Force and Pressure - Standard 8 - General Science - Balbharati
3. Force and Pressure
Exercise
Q. 1 Write proper word in the blank space.
(a) The SI unit of force is Newton. (Dyne, Newton, Joule)
(b) The air pressure on our body is equal to Atmospheric pressure. (Atmospheric, Sea bottom, Space)
(c) For a given object, the buoyant force in liquids of different density is different. (the same, density, different, area)
(d) The SI unit of pressure is N/m2. (N/m3, N/m2, kg/m2, Pa/m2)
Q. 2 Make a match.
| A Group | B Group |
|---|---|
| 1. Fluid | e. Same pressure in all directions |
| 2. Blunt knife | a. Higher pressure |
| 3. Sharp needle | d. Lower pressure |
| 4. Relative density | c. Specific gravity |
| 5. Hecto Pascal | b. Atmospheric pressure |
Q. 3 Answer the following questions in brief.
(a) A plastic cube is released in water. Will it sink or come to the surface of water?
Ans. If the density of the plastic cube is less than the density of water (1 g/cm³ or 1000 kg/m³), it will come to the surface (float). If its density is greater than water it will sink.
(b) Why do the load carrying heavy vehicles have large number of wheels?
Ans. More wheels increase the total area of contact with the road. For the same load (force), a larger contact area reduces the pressure on the road. This reduces damage to the road surface and spreads the load safely.
(c) How much pressure do we carry on our heads? Why don’t we feel it?
Ans. We are under about one atmosphere of pressure (~101 × 10³ Pa) from the air above us. We do not feel this pressure because the air inside our body cavities (lungs, sinuses, blood vessels) exerts an equal internal pressure — the forces balance, so there is no net crushing effect.
Q. 4 Why does it happen?
(a) A ship dips to a larger depth in fresh water as compared to marine water. Why?
Ans. Sea (marine) water is denser because of dissolved salts. Higher density gives a larger buoyant force for the same displaced volume, so the ship floats higher in sea water. In fresh water buoyant force is smaller, so the ship must displace more water and dips to a larger depth.
(b) Fruits can easily be cut with a sharp knife. Why?
Ans. A sharp knife has a very small contact area at the edge; for the same applied force the pressure is much greater, so the knife cuts easily. A blunt knife has larger contact area and produces less pressure, making cutting hard.
(c) The wall of a dam is broad at its base. Why?
Ans. Water pressure increases with depth. The broad base provides enough strength and area to resist the large pressure acting at lower depths, preventing collapse or cracking.
(d) If a stationary bus suddenly speeds up, passengers are thrown in the backward direction. Why?
Ans. Due to inertia of rest, the passengers tend to remain in their original state (at rest). When the bus moves forward suddenly the lower part (bus) moves forward first while the passengers’ bodies lag, appearing to be thrown backwards relative to the bus.
Q. 5 Complete the following tables.
(A) Density Table
| Mass (kg) | Volume (m³) | Density (kg/m³) |
|---|---|---|
| 350 | 175 | 2 |
| 760 | 190 | 4 |
Solution 1:
Given: mass = 350 kg, volume = 175 m³
density = mass ÷ volume
density = 350 ÷ 175 = 2
Answer: 2 kg/m³
Solution 2:
Given: mass = 760 kg, volume = 190 m³
density = mass ÷ volume
density = 760 ÷ 190 = 4
Answer: 4 kg/m³
(B) Relative Density Table
| Density of Metal (kg/m³) | Density of Water (kg/m³) | Relative Density |
|---|---|---|
| 103 | 5 | 20.6 |
| 8.5 × 10³ | 10³ | 8.5 |
Solution 1:
Given: density of metal = 103 kg/m³, density of water = 5 kg/m³
relative density = density of metal ÷ density of water
relative density = 103 ÷ 5 = 20.6
Answer: 20.6
Solution 2:
Given: density of metal = 8.5 × 10³ kg/m³, density of water = 10³ kg/m³
relative density = (8.5 × 10³) ÷ (10³) = 8.5
Answer: 8.5
(C) Pressure Table
| Weight (N) | Area (m²) | Pressure (N/m²) |
|---|---|---|
| 800 | 0.04 | 20,000 |
| 1500 | 500 | 3 |
Solution 1:
Given: area = 0.04 m², pressure = 20,000 N/m²
force = pressure × area
force = 20,000 × 0.04 = 800 N
Answer: 800 N
Solution 2:
Given: weight = 1500 N, area = 500 m²
pressure = weight ÷ area
pressure = 1500 ÷ 500 = 3 N/m²
Answer: 3 N/m²
Q. 6 The density of a metal is 10.8 × 10³ kg/m³. Find the relative density of the metal.
Ans.
Given: density of metal = 10.8 × 10³ kg/m³, density of water = 10³ kg/m³ (standard)
relative density = density of metal ÷ density of water
relative density = (10.8 × 10³) ÷ (10³) = 10.8
Answer: 10.8
Q. 7 Volume of an object is 20 cm³ and the mass is 50 g. Density of water = 1 g/cm³. Will the object float on water or sink in water?
Ans.
Given: mass = 50 g, volume = 20 cm³, density of water = 1 g/cm³
density = mass ÷ volume
density = 50 ÷ 20 = 2.5 g/cm³
2.5 g/cm³ > 1 g/cm³
The object is denser than water, so it will sink
Q. 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?
Ans.
Given: mass = 500 g, volume = 350 cm³, density of water = 1 g/cm³
density = mass ÷ volume = 500 ÷ 350 ≈ 1.4286 g/cm³
1.4286 g/cm³ > 1 g/cm³
So the box will sink
Mass of displaced water (when fully submerged) = volume × density of water = 350 × 1 = 350 g
