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DIGITAL TEXT ON THE CHAPTER " FORCE "
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(8th Standard Physics )

Submitted by,
AMALABHAS S
Roll no : 2
Physical Science
KUCTE, Kayamkulam
2019 - 2021

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Observe the pictures given below

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What do you observe in these two pictures? 🤔

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In fig. (a), a man is pulling a hand trolley.
In fig. (b), a man is pushing a car.

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Then, can you identify whether the following are push or pull?

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From all these things what did you learn?

When an object is pushed or pulled, a force is being applied on it.

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From all the examples given above, can you state what force is?

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" Force is that which changes or tends to change the shape, size, volume, state of rest or state of motion of a body.
The unit of force is newton. It is indicated by the letter 'N' "

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More information about the origin of the unit of force "newton" and the biography of Sir, Isaac Newton...👇

https://www.curriculumvisions.com/search/N/newton/newton.html

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Sir Isaac Newton.

The unit of force called the newton (small n) is named after the famous scientist Sir Isaac Newton.

Sir Isaac Newton lived between 1642 and 1726. He was an English physicist and mathematician. He is recognised as one of the most important scientists of all time and one of those who produced the scientific revolution. At the time this combination of physics and mathematics was called natural philosophy. That is why his famous book is called by the long title Philosophiæ Naturalis Principia Mathematica ("Mathematical Principles of Natural Philosophy"). It was published in 1687. Today we might call it 'how the world works'. It was a study of forces, and the world works as a result of forces. But he was also responsible for many new insights into Light and also was responsible for the development of a new branch of mathematics called calculus.

Newton's book laid out laws of movement and also explained the idea of gravity. No one had known about gravity before Newton. He also showed that the laws did not just apply here on Earth, but also everywhere in the Universe, and also demonstrated that the motion of objects on Earth and of celestial bodies could be described by the same laws. He even predicted that the Earth should be a slightly flattened globe, and this was later found to be the case.

Newton also built the first reflecting telescope and developed a theory of colour based on the way that a prism splits white light into the many colours of the spectrum (rainbow). He suggested a law of cooling, and worked on the speed of sound.

Newton was based at Trinity College Cambridge. He was a strange man, believing in both Christianity and alchemy, for this was before the days of chemistry as we know it. In his later life, Newton became president of the Royal Society. He also served the British government as Warden and Master of the Royal Mint.

In honour of his achievements the unit of force is called the newton (the amount needed to accelerate 1 kilogramme of mass at the rate of 1 metre per second squared).

Video: Balances.

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What is a newton? A newton is a measurement of force, named after the famous scientist, Sir Isaac Newton.

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Sir Isaac Newton. The unit of force called the newton (small n) is named after the famous scientist Sir Isaac Newton. Sir Isaac Newton lived between 1642 and 1726. He was an English physicist and mathematician. He is recognised as one of the most important scientists of all time and one of those who produced the scientific revolution.

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CONTACT AND NON CONTACT FORCE

Observe the pictures

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What is the difference that you can observe in each of these pictures?🤔

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The figures (a), (b) and (d) shows situations where force is applying by the contact between objects.

The figures (c), (e) and (f) shows situations where force is applying without the contact between objects.

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The can you state what are contact and non contact forces?

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"The force applied by the contact between objects is called contact force."

"The force applied on an object without contact on it is called non contact force."

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Video link for contact and non contact forces...👇
https://youtu.be/x1BR72OzQmE

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What are Contact Forces and Non Contact Forces in Physics? Easy Explanation for Kids

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FRICTIONAL FORCE

Observe the video well

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What you have learned?

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"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 known as friction."

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DIFFERENT TYPES OF FRICTION

Observe the following picture in which two children are trying to move a log.

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Which of these methods is easy in moving objects - by dragging or by rolling? 🤔

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Let's watch a video related to this.
Video link...👇
https://youtu.be/eNRqHWPn1iQ

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So what can we conclude?

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"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.

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For further study, watch the following video
Video link... 👇
https://youtu.be/zF0m8lZra-E

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THRUST AND PRESSURE

Observe the following cartoon.

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Why the boy is saying so? Have you ever think about this?

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Observe the following video

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What we learned?

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"The total normal force experienced by a surface is thrust."
"Thrust per unit area is pressure."

Pressure = Thrust / Area

From this equation we can conclude that the unit of pressure is N/m^2. This is known as pascal.

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LIQUID PRESSURE

Fix identical balloons A, B, C on a PVC pipe as shown in the figure given below. Observe the expansion of balloons on filling water in the pipe.

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Which balloon has expanded more?

What may be reason?

How can the expansion of balloon be related to the depth of the level of liquid in the pipe?

