Solution of Chapter Force and Pressure Class 8 Living Science
A. Multiple choices
1. (a) Gravitational force
3.(d)at the same depth, pressure is the same in all directions.
5. (b) A B acting in the direction of A
6. (d) Mass
7. (a) Gravitational force
8. (d) The pressure doubles. Pressure (P) = Force (F)/Area (A) = 10 N / 0.1 m2 = 100 N/m2 = 100 Pa
Now, according to the question,
New area, A' = A/2 and F' = F ∴ P' = F'/A' = 2F/A = 2P = 2 × 100 = 200 Pa
Thus, the pressure gets doubled.
B. V. Short types:
1. True. [Push or pull acting on a body are called force. The direction in which an object is pushed or pulled is called the direction of the force.]
2. False. [When we apply the brakes to slow down a bicycle it will bicycle will move forward. Thus, the bicycle will move in the opposite direction of the applied force.]
3. The net force acting on the object is A + B. [If two forces act on a body in the same direction, then the net force acting on object will be sum of the two forces.]
4. The net force acting on the object is A B.[If two forces are applied on an object in opposite directions, then the net force acting on the object is the difference between the two forces. ]
5. No. The net force of all forces determines the acceleration of the body
6. False. [Every object in the universe exerts gravitational force on other matter.]
7. Weight [An object’s weight is a measure of the Earth's gravitational pull acting on it. Weight = mg]
8. Magnetic force 9. Electrostatic force 10. Friction force
11. The larger the area over which a force acts, the lesser the pressure is.
12. Pressure exerted by a liquid increases with depth and acts in all directions.
13. True. [Due to the atmospheric pressure of air, we experience a force equal to that exerted by a mass of 1 kg on every square centimetre of our body.]
14. False. Atmospheric pressure is caused by the weight of air molecules above the atmosphere. As we go to higher altitudes, the air becomes thinner and the atmospheric pressure decreases
C. Short type (i)
1. Force can be described by two things: its magnitude and the direction in which it acts. Force is a vector quantity.
2. If several forces act on a body in different directions, the effect on the body will be due to the magnitude and direction of the net force acting on it. So, the body will move in the direction of the resultant force
3. Gravitational force is the force with which the Earth pulls everything towards itself. So, if an object is thrown up, it slows down and then comes down because of the gravitational force of the Earth.
The mass of a body is the quantity of matter contained in it
It is a scalar quantity that has only magnitude, no direction
The SI unit of mass is the kilogram (kg).
The weight of a body is the force with which it is attracted towards the centre of the Earth
Weight is a vector quantity having magnitude as well as direction
The SI unit of weight is the Newton (N).
5. Every magnetic substance is surrounded by its own invisible magnetic field, depending on its pole strength. A magnet can act from a distance because of the field of magnetic force.
6. Pressure is the trust acting on a unit area of an object. Pressure = Thrust/Area
Pressure depends on two factors: (i) Force applied (ii) Area over which the force acts
7. Atmospheric pressure is the force per unit area exerted on the surface of the Earth by the atmosphere. Atmospheric pressure is caused by the weight of air molecules in the atmosphere.
8. It is difficult to cut vegetables with a blunt knife because it has more area compared to a sharp knife, therefore the pressure exerted by the blunt knife is less than that of the sharp knife.
D. Short type (ii)
1. Force: Force is an agent that change or try to change the state of an object. The SI unit of force is kg m/s² or Newton (N).
Effects of force: 1. Change the state of rest and motion. 2. It can change the speed of a moving object. 3. It can change the direction of a moving object. 4. It can change the shape or size of an object.
2. Contact Force: The force which acts by touching objects is called Contact Force. For example muscular force, friction force
Non Contact Force: The force which act without touching objects is called non contact Force .. For example: Gravitational force, Electrostatic force
3. Friction is a kind force that always opposes motion of body. For example, when you stop pedalling a bicycle, it slows down and stops after some time because of the frictional force.
4. Caterpillar tracks are used in battle tanks instead because it distributes force over larger area and reduce the pressure over surface. This prevents tracks from sinking in areas where wheeled vehicles of the same weight can easily sink.
5 The pressure exerted by the liquid is given by the following expression:
P = ρgh
Here, ρ = Density of the liquid g = Acceleration due to gravity h = Height/depth of the liquid column
Here, pressure is directly proportional to the height of the liquid column. If we go down into the liquid, the depth (h) of the liquid column increases. Thus, the pressure in the liquid increases with increasing depth.
6. When we press a rubber sucker on a smooth surface, we remove some of the air between the sucker and the smooth surface. This deficiency of air lets the air outside to exert pressure on the surface of the sucker. Force due to the pressure keeps the sucker stuck to the surface.
1. False. If no external force is applied on the body, then there must be two equal and opposite forces acting on the body. Both these forces add up to zero.
