CBSE Important Questions Class 8 Science Chapter 5 – Exploring Forces
Class 8 Science Chapter 5 Exploring Forces is part of the new NCERT Curiosity textbook. It explains what a force is, the difference between contact and non-contact forces, and the everyday forces we meet: muscular force, friction, gravity, the buoyant force of liquids, and magnetic and electrostatic forces. Students also learn about weight, how a spring balance measures it, and why some objects float while others sink.
These CBSE important questions for Class 8 Science Chapter 5 cover the meaning of friction, assertion-reason questions on buoyancy and weight, what a spring balance actually measures, why an egg floats in salt water, why inflated arm bands help a swimmer, contact and non-contact forces, and the difference between mass and weight. Every question has a clear step-by-step answer in simple English, so students can practise on their own, parents can check the answers, and teachers can use them for class tests.
Q1. Choose the correct option. Friction is a force that ___________________.
(a) Accelerates objects towards each other
(b) Opposes the motion of an object through air
(c) Opposes the relative motion of an object when it is in contact with another surface
(d) Pulls objects towards the earth
Answer: (c) Opposes the relative motion of an object when it is in contact with another surface
Friction acts between two surfaces that are touching each other and always works against the direction of motion. Option (b) describes air resistance, which is one particular kind of friction, and option (d) describes gravity.
Q2. Read the Assertion (A) and Reason (R) and choose the correct option.
Assertion (A): A hollow object floats more easily than a solid object of the same material.
Reason (R): A hollow object displaces more water than a solid object of the same mass.
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true
Answer: (a) Both A and R are true, and R is the correct explanation of A
A hollow object takes up much more space for the same mass, so its average density is lower. Being bigger, it pushes aside more water, and the extra water displaced gives a larger upward buoyant force. That is exactly why it floats more easily, so R explains A.
Q3. Read the Assertion (A) and Reason (R) and choose the correct option.
Assertion (A): A larger object experiences greater buoyant force than a smaller object of the same material.
Reason (R): A larger object displaces more water.
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true
Answer: (a) Both A and R are true, and R is the correct explanation of A
The buoyant force on a body equals the weight of the liquid it pushes aside. A bigger object displaces a larger volume of water, so the weight of the displaced water is greater and the upward force on it is greater. So R is the correct reason for A.
Q4. Read the Assertion (A) and Reason (R) and choose the correct option.
Assertion (A): Buoyant force acts in the upward direction.
Reason (R): Water exerts pressure only from above.
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true
Answer: (c) A is true but R is false
The buoyant force does act upwards, so A is correct. But the reason is wrong: water presses on an object from all sides. Pressure increases with depth, so the upward push on the bottom surface is greater than the downward push on the top, and the difference between them is the buoyant force.
Q5. Read the Assertion (A) and Reason (R) and choose the correct option.
Assertion (A): Weight is a force acting on an object.
Reason (R): Weight is caused due to the gravitational pull of the Earth.
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true
Answer: (a) Both A and R are true, and R is the correct explanation of A
Weight is the force with which the Earth pulls an object towards itself, W = mg. Since it is produced by gravity, the Reason is exactly why weight is a force, and it is measured in newtons like any other force.
Q6. Rohit carried his school bag using a spring balance in the classroom. The balance showed a reading of 30 N. Apply the concept of weight to explain what the spring balance measures. What would happen to this reading on the Moon?
Answer: A spring balance measures weight, that is the force with which the Earth attracts the object. The gravitational force on the bag stretches the spring, and the amount of stretching is shown on the scale in newtons. So the bag weighs 30 N on Earth.
On the Moon the gravity is about one-sixth of that on Earth. The mass of the bag would stay the same, but its weight would become about 30 ÷ 6 = 5 N, so the spring balance would show a much smaller reading.
Q7. Ravi fills a bowl with water and tests different objects. A tomato floats, but when his mother adds salt and stirs, the tomato floats even higher. An egg, which normally sinks in plain water, starts to float when more salt is added. Why does this happen?
Answer: Adding salt increases the density of the water.
The tomato floats higher because the denser salt water gives a greater buoyant force, so the tomato has to push aside less liquid to balance its own weight, and more of it stays above the surface.
The egg was denser than plain water, so at first it sank. As more salt dissolves, the salt water becomes denser than the egg, so the upward buoyant force becomes equal to the weight of the egg and it floats. This is also why it is easier to float in the sea than in a swimming pool.
Q8. Neha visits a swimming pool and notices that her younger brother floats easily with inflated arm bands, while their heavy metal water bottle sinks to the bottom. Explain why.
Answer: The inflated arm bands are full of air, which has a very low density. When the boy wears them, the average density of the boy together with the bands becomes less than the density of water, so the upward buoyant force is able to support him and he floats.
