3. Force and Pressure
Complete solutions to the Exercises. Maharashtra State Board (Balbharati), Grade Eight General Science.
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Exercises
(a) The SI unit of force is Newton.
(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.
| A group | B group |
|---|---|
| 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 |
(a) A plastic cube is released in water. Will it sink or come to the surface of water?
It will come to the surface of water. The density of plastic is less than that of water. So the buoyant force is more than its weight.
(b) Why do the load carrying heavy vehicles have large number of wheels?
Many wheels increase the area of contact with the road. So the pressure on the road is reduced. The tyres do not burst and the road is not damaged.
(c) How much pressure do we carry on our heads? Why don’t we feel it?
We carry atmospheric pressure on our heads. It is about 101 × 10³ Pa.
We do not feel it because the air and blood in our body exert equal pressure from inside.
(a) A ship dips to a larger depth in fresh water as compared to marine water.
Marine water contains salts. So its density is more than fresh water. Fresh water gives less buoyant force. So the ship dips deeper in fresh water.
(b) Fruits can easily be cut with a sharp knife.
The edge of a sharp knife has a very small area. Less area gives more pressure for the same force. So fruits are cut easily.
(c) The wall of a dam is broad at its base.
Pressure of a liquid increases with depth. So the pressure of water is highest at the bottom of the dam. Hence, the base is made broad to bear this pressure.
Water pressure on a dam increases with depth, so the wall is made broad at the base.
(d) If a stationary bus suddenly speeds up, passengers are thrown in the backward direction.
This happens due to inertia of rest. The lower part of the body moves with the bus. The upper part tends to stay at rest. So passengers are thrown backward.
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Table 1: Density = Mass ÷ Volume
| Mass (kg) | Volume (m³) | Density (kg/m³) |
|---|---|---|
| 350 | 175 | 2 |
| 760 | 190 | 4 |
Table 2: Relative density = Density of metal ÷ Density of water
| Density of metal (kg/m³) | Density of water (kg/m³) | Relative density |
|---|---|---|
| 5 × 10³ | 10³ | 5 |
| 8.5 × 10³ | 10³ | 8.5 |
Table 3: Pressure = Weight ÷ Area
| Weight (N) | Area (m²) | Pressure (N/m²) |
|---|---|---|
| 800 | 0.04 | 20,000 |
| 1500 | 500 | 3 |
Relative density = Density of metal ÷ Density of water
= (10.8 × 10³) ÷ 10³ = 10.8
The relative density of the metal is 10.8.
Density = Mass ÷ Volume = 50 ÷ 20 = 2.5 g/cm³
The density of the object is more than that of water. So the object will sink.
Density = 500 ÷ 350 = 1.43 g/cm³
Mass of water displaced = 350 cm³ × 1 g/cm³ = 350 g
The density of the box is more than that of water. So the box will sink.
The mass of water displaced is 350 g.