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Kerala State Syllabus 8th Standard Basic Science Solutions Chapter 10 Force
When an object is pushed or pulled, a force is being applied on it. The unit of force is newton. It is indicated by the letter N.
Force is that which changes or tends to change the shape, size, volume, state of rest or state of motion of a body.
There are various types of forces such as muscular force, magnetic force, electric force, gravitational force, frictional force etc.
Contact force and non contact force
The force applied by the contact between objects is contact force. The force applied on an object without contact on it is non contact force.
Ex:pushing a trolley- contact force, Magnet attracting a nail-non contact force
Kerala Syllabus 8th Standard Physics Notes Frictional force:
When a ball is rolling on the floor after some time it stops. This is because of the frictional force between the ball and floor. On the surface of the objects which are on contact with each other there are many mounts and pits get interlocked.
When we try to move a body by applying a force parallel to their surfaces an opposing force is experienced.
When a body moves or tends to move on the surface of another body, a force is experienced parallel to the surface which opposes the relative motion between them. This is friction.
Rolling friction and sliding friction
When a body rolls over the surface of another body, the force of friction which originates is rolling friction. When a body slides on the surface of another body the force of friction which originates is sliding friction.
Rolling friction is less than sliding friction. Striking a match stick on the side of a match box, wear and tear of machines, ability to hold objects, walking etc are advantages of the friction and treading of tyres of vehicles, wearing out of tyres, fuel loss are disadvantages of friction.
The materials like oil, grease which are used to reduce friction are the lubricants. Graphite is a solid lubricant.
Force Class 8 Kerala Syllabus Thrust and Pressure
The total normal force experienced by a ‘ surface is thrust. Thrust per unit area is pressure.
Pressure = \(\frac{Thrust}{area}\)
Unit of pressure is N/m2. This is known as pascal. The force acting on a larger area exerts a smaller pressure, while a force acting on a smaller area exerts a large pressure.
Hsslive Guru 8th Class Physics Kerala Syllabus Liquid Pressure
The pressure is experienced in liquids also. As the height of liquid column increases the pressure exerted by it increases.
The pressure exerted by a liquid column increases with increase in height. The thrust acting per unit area by a liquid is liquid pressure.
Liquids exert force on all sides of the container in which they are taken.
If the height of the liquid column is Ti’, density of liquid ‘d’ and acceleration due to gravity ‘g’, then liquid pressure is P = hdg
Class 8 Physics Kerala Syllabus Atmospheric pressure
Atmospheric air also can exert pressure. An envelope of air surrounds the earth. This is earth’s atmosphere. The density of atmospheric air near the surface of earth is greater and it decreases when we go up. The weight of air column over the unit area of earth’s surface is atmospheric pressure. One atmospheric pressure is the weight of mercury column of 0.76 m height and unit area (1m²). This is standard atmospheric pressure .
The unit of atmospheric pressure is* bar. The instrument used to measure atmospheric pressure is Barometre
Force Textbook Questions and Answers
Kerala Syllabus 8th Standard Physics Notes Pdf Question 1.
Classify the following situations into contact and noncontact forces.
a. Applying break in a bicycle.
b. A mango falling from a mango tree.
c. The earth revolving around the sun.
d. The speed of a hall rolling on ground is reduced.
Answer:
Contact force
a. Applying break in a bicycle.
d. The speed of a ball rolling on ground is reduced
Non contact force
b. A mango falling from a mango tree.
c. The earth revolving around the sun.
Class 8 Physics Notes Kerala Syllabus Question 2.
State reason
a. We are able to walk on the ground without slipping.
b. It is easy to cut vegetables using a sharp knife.
c. The number of tyres is more for goods vehicles.
d. The moving parts of machines experience wear and tear.
Answer:
a. Because of the friction between ground and feet
b. When the area decreases pressure increases. As the area of the edge of the knife is veiy less, it is easy to cut
c. When the area of surface increases pressure decreases, so when more wheels are used it won’t depressed in soil.
d. Because of the friction between the area of contacts.
Hss Live Guru 8th Physics Kerala Syllabus Question 3.
Match the following
| A | B | C |
| Atmospheric pressure | pascal | coconut oil |
| Lubricant | attration | barometer |
| Coconut falling down | bar | thermometer |
| Magnet | friction | noncontact |
| gravitation | repulsion |
Answer:
| Atmospheric pressure | bar | barometre |
| Lubricant | friction | Coconut oil |
| Coconut falling down | Gravitational force | Non contact |
| Magnet | attraction | repulsion |
Hsslive Guru Physics 8th Standard Kerala Syllabus Question 4.