The metal bottle sinks because metal is much denser than water. Its weight is greater than the buoyant force acting upwards, so the net force is downwards and it goes to the bottom of the pool.
Q9. What is a force? Write two effects that a force can produce.
Answer: A force is a push or a pull acting on an object. It is measured in newtons (N).
Effects of a force:
1. It can change the state of motion of an object: it can move a body at rest, stop a moving body, make it faster or slower, or change its direction. For example, a kick sets a football moving.
2. It can change the shape or size of an object. For example, pressing a lump of dough flattens it, and stretching a rubber band makes it longer.
Q10. What is the difference between contact forces and non-contact forces? Give two examples of each.
| Contact force | Non-contact force |
|---|---|
| Acts only when the two objects are touching each other. | Acts even when the two objects are far apart, without touching. |
| Examples: muscular force (pushing a trolley), friction (a ball slowing down on the ground). | Examples: gravitational force (a fruit falling from a tree), magnetic force (a magnet attracting an iron nail). |
Q11. Write any two advantages and two disadvantages of friction.
Answer: Advantages:
1. Friction lets us walk without slipping, because our shoes grip the ground.
2. Friction makes brakes work, so vehicles can slow down and stop.
Disadvantages:
1. Friction wears out the soles of shoes, tyres and machine parts over time.
2. Friction produces heat and wastes energy in machines, which reduces their efficiency.
Friction is reduced by using lubricants such as oil and grease, and by using ball bearings and wheels.
Q12. What is the difference between mass and weight?
| Mass | Weight |
|---|---|
| It is the amount of matter contained in a body. | It is the force with which the Earth attracts the body. |
| Its SI unit is the kilogram (kg). | Its SI unit is the newton (N). |
| It is the same everywhere, on the Earth or on the Moon. | It changes from place to place, because the value of g changes. |
| It is measured with a beam balance. | It is measured with a spring balance. |
Related Study Material on Extramarks
- CBSE Important Questions Class 8 Science Chapter 1
- CBSE Important Questions Class 8 Science Chapter 2
- CBSE Important Questions Class 8 Science Chapter 3
- CBSE Important Questions Class 8 Science Chapter 4
- CBSE Important Questions Class 8 Science Chapter 6
- CBSE Important Questions Class 8 Science Chapter 7
- CBSE Important Questions Class 8 Science Chapter 8
- CBSE Important Questions Class 8 Science Chapter 9
- CBSE Important Questions Class 8 Science Chapter 10
- CBSE Important Questions Class 8 Science Chapter 11
- CBSE Important Questions Class 8 Science Chapter 12
- CBSE Important Questions Class 8 Science Chapter 13
Q.1 Which organisation in India, is associated with the conservation of petrol and diesel?
Ans
Petroleum Conservation Research Association (PCRA) is associated with the conservation of petrol and diesel.
Q.2 What are natural resources?
Ans
Resources obtained from the nature are called natural resources. E.g., coal, petroleum, etc.
Q.3 What are the various constituents of petroleum? Give their uses.
Ans
Constituents of petroleum are petroleum gas, petrol, kerosene, diesel, lubricating oil, paraffin wax and bitumen.
| S.No. |
Constituents of petroleum |
Uses
|
| 1. | Petroleum Gas in Liquid form (LPG) | Fuel for home and industry |
| 2. | Petrol | Motor fuel, aviation fuel, solvent for dry cleaning |
| 3. | Kerosene | Fuel for stoves, lamps and for jet aircrafts |
| 4. | Diesel | Fuel for heavy motor vehicles, electric generators |
| 5. | Lubricating oil | Lubrication |
| 6. | Paraffin wax | Ointments, candles, vaseline etc. |
| 7. | Bitumen | Paints, road surfacing |
Q.4 Why petroleum is known as black gold?
Ans
Petoleum is a black oily liquid. Its fractional distillation yields a range of combustible fuels, petrochemicals and lubricants. These petrochemicals are used in the manufacture of detergents, synthetic fibres (polyester, nylon, acrylic etc.), polythene and other man-made plastics. Also, hydrogen gas obtained from natural gas, is used in the production of fertilisers such as urea.
Hence, petroleum is called ‘black gold’ due to its great commercial importance and appearance.
Q.5 Write short note on natural gas.
Ans
Natural gas is a very important fossil fuel consisting primarily of methane. It is stored under high pressure as compressed natural gas (CNG). CNG is used for power generation and also as a fuel for transport vehicles because it is less polluting. Natural gas is also used as a starting material for the manufacture of a number of chemicals and fertilisers.
It is a cleaner fuel. The greatest advantage of CNG is that it can be used directly for burning in homes and factories where it can be supplied through pipes.