2. False. Contact force is the force in which a direct contact is required between two bodies. The contact may also be with the help of a stick or a piece of rope. If we use a stick to push an object, we make the contact with the help of stick. This push or force is caused by the action of the muscles in our body. This muscular force is a contact force.
3. The magnitude of gravitational force depends on the masses of two objects and the distance between them.
Gravitational force generally felt between objects of greater masses. Since mass of building and our body smaller as compared to the earth, we are not pulled towards the building.
4. It is necessary to define pressure as a separate quantity to find the impact of force on a surface.
As we know that the impact of force depends on both the amount of force applied and the area over which it is applied. So, only force is not enough for this purpose.
5. If one of the windows in a jet plane breaks due to an accident, the air pressure inside plane decreases. At such low pressure, we can have problems like nose bleeding, because the pressure exerted by the blood in our body will be much higher than the pressure outside. This can cause blood vessels to burst.
6. If we press a rubber sucker on a rough surface, it will not stick to the surface because an airtight seal cannot be achieved between the surfaces. It is so because air rushes into the space between the rubber sucker and the rough surface.
FOR FORMATIVE AND SUMMATIVE ASSESSMENT
A. MULTIPLE-CHOICE QUESTIONS: Choose the most appropriate answer.
Which of these is not a cereal?
a. maize b. gram c. wheat d. rice
Which of these is not a kharif crop?
a. wheat b. rice c. maize d. groundnut
Which of these nutrients is compost rich in?
a. organic nutrients b. nitrogen c. phosphorus d. potassium
Which of these is not a fungal disease of plants?
a. wilt b. rust c. smut d. blight (of potatoes)
Which one of the following is not true about ploughing?
a. It loosens the soil. b. It aerates the soil. c. It prevents soil erosion. d. It allows easy penetration of roots into the soil.
Compost lacks which of the following nutrients?
a. nitrogen b. phosphorous c. potassium d. all of these
7. Which one of the following does not add nutrients to the soil?
a. crop rotation b. field fallow c. manures d. ploughing
8. Which of the following machines can be used to harvest a crop and also to beat out the grain from the chaff? a. harvester b. combine c. thresher d. harrow
B. VERY SHORT-ANSWER QUESTIONS: Give one-word answers.
1. Name three natural methods of adding nutrients to soil.
2. What are weeds?
3. Why should grains be dried before storage?
4. Why does the government usually maintain a buffer stock of grains?
5. What is hybridization?
6. List the steps involved in the nitrogen cycle.
7. What do you mean by nitrogen fixation? S. What is nitrogen assimilation?
9. What enables leguminous plants to fix nitrogen?
10. What is animal husbandry?
D. SHORT-ANSWER QUESTIONS (TYPE II): Answer in about 30 words.
1. Distinguish between kharif and rabi crops, giving two examples of each.
2. List the steps involved in crop production, in sequential order.
3. Why is it important to plant seeds at the correct distances from each other?
4. Why do farmers normally use a mixture of manures and fertilizers in their fields?
5. What method of irrigation will you use if you live in a dry area with shortage of water? Explain the method.
6. What is water logging? How does it harm the crops? How can it be prevented?
7. Discuss two methods of weeding in which poisonous chemicals are not used.
8. Differentiate between insecticides, rodenticides and fungicides. Which of these are pesticides?
9. Discuss with an example how hybridization is used for developing new varieties of crops with desired characteristics.
10. Differentiate between nitrification and denitrification.
LONG ANSWER QUESTIONS: Answer in about 60 words.
1. What are the advantages of ploughing the soil before sowing seeds?
2. What precautions must be kept in mind while sowing seeds in a field?
3. Discuss two natural methods of manuring.
4. Draw a labelled diagram to illustrate the nitrogen cycle.
HOTS QUESTIONS: Think and answer.
1. Which method, according to you, can bring about the maximum increase in crop produce in our country in future?
a. increasing land under cultivation c. less wastage in storage
b. using more manures and fertilizers d. using better varieties of crop plants
2. What do you think are the drawbacks of seeds being unevenly distributed in a field while sowing?
3. Do you expect to find fully-grown crop plants in a nursery? Why? Why not?
4. Crop rotation is practised by farmers so that they have different products to sell in the market in every season. Do you agree? Give reasons.
5. I have grown a few wheat plants in my garden because I like the plants. Am I growing a wheat crop? Give reasons.
6. It is observed that melon fruits growing in the fields sometimes crack, exposing the contents inside. What do you think can be the reason? (HINT: This generally happens in fields where drainage is not proper.)
8th Crop Production _Living Science Solution
A. Multiple choice questions:
1. (a) Gram 2.(a) wheat 3.(a) organic nutrients 4.(a) wilt 5. (c) it prevent soil erosion 6. (d) All of these 7. (d) Ploughing 8. (b) Combine.