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By answering the above questions, what idea you got finally?

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.

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Now observe the following video
Video link👇
https://youtu.be/Cvp6mLWbgaM

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Do Liquids Exert Pressure? | Physics | Don't Memorise

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What do you understood from the video?

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If the height of the liquid column is 'h', density of liquid 'd' and acceleration due to gravity 'g', then liquid pressure is P = hdg

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ATMOSPHERIC PRESSURE

Observe the following video
Video link... 👇
https://youtu.be/KndNN28OcEI

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Atmospheric Pressure | Force and Pressure | Don't Memorise

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So what can we sum up?

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An envelope of air surrounds the earth. This is the earth's atmosphere. The density of atmospheric air near the surface of the earth is greater and it decreases as we go up.
The weight of the air column over unit area of the earth's surface is atmospheric pressure.

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FOR DETAILED STUDY

https://en.m.wikipedia.org/wiki/Atmospheric_pressure

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Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa; 1,013.25 mbar), which is equivalent to 760 mm Hg, 29.9212 inchesHg, or 14.696 psi.[1] The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth, that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.

In most circumstances, atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. As elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation. Pressure measures force per unit area, with SI units of Pascals (1 pascal = 1 newton per square metre, 1N/m ²). On average, a column of air with a cross-sectional area of 1 square centimetre (cm ²), measured from mean (average) sea level to the top of Earth's atmosphere, has a mass of about 1.03 kilogram and exerts a force or "weight" of about 10.1 newtons, resulting in a pressure of 10.1 N/cm ² or 101 kN/m ² (101 kilopascals, kPa). A column of air with a cross-sectional area of 1in ² would have a weight of about 14.7lb f, resulting in a pressure of 14.7lb f/in ².

Atmospheric pressure is caused by the gravitational attraction of the planet on the atmospheric gases above the surface and is a function of the mass of the planet, the radius of the surface, and the amount and composition of the gases and their vertical distribution in the atmosphere.[2][3] It is modified by the planetary rotation and local effects such as wind velocity, density variations due to temperature and variations in composition.[4]

Surface pressure is the atmospheric pressure at a location on Earth's surface ( terrain and oceans). It is directly proportional to the mass of air over that location.

For numerical reasons, atmospheric models such as general circulation models (GCMs) usually predict the nondimensional logarithm of surface pressure.

The average value of surface pressure on Earth is 985 hPa.[7] This is in contrast to mean sea-level pressure, which involves the extrapolation of pressure to sea-level for locations above or below sea-level. The average pressure at mean sea-level ( MSL) in the International Standard Atmosphere ( ISA) is 1013.25 hPa, or 1 atmosphere (atm), or 29.92 inches of mercury.

Pressure (p), mass (m), and the acceleration due to gravity (g), are related by P = F/A = (m*g)/A, where A is surface area. Atmospheric pressure is thus proportional to the weight per unit area of the atmospheric mass above that location.

Pressure on Earth varies with the altitude of the surface; so air pressure on mountains is usually lower than air pressure at sea level. Pressure varies smoothly from the Earth's surface to the top of the mesosphere. Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round. As altitude increases, atmospheric pressure decreases. One can calculate the atmospheric pressure at a given altitude.[8] Temperature and humidity also affect the atmospheric pressure, and it is necessary to know these to compute an accurate figure. The graph above was developed for a temperature of 15 °C and a relative humidity of 0%.

At low altitudes above sea level, the pressure decreases by about 1.2 kPa (12 hPa) for every 100 metres. For higher altitudes within the troposphere, the following equation (the barometric formula) relates atmospheric pressure p to altitude h:

where the constant parameters are as described below:

Parameter

Description

Value

p ₀
Sea level standard atmospheric pressure

101325Pa

L
Temperature lapse rate, = g/ c p for dry air

~ 0.00976K/m

c p
Constant-pressure specific heat

1004.68506J/(kg·K)

T ₀
Sea level standard temperature

288.16K

g
Earth-surface gravitational acceleration

9.80665m/s ²

M
Molar mass of dry air

0.02896968kg/mol

R Universal gas constant ₀
8.314462618J/(mol·K)

Atmospheric pressure varies widely on Earth, and these changes are important in studying weather and climate. See pressure system for the effects of air pressure variations on weather.

Atmospheric pressure shows a diurnal or semidiurnal (twice-daily) cycle caused by global atmospheric tides. This effect is strongest in tropical zones, with an amplitude of a few millibars, and almost zero in polar areas. These variations have two superimposed cycles, a circadian (24 h) cycle and semi-circadian (12 h) cycle.

