CHAPTER-11

FORCE AND PRESSURE

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BY: Mrs. Suchismita sahoo

TGT (Science), Jnv ,nayagarh,odisha

NAVODAYA VIDYALAYA SAMITI, NOIDA

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E-Content for Class VIII Science

CONTENTS

- definition and unit of force

- forces are due to an interaction

- exploring force

- effects of force

- types of force

- definition and unit of pressure

- pressure exerted by liquids and gases

- atmospheric pressure

OBJECTIVES

At the end of this lesson, students will be able to :

• Define force.
• List the properties of force.
• Understand the effects of force.
• Distinguish contact and non-contact forces.
• List properties of pressure exerted by solids, liquids and gases.
• List the effects of pressure on human activities.
• Define atmospheric pressure.

Previous Knowledge TestPPbg

• 11. How an object move ?
• 2. How you decide whether an object is moving faster or slower ?
• 3. What does the distance moved by an object in unit time indicate ?
• 4. What makes an object slow down or go faster or change its direction of motion ?

Push- A force to move an object away from our body.

Pull- A force to move an object towards our body .

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FORCE:-

A force is a push or pull on an object.

examples:-A man pushing cart,

kicking a ball

FORCE AND PRESSURE

• pull or push

Forces are due to an interaction

EXPLORING FORCE

1. If two forces act on a body in same direction, the net force is the sum of two forces.

F1

F2

Net force (F)=F1+F2

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If two forces act on a body in opposite direction, the net force is the difference between two forces

F1 F2

Net force = F2 – F1 (F2>F1)

The effect of force on the object depends on the net force acting on it.

Force has Magnitude as well as Direction.

S.I. Unit of Force - Newton (N)

C.G.S. Unit of Force – Dyne (Dyn)

SSState of Motion

A change in either the speed of an object , or its direction of motion , or both is described as a change in its state of motion.

An object may be at rest or in motion ; both are its state of motion.

Thus , a force may bring a change in the state of motion of an object.

EFFECTS OF A FORCE

MAKES A STATIONARY OBJECT MOVES

CHANGE THE DIRECTION OF A MOVING OBJECT

CHANGES THE SHAPE PERMANENTLY

STOPS A MOVING OBJECT

CHANGES THE SHAPE TEMPORARILY

CHANGE THE SPEED OF A MOVING OBJECT

• TYPE OF FORCES
• CONTACT FORCES
• MUSCULAR FORCES
• FRICTIONS

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• NON CONTACT FORCES
• MAGNETIC FORCE
• ELECTROSTATIC FORCE
• GRAVITATIONAL FORCE

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CONTACT FORCE

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• A FORCE WHICH CAN BE EXERTED BY AN OBJECT ON ANOTHER OBJECT ONLY THROUGH PHYSICAL TOUCHING IS CALLED A CONTACT FORCE.

• THE FORCE EXERTED BY THE MUSCLES OF THE BODY IS CALLED MUSCULAR FORCE

MUSCULAR FORCE

THE FORCE EXERTED BY THE MUSCLESOF THE BODY IS CALLED MUSCULAR FORCE

NON-CONTACT FORCE�

• A FORCE WHICH CAN BE EXERTED BY AN OBJECT ON ANOTHER OBJECT EVEN FROM A DISTANCE(WITHOUT TOUCHING EATH OTHER) IS CALLED A NON-CONTACT FORCE

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Electrically charged comb attracts tiny pieces of paper

Gravitational Force

The force of attraction between any two objects possessing mass is called gravitational force.

It is the gravitational force between the sun and the earth which holds the earth in its orbit around the sun.

The force with which the earth pulls the objects towards it , is called the force of gravity .

A ball thrown upward falls down,

A ripened fruit falling from a tree

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PRESSURE

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The force acting on unit area of a surface is called pressure.

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Pressure= Force /Area on which it acts

S.I. unit of pressure is Pascal.

Applications of Pressure

LIQUIDS EXERTS PRESSURE

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Liquids exerts pressure on the walls of the container.

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GASES EXERTS PRESSURE

• Gases exert pressure on the walls of the container.

• The pressure exerted by the air around us is

ATMOSPHERIC PRESSURE

• The pressure exerted by the air around us known as atmospheric pressure. Atmospheric pressure is due to the weight of air present in the atmosphere.

Things to Think about

• 1. Air planes have pressurized cabins . Why ?
• 2. How the vacuum cleaner works ?
• 3. Camel can run or walk on sand very easily but humans not ? Why ?
• 4. Have you ever think about atmospheric pressure on other planets . Do other planets have atmospheric pressure ?

FUN TIME

Write three non-contact forces that you have studied .Now take the first alphabet from each of the forces and make a word that means ‘something precious ‘.

Suggested Project

• Make a chart with colourful pictures of various things like a car, a glass of water, a person standing , a boy writing , a tailor stitching , someone cutting vegetables, a doctor giving an injection, etc. Make a caption under each picture to say where the pressure is being applied and write a few lines about it .
• Outdoor Activities
• Next time you go to the petrol station , take time to talk to the person who fills air in the tyre.Ask him to show you the pressure gauge he uses.Find out how much pressure is needed for each vehicle (scooter,auto,car,truck,bus, etc. )

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Let us summarize what have we learnt:-

• Force is a push or pull resulting in motion of a body.
• Force can change the shape of an object, move an object from the position of rest, change the direction of a body in motion and change the speed of the moving object.
• There are two kinds of force: contact force and non-contact force.
• Muscular force and force of friction are the examples of contact forces.
• Gravitational force, electro static force and magnetic force are the examples of non-contact forces.

SUMMARY

• Force per unit area is known as pressure.
• Pressure from solids is due to their weight and acts downward, but in the case of liquids and gases the pressure exerted by them is downard,sideways and upward also, that means in all directions.
• The envelope of air around the earth is known as atmosphere.
• The pressure of the air upon the surface of the earth is known as the atmospheric pressure.

THANK YOU