3. Force and Pressure Class 8 Science Solutions | Maharashtra State Board
3. Force and Pressure
Class 8 Science – Maharashtra State Board Solutions
Force and Pressure is an important chapter of Class 8 Science. In this chapter, students will learn about force, pressure, atmospheric pressure, buoyant force, density, relative density and the effects of pressure in daily life.
This page provides complete, easy-to-understand and exam-ready solutions for the Maharashtra State Board Class 8 Science chapter “Force and Pressure”.
⭐ Useful for students' revision, homework and exam preparation.
1. Write the proper word in the blank space.
a. The SI unit of force is ...............
(Dyne, Newton, Joule)
Answer: Newton
b. The air pressure on our body is equal to ........... pressure.
(Atmospheric, Sea bottom, Space)
Answer: Atmospheric
c. For a given object, the buoyan force in liquids of different .... is ............... .
(the same, density, different, area)
Answer: density, different
d. The SI unit of pressure is ...............
(N/m3, N/m2, kg/m2, Pa/m2)
Answer: N/m2 (Pascal)
2. Make a Match
| Group A | Group B | Answer |
|---|---|---|
| 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 |
Final Answers:
1 → e
2 → d
3 → a
4 → c
5 → b
Q.3 Answer the following questions in brief.
1. A plastic cube is released in water. Will it sink or come to the surface of water?
Answer:
The plastic cube will float on the surface of water because its density is less than the density of water.
2. Why do the load-carrying heavy vehicles have a large number of wheels?
Answer:
We know,
Pressure = Force / Area
A large number of wheels increases the area of contact with the road and hence reduces the pressure on the road. The load is also distributed among all the tyres.
3. How much pressure do we carry on our heads? Why don't we feel it?
Answer:
We carry atmospheric pressure of about 105 Pa on our heads. We do not feel it because the atmospheric pressure is balanced by the pressure of air and blood inside our body.
Q.4 Why does it happen?
1. A ship dips to a larger depth in fresh water as compared to marine water.
Answer:
Marine water is denser than fresh water. Therefore, the buoyant force in marine water is greater. Hence, a ship dips deeper in fresh water than in marine water.
2. Fruits can easily be cut with a sharp knife.
Answer:
A sharp knife has a smaller area of contact. Therefore, it exerts greater pressure on the fruit. Hence, fruits can easily be cut with a sharp knife.
3. The wall of a dam is broad at its base.
Answer:
The pressure exerted by a liquid increases with depth. Therefore, the bottom of a dam experiences greater pressure. Hence, the wall of a dam is made broader at its base to withstand the greater pressure.
4. If a stationary bus suddenly speeds up, passengers are thrown in the backward direction.
Answer:
Initially, the passengers are at rest. When the bus suddenly moves forward, their lower body moves with the bus, but the upper body tends to remain at rest due to inertia of rest. Hence, passengers are thrown backward.
⭐ Quick Revision
1. Plastic cube → Floats because its density is less than water.
2. More wheels → Less pressure on the road.
3. Atmospheric pressure → About 105 Pa.
4. Marine water → Denser → Greater buoyant force.
5. Sharp knife → Small area → Greater pressure.
6. Dam → Broad at base because liquid pressure increases with depth.
7. Bus suddenly moves → Passengers move backward due to inertia of rest.
Complete the Following Tables
Formulae:
Density = Mass / Volume
Mass = Density × Volume
Relative Density = Density of metal / Density of water
Pressure = Weight / Area
1. Complete the following table.
| Mass (kg) | Volume (m3) | Density (kg/m3) |
|---|---|---|
| 350 | 175 | — |
| — | 190 | 4 |
Answer:
| Mass (kg) | Volume (m3) | Density (kg/m3) |
|---|---|---|
| 350 | 175 | 2 |
| 760 | 190 | 4 |
Density = Mass / Volume
= 350 / 175
= 2 kg/m3
Mass = Density × Volume
= 4 × 190
= 760 kg
2. Complete the following table.
| Density of Metal (kg/m3) |
Density of Water (kg/m3) |
Relative Density |
|---|---|---|
| — | 103 | 5 |
| 8.5 × 103 | 103 | — |
Answer:
| Density of Metal (kg/m3) |
Density of Water (kg/m3) |
Relative Density |
|---|---|---|
| 5 × 103 | 103 | 5 |
| 8.5 × 103 | 103 | 8.5 |
Density of metal = Relative density × Density of water
= 5 × 103
= 5 × 103 kg/m3
Relative density = (8.5 × 103) / 103
= 8.5
3. Complete the following table.
| Weight (N) | Area (m2) | Pressure (N/m2) |
|---|---|---|
| — | 0.04 | 20,000 |
| 1500 | 500 | — |
Answer:
| Weight (N) | Area (m2) | Pressure (N/m2) |
|---|---|---|
| 800 | 0.04 | 20,000 |
| 1500 | 500 | 3 |
Weight = Pressure × Area
= 20,000 × 0.04
= 800 N
Pressure = Weight / Area
= 1500 / 500
= 3 N/m2
Q.6 Find the Relative Density
Question:
The density of a metal is 10.8 × 103 kg/m3. Find the relative density of the metal.
Solution:
We know,
Relative Density = Density of Metal / Density of Water
Density of metal = 10.8 × 103 kg/m3
Density of water = 1 × 103 kg/m3
Therefore,
Relative Density = (10.8 × 103) / (1 × 103)
= 10.8
Answer: Relative density of the metal = 10.8
Q.7 Float or Sink?
Question:
Volume of an object is 20 cm3 and its mass is 50 g. Density of water is 1 g/cm3. Will the object float on water or sink in water?
Solution:
We know,
Density = Mass / Volume
Mass of object = 50 g
Volume of object = 20 cm3
Therefore,
Density of object = 50 / 20
= 2.5 g/cm3
Density of water = 1 g/cm3
Since,
Density of object > Density of water
Therefore, the object will sink in water.
Answer: The object will sink in water.
Q.8 Float or Sink and Mass of Water Displaced
Question:
The volume of a plastic-covered sealed box is 350 cm3 and the box has a mass of 500 g. Will the box float on water or sink in water? What will be the mass of water displaced by the box?
Solution:
Step 1: Find the density of the box.
We know,
Density = Mass / Volume
Mass of box = 500 g
Volume of box = 350 cm3
Therefore,
Density of box = 500 / 350
= 1.43 g/cm3 (approximately)
Density of water = 1 g/cm3
Since,
Density of box > Density of water
Therefore, the box will sink in water.
Step 2: Find the mass of water displaced.
Since the box sinks, it is completely immersed in water.
Volume of water displaced = Volume of box
= 350 cm3
We know,
Mass = Density × Volume
Mass of water displaced = 1 × 350
= 350 g
Answer: The box will sink in water and the mass of water displaced by it is 350 g.
Quick Revision
Q.6 → Relative Density = 10.8
Q.7 → Density of object = 2.5 g/cm3 → Sink
Q.8 → Density of box = 1.43 g/cm3 → Sink
Mass of water displaced = 350 g
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