Bubbles rising from the bottom of the water filled in a bottle are depicted in the figure. Which is the correct figure? Justify your answer.
Answer:
Fig.c. There will be more pressure in the lower side of the liquid. The pressure decreases when it comes upward.
Hss Live Guru 8 Physics Kerala Syllabus Question 5.
A toy car of about 50 g placed on a polished table with threads attached to it carrying two pans passed through pulleys is depicted.
(a) What do you observe if too g each is placed on both pans?
(b) What do you observe if too g is placed on pan A and 200 g in pan B
(c) Justify your answers.
Answer:
a. The car will be stationary because the force excreted from both sides are equal.
b. The pan of weight 200 will pull with more force and it moves that direction.
c. In the first phase same force is excerted. But in second phase force is exerted to one direction.
Force Additional Questions and Answers
Force And Pressure Class 8 Worksheets With Answers Pdf Kerala Syllabus Question 1.
Tabulate the following as contact force and non contact force.
1. pushing a trolley
2. falling coconut from coconut tree
3. magnet attract iron nail
4. Drowing water from the well
5. pushing the car
6. earth is moving around the sun
Answer:
contact force – 1,4,5
non contact force – 2,3,6
8th Standard Physics Notes Kerala Syllabus Question 2.
Ramu tried to push a round stone and rectangle stone of same weight. Which is easy to push.
Answer:
It is easy to push round stone because rolling friction is less than sliding friction.
3. Which are the following occation friction is beneficial and non beneficial
1. striking a match stick on the side of a match box
2. wear and tear of machines.
3. ability to hold objects
4. wearing out of tyres
5. walking 6. loss of fuel
Answer:
Beneficial : 1, 3, 5
non beneficial : 2, 4, 6
8th Std Physics Notes Kerala Syllabus Question 4.
Why does the design of ship and aeroplane are streamlining?
Answer:
To reduce friction while moving through air and water.
Kerala Syllabus 8th Standard Physics Question 5.
Write one solid lubricant and liquid lubricant
Answer:
Solid-graphite
Liquid-oil
Kerala Syllabus 8th Standard Science Notes Question 6.
Fill up suitably.
Force – newton
Pressure – ………..
Answer:
Pressure – pascal
8th Standard Physics Notes Pdf Kerala Syllabus Question 7.
Write the reason
1. Constructing a knife with sharp edge
2. Constructing the building with wide basement
Answer:
1. When the area of contact decreases the pressure increases
2. When area increases the chance to depress the base in soil is very less.
Class 8 Basic Science Solutions Kerala Syllabus Question 8.
Given figures, one can filled with water and other can filled with kerosine.
Which balloon is filled with water
Answer:
fig 2. When density of liquid increases the pressure increases, water is denser than kerosene.
Physics Class 8 Kerala Syllabus Question 9.
What are the factors effecting liquid pressure? Write the mathematical equation.
Answer:
The height of the liquid column is ‘h’, density of liquid ‘d’ and acceleration due to gravity ‘g’ are the factors. The equation is liquid pressure P = hdg
Basic Science Class 8 Kerala Syllabus Question 10.
As mountaineers climb higher altitude there is a possibility of nasal bleeding. Why?
Answer:
Going higher altitude the atmospheric pressure decreases. So blood veins are broken and blood is oozing out.
Question 11.
What is standard atmospheric pressure
Answer:
Atmospheric pressure at the sea level is equal to the pressure exerted by 0.76m of mercury column. This is standard atmospheric pressure.
Question 12.
When a hole is formed at the bottom of the water bottle, Anu closed the lid of the bottle tightly. The flow of water stops. Explain
Answer:
When the lid is closed tightly, the effect of atmospheric pressure is not experienced on the surface of the water in the bottle. Hence water cannot flow out from the bottle.
Question 13.
Mercury remains at a height of 76cm in a barometer. Raju made a hole at the top end of the tube. What will you observe? How can it be explained?
Answer:
Mercury will flow downwards when a hole is made at the top part of the tube. The atmospheric pressure exerted on the mercury in the vessels supports the mercury level in the tube. When a hole is made at the top, air enters the tube and the pressure is distributed throughout uniformly and hence the mercury level comes down.
Question 14.
A sharp nail pierces more easily through wood than a blunt nail. What is the reason?
Answer:
The surface area of the sharp tip of a nail is very small. Hence the pressure that it can exert on the wood is very great. That is why it pierces easily. For a blunt nail, the surface area of the tip is greater. Hence it can exert only a small amount of pressure.
Question 15.
What is meant by limiting friction.
Answer:
Limiting friction is the force of static friction acting just before the body starts moving.