Q.6 Coal is a fossil fuel. Explain.
Ans
Due to natural processes, like flooding, dead plants got buried under the soil. As more soil deposit over them, they were compressed. The temperature also rose as they sank deeper and deeper. Under high pressure and high temperature, dead plants got slowly converted to coal. As coal contains mainly carbon, the slow process of conversion of dead vegetation into coal is called carbonisation. As it was formed from the remains of vegetation, coal is also called a fossil fuel.
Q.7 What are the advantages of using CNG and LPG as fuels?
Ans
Advantages of using CNG and LPG as fuels are as follows:
1. They are less polluting and are cleaner fuels.
2. Their cost is low.
3. The great advantage of CNG is that it can be used directly for burning in homes and factories where it can be supplied through pipes.
Q.8 Give one use of each of the following:
(i) Coke
(ii) Coal tar
(iii) Coal gas
Ans
(i) Coke is used in the manufacture of steel and in the extraction of many metals.
(ii) Coal tar is used for metalling the roads.
(iii) Coal gas is used as a fuel in many industries situated near the coal processing plants.
Q.9 Write three uses of coal.
Ans
Uses of coal are as follows:
1. In thermal power plants to produce electricity
2. In the manufacture of coke, coal tar and other useful products
3. As domestic fuels for cooking purposes
Q.10 Give a brief account on
(i) Exhaustible natural resources
(ii) Inexhaustible natural resources
Ans
Exhaustible natural resources: The amount of these resources in nature is limited. They can be exhausted by human activities. Examples of these resources are forests, wildlife, minerals, coal, petroleum, natural gas etc.
Inexhaustible natural resources: These resources are present in unlimited quantity in nature and are not likely to be exhausted by human activities. E.g., sunlight, air etc.
Q.11 Write the constituents of petroleum.
Ans
The constituents of petroleum are petroleum gas, petrol, kerosene, diesel, lubricating oil, paraffin wax and bitumen.
Q.12 How is carbon black prepared?
Ans
Carbon black is prepared by burning oils and the petroleum gas in insufficient supply of air.
Q.13 Write the products obtained by processing of coal.
Ans
Coke, coal gas and coal tar are the three products obtained by processing of coal.
Q.14 Give two examples of exhaustible and inexhaustible natural resources.
Ans
Coal and petroleum are examples of exhaustible natural resources.
Sunlight and air are examples of inexhaustible natural resources.
Q.15 What are the constituents of natural gas?
Ans
Natural gas contains 95% methane and 5% of other hydrocarbons.
Q.16 What is the full form of PCRA?
Ans
PCRA is Petroleum Conservation Research Association.
Q.17 What are inexhaustible natural resources?
Ans
Inexhaustible natural resources are the resources which are present in unlimited quantity in nature and are not likely to be exhausted by human activities.For example, sunlight, air, etc.
Q.18 What are exhaustible natural resources?
Ans
Exhaustible natural resources are the resources whose amount in the nature is limited. They can be exhausted by human activities. Examples: coal, petroleum and natural gas.
Q.19 What are the fossil fuels?
Ans
The fuels which are obtained from the dead remains of living organisms are known as fossil fuels.
Q.20 How dead plants are converted into coal?
Ans
Under high pressure and high temperature, dead plants got slowly converted into coal. This process is known as carbonisation.
Q.21 Explain the term carbonisation.
Ans
The slow process of conversion of dead vegetation into coal is called carbonisation.
Q.22 What is bitumen?
Ans
Bitumen is a petroleum product. These days, bitumen is used in place of coal tar for metalling the roads.
Q.23 How can we obtain the coal gas?
Ans
Coal gas is obtained as a by-product during the processing of coal to get coke.
Q.24 Explain the formation of petroleum.
Ans
Petroleum was formed from organisms living in the sea. The remains of these organisms got settled at the bottom of the sea and subsequently got covered with sand and clay. Over millions of years, absence of air, high temperature and high pressure, transformed these dead organisms into petroleum.
Q.25 Define the term coal tar.
Ans
Coal tar is a black, thick liquid with unpleasant smell. It is a mixture of about 200 substances.
Q.26 Explain the term petroleum refining.
Ans
The process of separating the various constituents/ fractions of petroleum is known as refining. It is carried out in a petroleum refinery.
Q.27 Why should we use the fossil fuels economically and wisely?
Ans
The deposits of fossil fuels are limited. It requires millions of years for the dead organisms to get converted into these fuels. On the other hand, the known reserves of these will last atmost a few hundred years.
Moreover, burning of these fuels is a major cause of air pollution. Their use is also linked to global warming.
So, we should use the fossil fuels economically and wisely.
Q.28 Give tips to save petrol/diesel while driving.