The highest adjusted-to-sea level barometric pressure ever recorded on Earth (above 750 meters) was 1084.8 hPa (32.03 inHg) measured in Tosontsengel, Mongolia on 19 December 2001.[9] The highest adjusted-to-sea level barometric pressure ever recorded (below 750 meters) was at Agata in Evenk Autonomous Okrug, Russia (66°53'N, 93°28'E, elevation: 261 m, 856 ft) on 31 December 1968 of 1083.8 hPa (32.005 inHg).[10] The discrimination is due to the problematic assumptions (assuming a standard lapse rate) associated with reduction of sea level from high elevations.[9]

The Dead Sea, the lowest place on Earth at 430 metres (1,410 ft) below sea level, has a correspondingly high typical atmospheric pressure of 1065hPa.[11] A below-sea-level surface pressure record of 1081.8 hPa (31.95 inHg) was set on 21 February 1961.[12]

The lowest non-tornadic atmospheric pressure ever measured was 870 hPa (0.858 atm; 25.69 inHg), set on 12 October 1979, during Typhoon Tip in the western Pacific Ocean. The measurement was based on an instrumental observation made from a reconnaissance aircraft.[13]

One atmosphere (101.325 kPa or 14.7 psi) is also the pressure caused by the weight of a column of fresh water of approximately 10.3 m (33.8 ft). Thus, a diver 10.3 m underwater experiences a pressure of about 2 atmospheres (1 atm of air plus 1 atm of water). Conversely, 10.3 m is the maximum height to which water can be raised using suction under standard atmospheric conditions.

Low pressures such as natural gas lines are sometimes specified in inches of water, typically written as w.c. (water column) gauge or w.g. (inches water gauge). A typical gas-using residential appliance in the US is rated for a maximum of 1/2 psi, which is approximately 14 w.g. (3487 Pa or 34.9 millibars). Similar metric units with a wide variety of names and notation based on millimetres, centimetres or metres are now less commonly used.

Pure water boils at 100 °C (212 °F) at earth's standard atmospheric pressure. The boiling point is the temperature at which the vapor pressure is equal to the atmospheric pressure around the water.[14] Because of this, the boiling point of water is lower at lower pressure and higher at higher pressure. Cooking at high elevations, therefore, requires adjustments to recipes[15] or pressure cooking. A rough approximation of elevation can be obtained by measuring the temperature at which water boils; in the mid-19th century, this method was used by explorers.[16]

An important application of the knowledge that atmospheric pressure varies directly with altitude was in determining the height of hills and mountains thanks to the availability of reliable pressure measurement devices. In 1774, Maskelyne was confirming Newton's theory of gravitation at and on Schiehallion mountain in Scotland, and he needed to accurately measure elevations on the mountain's sides. William Roy, using barometric pressure, was able to confirm Maskelyne's height determinations, the agreement being to within one meter (3.28 feet). This method became and continues to be useful for survey work and map making.[17]

1. International Civil Aviation Organization. ^ Manual of the ICAO Standard Atmosphere, Doc 7488-CD, Third Edition, 1993. ISBN 92-9194-004-6.

2. ^ Jacob, Daniel J. (1999). Introduction to Atmospheric Chemistry. Princeton University Press. ISBN 9780691001852.

3. ^ Sample METAR of CYVR Nav Canada

4. Jacob, Daniel J. ^ Introduction to Atmospheric Chemistry. Princeton University Press, 1999.

5. ^ A quick derivation relating altitude to air pressure Archived 2011-09-28 at the Wayback Machine by Portland State Aerospace Society, 2004, accessed 05032011

6. ^ a World: Highest Sea Level Air Pressure Above 750 m, Wmo.asu.edu, 2001-12-19, archived from the original on 2012-10-17 b

7. ^ World: Highest Sea Level Air Pressure Below 750 m, Wmo.asu.edu, 1968-12-31, archived from the original on 2013-05-14

8. ^ Kramer, MR; Springer C; Berkman N; Glazer M; Bublil M; Bar-Yishay E; Godfrey S (March 1998). "Rehabilitation of hypoxemic patients with COPD at low altitude at the Dead Sea, the lowest place on earth" (PDF). Chest. 113 (3): 571-575. doi:10.1378/chest.113.3.571. PMID 9515826. Archived from the original (PDF) on 2013-10-29.

9. ^ Court, Arnold (1969). "Improbable Pressure Extreme: 1070 Mb". Bulletin of the American Meteorological Society. 50 (4): 248-50. JSTOR 26252600.

10. ^ Chris Landsea (2010-04-21). "Subject: E1), Which is the most intense tropical cyclone on record?". Atlantic Oceanographic and Meteorological Laboratory. Archived from the original on 6 December 2010.