B. Very short type Questions
1. Ten thousands yrs ago
3.Loosing and turning
5.Fielf fallow, nutrients
6. False. All crops require different amounts of water. For example, a paddy field needs more water compared to a wheat field.
7. BHC or Malathion
8.False Cause irritation of skin and respiratory problems, So fruits and vegetable washed before use.
9. Rhizobium 10.Ammonification
11.True they are called denitrifying bacteria
C. Short type Questions - I
1. The three natural methods of adding nutrients to soil are: Field fallow, Crop rotation and Mixed cropping
2. The unwanted plants that grow along with the crops and share nutrients are called weeds. Weeds decrease the crop yield as they compete with crops for water, nutrients and sunlight and limit their growth.
3. Grains should be dried before storage to remove moisture. This excess moisture helps growth of microbes that may destroy the grains.
4. The government usually maintains a buffer stock of grains to avoid shortage of grains during natural disaster.
5. Hybridisation is a technique to develop new varieties of crops by crossbreeding two different varieties.
6. The steps involved in the nitrogen cycle are as follows:
Nitrogen fixation ® Nitrogen assimilation ® Ammonification ® Nitrification ® Denitrification
7. Nitrogen fixation is the process of converting free atmospheric nitrogen gas into nitrogen compounds nitrites and nitrates .
8. Nitrogen assimilation is the process by which inorganic nitrogen compounds are converted into organic compounds such as proteins that become a part of living beings.
9. The nitrogen fixing bacterium Rhizobium ive in the root nodules of leguminous plants enables them to fix nitrogen.
10. Animal husbandry is defined as the rearing of domestic animals to obtain food and other products from them.
D. Short type Questions- II
They are sown during June or July
They are sown from October to December
They are harvested in September or October
They are harvested in March or April
Examples: rice, maize
Examples: wheat, pea
2. The steps involved in crop production are as follows: Preparation of soil, selection of seed and sowing Manuring, irrigation and weeding, Protection from pests and diseases, Harvesting, threshing and winnowing and Storage
3. Seeds should be planted at a proper distance from each other so that they cannot compete for water, nutrients and sunlight.
4. Farmers normally use a mixture of manure and fertilizers in their fields to increase the yield and production of crops. Manure is good for plants as it provides them with organic nutrients but it lacks some important nutrients like nitrogen, phosphorous and potassium. So, along with manure, farmers also use fertilizers such as NPK fertilizers that provide nitrogen, phosphorous and potassium.
5. Drip irrigation is the method of irrigation useful for dry areas which have a shortage of water. In this method, water is. Water is allowed to fall drop by drop from a pipe placed near the roots of plant.
6. When an excess of water is supplied to the field it is called water logging. As a result of this, the air supply to the roots reduces and in extreme cases, gets totally cut off. This results in the retarded growth of plants. Water logging can be prevented by supplying just the adequate amount of water to the field using a proper drainage system.
7. The two methods of weeding in which chemicals are not used are:
Manual method: Weeds are pulled out by hand or uprooted using a trowel or a harrow.
Introducing Insect: Insects that feed upon only specific weeds are used control weeds.
8. Insecticides are chemicals used to kill insects like locusts. Rodenticides are chemicals used to kill rodents like rats and Fungicides are chemicals used to destroy fungi.
Insects, rodents and fungi are pests. So insecticides, rodenticides and fungicides are all pesticides.
9. Hybridization method of obtaining new variety of desirable traits by crossbreeding two different varieties of crops for example if two varieties of crops, one with high yield and other with disease resistance, can be crossbred to obtain a variety that has both the traits, i.e., high yield and disease resistance.
10. Nitrification is the process of conversion of ammonia into nitrates .It is carried out by nitrifying bacteria.
Denitrification is the process of conversion of nitrates into free nitrogen gas. It is carried out by denitrifying bacteria.
1. (d) using better varieties of crop plants. Better varieties of crop plants will result in increased production as they give higher yield as well as high resistance to diseases.
2. The drawbacks of uneven distribution of seeds in a field, while sowing, are:
(i) They will compete for water, nutrients and sunlight that result limited growth. (ii) If they are sown far apart, it will result in the wastage of field area. (iii) If seeds are not sown at the proper depth and covered by soil, they may be eaten up by birds.
3. We won't find fully grown crop plants in a nursery like rice, tomato, onion, etc because they are transferred to the fields from nurseries in their seedling stage.
4. No, farmers adopt crop rotation to maintain the fertility of the soil. For examples , wheat and rice use large amount of nitrogen from soil therefore legumes plants are grown next to the wheat or rice to replenish nitrogen naturally in field.
5. No. Growing a few plants in a garden is not crop production. Crop production is the practice of growing wheat on a large scale in a field to cultivate grains
6. Melons growing in waterlogged fields with poor drainage tend to absorb more water than they require. Melons cannot hold this excess water, therefore, they crack.
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