|---|
| 1. Fluid | e. Same pressure in all directions |
| 2. Blunt knife | d. Lower pressure |
| 3. Sharp needle | a. Higher pressure |
| 4. Relative density | c. Specific gravity |
| 5. Hecto Pascal | b. Atmospheric pressure |
3. Answer the following questions in brief.
a) A plastic cube is released in water. Will it sink or come to the surface?
- If the density of the plastic cube is less than water, it will float on the surface.
- If the density is more than water, it will sink to the bottom.
b) Why do heavy vehicles have large numbers of wheels?
- Heavy vehicles have many wheels to distribute their weight over a larger surface area.
- This reduces pressure on the ground, preventing damage to roads.
c) How much pressure do we carry on our heads? Why don’t we feel it?
- We carry atmospheric pressure of about 101,000 Pascal (Pa) on our heads.
- We do not feel it because the air inside our body balances the pressure outside.
4. Why does it happen?
a) A ship dips to a larger depth in freshwater than in seawater.
- Seawater is denser than freshwater due to dissolved salts.
- This gives greater buoyant force, so the ship does not sink as deep in seawater.
b) Fruits can be easily cut with a sharp knife.
- A sharp knife has a smaller surface area, so more pressure is applied when cutting.
- A blunt knife has a larger surface area, so it applies less pressure, making cutting difficult.
c) The wall of a dam is broader at its base.
- Water pressure increases with depth, so the bottom of the dam experiences more pressure.
- A broader base provides strength to withstand this high pressure.
d) If a stationary bus suddenly speeds up, passengers are thrown backward.
- This happens due to the inertia of motion.
- The passengers want to remain at rest, but the bus moves forward, making them feel pushed backward.
5. Complete the following tables.
(A) Density Calculation Table
| Mass (kg) | Volume (m³) | Density (kg/m³) |
|---|---|---|
| 350 | 175 | 2 |
| – | 190 | 4 |
(B) Relative Density Table
| Density of Metal (kg/m³) | Density of Water (kg/m³) | Relative Density |
|---|---|---|
| 10³ | 10³ | 1 |
| 8.5 × 10³ | 10³ | 8.5 |
(C) Pressure Calculation Table
| Weight (N) | Area (m²) | Pressure (N/m²) |
|---|---|---|
| – | 0.04 | 20,000 |
| 1500 | 500 | – |
6. Calculate the relative density of a metal if its density is 10.8 × 10³ kg/m³.
- Relative Density = Density of Metal ÷ Density of Water
- = (10.8 × 10³ kg/m³) ÷ (10³ kg/m³)
- = 10.8
Answer: The relative density of the metal is 10.8.
7. Volume of an object is 20 cm³ and the mass in 50 g. Density of water is 1 gcm³. Will the object float on water or sink in water?
- Density = Mass ÷ Volume = 50 g ÷ 20 cm³ = 2.5 g/cm³
- Since 2.5 g/cm³ > 1 g/cm³ (density of water), the object will sink.
Answer: The object will sink in water.
8. The volume of a plastic covered sealed box is 350 cm³ and the box has a mass 500 g. Will the box float on water or sink in water? What will be the mass of water displaced by the box?
- Density = Mass ÷ Volume = 500 g ÷ 350 cm³ = 1.43 g/cm³
- Since 1.43 g/cm³ > 1 g/cm³ (density of water), the box will sink.
- Mass of displaced water = 350 g (same as the volume of the box).
Answer: The box will sink, and the mass of displaced water is 350 g.
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