11. ^ High Altitude Cooking, Crisco.com, 2010-09-30, archived from the original on 2012-09-07

12. ^ Berberan-Santos, M. N.; Bodunov, E. N.; Pogliani, L. (1997). "On the barometric formula". American Journal of Physics. 65 (5): 404-412. Bibcode:1997AmJPh..65..404B. doi:10.1119/1.18555.

13. Hewitt, Rachel, ^ Map of a Nation - a Biography of the Ordnance Survey ISBN 1-84708-098-7

Experiments Edit

The mean sea-level pressure (MSLP) is the atmospheric pressure at mean sea level (PMSL). This is the atmospheric pressure normally given in weather reports on radio, television, and newspapers or on the Internet. When barometers in the home are set to match the local weather reports, they display pressure adjusted to sea level, not the actual local atmospheric pressure.

The altimeter setting in aviation is an atmospheric pressure adjustment.

Average sea-level pressure is 1013.25 mbar (101.325 kPa; 29.921 inHg; 760.00 mmHg). In aviation, weather reports ( METAR), QNH is transmitted around the world in millibars or hectopascals (1 hectopascal = 1 millibar), except in the United States, Canada, and Colombia where it is reported in inches of mercury (to two decimal places). The United States and Canada also report sea-level pressure SLP, which is adjusted to sea level by a different method, in the remarks section, not in the internationally transmitted part of the code, in hectopascals or millibars.[5] However, in Canada's public weather reports, sea level pressure is instead reported in kilopascals.[6]

In the US weather code remarks, three digits are all that are transmitted; decimal points and the one or two most significant digits are omitted: 1013.2 mbar (101.32 kPa) is transmitted as 132; 1000.0 mbar (100.00 kPa) is transmitted as 000; 998.7mbar is transmitted as 987; etc. The highest sea-level pressure on Earth occurs in Siberia, where the Siberian High often attains a sea-level pressure above 1050 mbar (105 kPa; 31 inHg), with record highs close to 1085 mbar (108.5 kPa; 32.0 inHg). The lowest measurable sea-level pressure is found at the centers of tropical cyclones and tornadoes, with a record low of 870 mbar (87 kPa; 26 inHg).

Surface pressure Edit

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Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa; 1,013.25 mbar), which is equivalent to 760 mm Hg, 29.9212 inchesHg, or 14.696 psi.[1] The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth, that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.

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SIGNIFICANT LEARNING OUTCOMES

The learner can

explain what force is and what the results of applying a force are.

identify different types of forces and classify them into contact and non contact forces.

explain friction and identify its advantages and disadvantages and use it in daily life situations.

explain thrust and pressure.

use the relation between area and pressure in appropriate situations.

explain liquid pressure and engage in experiments that show the relation between pressure and height.

explain what atmospheric pressure is and use it appropriately in daily life situations.

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ANSWER THE FOLLOWING QUESTIONS

1) Classify the following situations into contact and non contact forces.

Applying break in a bicycle.

A mango falling from a mango tree.

The earth revolving around the sun.

The speed of a ball rolling on ground is reduced.

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2) State reason.

We are able to walk on the ground without slipping.

It is easy to cut vegetables using a sharp knife.

The number of tyres is more for goods vehicles.

The moving parts of machines experience wear and tear.

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3) Fill in the blank.

FORCE : newton

_______ : pascal

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4) List the situations in daily life where atmospheric pressure is used.

drinking juice using a straw.

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5) List the factors influencing liquid pressure.

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6 ) Match the following.

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SOLVED QUESTIONS

https://www.hsslive.guru/kerala-syllabus-8th-standard-basic-science-solutions-chapter-10/

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You can Download Force Questions and Answers, Summary, Activity, Notes, Kerala Syllabus 8th Standard Basic Science Solutions Chapter 10 help you to revise complete Syllabus and score more marks in your examinations.

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

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.

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.

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

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

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.

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.

Question 3.
Match the following

Answer:

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.

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

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

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

Question 5.
Write one solid lubricant and liquid lubricant
Answer:
Solid-graphite
Liquid-oil

Question 6.
Fill up suitably.
Force - newton
Pressure - ...........
Answer:
Pressure - pascal

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.

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.

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Kerala Syllabus 8th Standard Basic Science Solutions Chapter 10 Force

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You can Download Force Questions and Answers, Summary, Activity, Notes, Kerala Syllabus 8th Standard Basic Science Solutions Chapter 10 help you to revise complete Syllabus and score more marks in your examinations. When an object is pushed or pulled, a force is being applied on it. The unit of force is newton.