Exercises
1. Choose proper word in the blank spaces.
a. The SI unit of force is …………….
(Newton, Newton Joule)
Answer:
The SI unit of force is Newton.
b. The air pressure on our body is equal to ……………. pressure.
(Atmospheric, Sea bottom, Space)
Answer:
The air pressure on our body is equal to atmospheric pressure.
c. For a given object, the buoyant force in liquids of different ……………. is ……………. .
(the same, density, different, zero)
Answer:
For a given object, the buoyant force in liquids of different density is different.
d. The SI unit of pressure is …………….
(N/m², Nm², kg m, Pa/m²)
Answer:
The SI unit of pressure is N/m².
2. Make a match.
| A group | B group | Correct Match |
| 1. Fluid | e. Same pressure in all directions | 1 – e |
| 2. Blunt knife | d. Lower pressure | 2 – d |
| 3. Sharp needle | a. Higher pressure | 3 – a |
| 4. Relative density | c. Specific gravity | 4 – c |
| 5. Hecto Pascal | b. Atmospheric Pressure | 5 – b |
Answer:
- Fluid → Same pressure in all directions
- Blunt knife → Lower pressure
- Sharp needle → Higher pressure
- Relative density → Specific gravity
- Hecto Pascal → 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 of water?
Answer:
A plastic cube will usually come to the surface and float if its density is less than the density of water.
- An object that is less dense than water floats.
- An object that is denser than water sinks.
Therefore, a normal plastic cube generally floats on the surface of water.
b. Why do the load carrying heavy vehicles have large number of wheels?
Answer:
Heavy vehicles have a large number of wheels to reduce the pressure on the road.
- The weight of the vehicle is distributed over a larger area.
- More wheels provide more contact area with the ground.
- This reduces pressure on each wheel and on the road.
- It also helps the vehicle carry heavy loads safely.
Therefore, heavy vehicles use many wheels to spread their weight and reduce pressure.
c. How much pressure do we carry on our heads? Why don’t we feel it?
Answer:
We carry atmospheric pressure on our heads and all over our bodies.
We do not feel this pressure because:
- Air pressure acts on our body from all directions.
- The fluids and air inside our body produce an outward pressure.
- This internal pressure balances the atmospheric pressure.
Therefore, we do not normally feel atmospheric pressure.
4. Why does it happen?
a. A ship dips to a larger depth in fresh water as compared to marine water.
Answer:
Marine water contains more dissolved salts than fresh water, so it is denser.
- Denser water produces a greater buoyant force.
- Therefore, a ship floats higher in marine water.
- Fresh water is less dense and provides less buoyant force.
- So, the ship has to displace more water and dips deeper.
Therefore, a ship dips deeper in fresh water than in marine water.
b. Fruits can easily be cut with a sharp knife.
Answer:
A sharp knife has a very small edge area.
- The same force acts over a smaller area.
- This produces greater pressure.
- The greater pressure helps the knife cut through the fruit easily.
Therefore, fruits can be cut easily with a sharp knife.
c. The wall of a dam is broad at its base.
Answer:
Water pressure increases with depth.
- The lower part of a dam experiences greater water pressure.
- Therefore, the base must be stronger and thicker.
- A broad base helps the dam withstand this greater pressure.
Therefore, the wall of a dam is made broad at its base.
d. If a stationary bus suddenly speeds up, passengers are thrown in the backward direction.
Answer:
This happens because of inertia of rest.
- When the bus suddenly starts moving forward, the lower part of the passenger’s body moves with the bus.
- However, the upper part of the body tends to remain at rest for a moment.
- As a result, the passenger appears to fall or move backward.
Therefore, passengers are thrown backward when a stationary bus suddenly speeds up.
5. Complete the following tables.
Table 1
| Mass (kg) | Volume (m³) | Density (kg/m³) |
| 350 | 175 | 2 |
| 760 | 190 | 4 |
Explanation:
We know:
Density = Mass ÷ Volume
For the first row:
Density = 350 ÷ 175 = 2 kg/m³
For the second row:
Mass = Density × Volume
Mass = 4 × 190 = 760 kg
Table 2
| Density of Metal (kg/m³) | Density of Water (kg/m³) | Relative Density |
| 5 × 10³ | 10³ | 5 |
| 8.5 × 10³ | 10³ | 8.5 |
Explanation:
We know:
Relative Density = Density of substance ÷ Density of water
For the first row:
Density of metal = 5 × Density of water
= 5 × 10³ kg/m³
For the second row:
Relative Density = (8.5 × 10³) ÷ 10³
= 8.5
Table 3
| Weight (N) | Area (m²) | Pressure (N/m²) |
| 800 | 0.04 | 20,000 |
| 1500 | 500 | 3 |
Explanation:
We know:
Pressure = Weight ÷ Area
For the first row:
Weight = Pressure × Area
= 20,000 × 0.04
= 800 N
For the second row:
Pressure = 1500 ÷ 500
= 3 N/m²
6. The density of a metal is 10.8 × 10³ kg/m³. Find the relative density of the metal.
Answer:
Given:
- Density of metal = 10.8 × 10³ kg/m³
- Density of water = 10³ kg/m³
Formula:
Relative Density = Density of substance ÷ Density of water
Therefore:
Relative Density = (10.8 × 10³) ÷ 10³
Relative Density = 10.8
Final Answer: 10.8
7. Volume of an object is 20 cm³ and the mass is 50 g. Density of water is 1 g/cm³. Will the object float on water or sink in water?
Answer:
Given:
- Mass of object = 50 g
- Volume of object = 20 cm³
Formula:
Density = Mass ÷ Volume
Therefore:
Density = 50 ÷ 20
Density = 2.5 g/cm³
The density of water is 1 g/cm³.
Since:
2.5 g/cm³ > 1 g/cm³
The object is denser than water.
Final 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?
Answer:
Given:
- Volume of the box = 350 cm³
- Mass of the box = 500 g
- Density of water = 1 g/cm³
First, let us find the density of the box.
Density = Mass ÷ Volume
Density = 500 ÷ 350
Density = 1.43 g/cm³ approximately
The density of water is 1 g/cm³.
Since the density of the box is greater than the density of water, the box will sink.
When the box sinks completely, it displaces water equal to its own volume.
Volume of water displaced = 350 cm³
Since the density of water is 1 g/cm³:
Mass of water displaced = 350 × 1
Mass of water displaced = 350 g
Final Answer:
- The box will sink in water.
- The mass of water displaced will be 350 g.