Ans
Tips to save petrol/diesel while driving are as follows:
(i) Drive at a constant and moderate speed as far as possible.
(ii) Switch off the engine at traffic lights or at a place where we have to wait.
(iii) Ensure correct tyre pressure.
(iv) Ensure regular maintenance of the vehicle.
Q.29 Can the fossil fuels be prepared in the laboratory? If not, Why?
Ans
Fossil fuels cannot be prepared in the laboratory because their formation is very slow process. It requires very high temperature and pressure and takes millions of years. The conditions for the formation of fossil fuels cannot be created in the laboratory.
Q.30 Draw a neat and clean diagram of petroleum refinery.
Ans
Diagram is as follows:
Q.31 How petroleum is mined from their deposits?
Ans
Petroleum is brought to the surface by drilling a hole in the Earth’s crust and sinking pipes deep down through the impervious cap rock. Natural gas first comes out under high pressure. Then, petroleum is pumped out, collected in tanks and transported for further processing.
Q.32 (i) What is carbonisation?
(ii) Name three products obtained after the processing of coal. Also mention their uses.
Ans
(i)The slow process of conversion of wood into coal is called carbonisation.
(ii)The three products obtained by processing of coal are:
1) Coke: It is a pure form of carbon, which is tough, porous and black in colour. It is used in the manufacture of steel and extraction of many metals.
2) Coal gas: It is used for street lighting.
3) Coal tar: It is a black thick liquid with unpleasant smell. Coal tar is a mixture of several substances.
It is used for manufacturing dyes, drugs, perfumes, naphthalene balls etc.
Q.33 Define refining of petroleum. Where it is carried out?
Ans
The process of separating the various fractions by fractional distillation is known as refining of petroleum. It is carried out in a petroleum refinery.
Q.34 Write four uses of petrochemicals.
Ans
1) Petrochemicals are used in the manufacture of detergents.
2) They are used in the manufacture of fibres (polyester, nylon and acrylic).
3) They are used in the manufacture of polythene.
4) They are used in the manufacture of man-made plastics.
Q.35 Write two advantages of using CNG as a fuel.
Ans
1) It can be supplied by pipes and used directly at home and factories.
2) It is used as a fuel for transport vehicles because it is less polluting.
Q.36 (i) What are petrochemicals?(ii) Why petroleum is called black gold?
Ans
(i)The useful substances that are obtained from petroleum and natural gas are called petrochemicals.
(ii) Due to its great commercial importance, petroleum is called black gold.
Q.37 Name the places in India where reserves of natural gas are found.
Ans
Natural gas is found in Tripura, Rajasthan, Maharashtra and Krishna Godavari delta.
Q.38 Where do we find reserves of petroleum and natural gas – above water or below water? Why?
Ans
Petroleum and natural gas reserves are found above water because petroleum and natural gas are lighter than water.
Q.39 Write two advantages of using CNG as a fuel.
Ans
Following are the advantages of using CNG as a fuel:
1) It can be supplied by pipes and used directly at home and factories.
2) It is used as a fuel for transport vehicles because it is less polluting.
Q.40 Write four uses of petrochemicals.
Ans
Uses of petrochemicals are as follows:
1) They are used in the manufacture of detergents.
2) They are used for making synthetic fibres (polyester, nylon and acrylic).
3) They are used in the manufacture of polythene.
4) They are used in making plastics.
Q.41 Define refining of petroleum. Where is it carried out?
Ans
The process of separating the useful fractions from petroleum by fractional distillation is known as refining. It is carried out in a petroleum refinery.
Q.42 What is carbonisation? Name three products obtained after the processing of coal. Also mention their uses.
Ans
The slow process of conversion of wood into coal is called carbonisation.
The three products obtained by processing of coal are:
1) Coke: It is a pure form of carbon, which is tough, porous and black in colour. It is used in the manufacture of steel and extraction of many metals.
2) Coal gas: It is used for street lighting.
3) Coal tar: It is a black thick liquid with unpleasant smell. It is a mixture of several substances. It is used for manufacturing dyes, drugs, perfumes, naphthalene balls etc.
Q.43 (i)What are petrochemicals?
(ii) Why petroleum is called black gold?
Ans
(i)The useful substances that are obtained from petroleum and natural gas are called petrochemicals.
(ii) Due to great commercial importance of petroleum, it is called black gold.
Q.44 Name the places in India where reserves of natural gas are found.
Ans
Natural gas is found in Tripura, Rajasthan, Maharashtra and Krishna Godavari delta.
Q.45 Where do we find reserves of petroleum and natural gas – above water or below water? Why?
Ans
Petroleum and natural gas reserves are found above water because petroleum and natural gas are lighter than water.