NCERT Class 8 Science Chapter 8 ‘Force and Pressure’: Notes and Solutions (Free PDF)

To understand a chapter well, organised notes are of great importance. They not only help you go through the chapter thoroughly but also serve as a mode of quick revision of the same. And especially during the exam season, students understand the value of notes. Keeping this in mind, we have prepared NCERT Solutions and Notes for Class 8 Science Chapter 8: Force and Pressure. These notes have been prepared in a point-wise manner thus making it easy for you to understand and memorise them. After the notes, we have also provided NCERT Solutions to the textbook questions. A PDF of the same is also available for you to download. Now, without any delay, let’s explore the notes!

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Class 8 Science Chapter 8 Force and Pressure: Force Notes: Introduction

In this chapter, we will understand and learn concepts such as force, its effects, contact and non-contact forces, electrostatic and gravitational forces and other related concepts. Let’s start with the notes.

Force is defined as a push or pull that is applied to an object and causes a change in its state. 

Explore: Branches of Physics

Forces are due to an Interaction

• For a force to come into play, at least two objects must interact with each other.
• It is the interaction of the two objects that leads to a force being applied between them. 

There are several effects that a force may have on an object. Some of them are listed below:-

• It may make an object move from rest. 
• It may change the speed of an object if it is moving. 
• It may change the direction of motion of an object. 
• It may bring about a change in the shape of an object. 

Exploring Forces

• The strength of the force that is applied is known as its magnitude.
• In order to find the magnitude of the net force, the direction in which the forces are being applied has to be specified. 
• When forces on an object are applied in the same direction, the net resultant force will be the sum of the applied forces.
• When forces on an object are applied in the opposite direction, the net resultant force will be the difference of the forces applied. 

A Force can Change the State of Motion

• A change in either the speed of an object, its direction of motion, or both, is described as a change in its state of motion. The state of motion is described by its speed and the direction of motion.
• An applied force can bring this change.
• When an object is at rest, and a force is applied to it, it may start moving.
• To an already moving object, if a force is applied in the direction of its motion, the net speed may increase and if that force is applied in the direction opposite to that of its motion, the net speed may decrease or the object may even come to rest. 
• But sometimes, the force may not have any effect at all. For example:- when a force is applied to a heavy box, it may not move at all.

Force can Change the Shape of an Object

• The application of force on an object may change its shape.
• For example, when force is applied to a rubber ball that is placed on the table, its shape changes. 

Class 8 Science Chapter 8: Force and Pressure: Contact and Non-Contact Forces Notes

Let us now explore the different types of forces that come under the category of contact and non-contact forces.

Contact Forces

A force that is applied to an object while being in contact with the object is known as contact force. It includes muscular force and friction.

Muscular Force

• Force that is applied because of the action of muscles of our body is known as muscular force.    
• For Example: When we push an object like a school bag or lift a bucket of water.

Friction

• The force of friction comes into effect when the surfaces of two objects come in contact. It opposes the motion.
• The force responsible for changing the state of motion of an object is the force of friction.
• The force of friction always acts on all the moving objects and its direction is always opposite to the direction of motion.

Non-Contact Forces

A force that is applied to an object without the two objects being in contact with each other is known as a non-contact force. It includes magnetic force, electrostatic force and gravitational force.

Magnetic Force

• Like poles of two magnets repel each other and unlike poles of a magnet attract each other.
• This can also be seen as another form of push or pull. 
• These forces applied by the magnets don’t require the magnets to be in contact with each other. Hence, it is a non-contact force. 

Electrostatic Force

• The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.
• The electrostatic force acts even when the objects are not in contact. 

Also Read: Save Electricity Essay: Format & Samples

Gravitational Force

• The force of gravity is defined as the force that is applied by the Earth on an object that falls towards it in the form of a pull. 
• The force of gravity acts on all objects. 
• The force of gravity acts on all of us all the time without our being aware of it.

Class 8 Science Chapter 8 Force and Pressure: Pressure Notes

Let us now explore what is pressure, its different types, its effects, etc in this section.

• Pressure is defined as the force acting on a unit area of a surface. 

Pressure= Force / Area of a surface

• Pressure is mostly considered perpendicular to the surface.
• The smaller the area, the more will be the force of pressure and vice-versa.

Pressure Exerted by Liquids and Gases

• Liquids and gases exert pressure on the walls of the container they are stored in.

Atmospheric Pressure

• There is an envelope of air all around us. This envelope of air is known as the atmosphere. The atmospheric air extends up to many kilometres above the surface of the earth.
• The pressure exerted by this air is known as atmospheric pressure.
• The pressure inside our bodies is also equal to the atmospheric pressure and balances the pressure from outside. Hence, our bodies do not get crushed because of the atmospheric pressure. 

Also Read:- How to Become a Physicist?

Important Definitions in NCERT Class 8 Science Chapter ‘Force and Pressure’

Some important definitions covered in the chapter are mentioned below:-

• Force: Force is defined as a push or pull that is applied to an object and causes a change in its state.
• Magnitude of a Force: The strength of the force that is applied is known as its magnitude.
• Contact Force: A force that is applied to an object while being in contact with the object is known as contact force.
• Muscular Force: Force that is applied because of the action of muscles of our body is known as muscular force.
• Non-Contact Force: A force that is applied to an object without the two objects being in contact with each other is known as a non-contact force.
• Electrostatic Force: The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.
• Gravitational Force: The force of gravity is defined as the force that is applied by the Earth on an object that falls towards it in the form of a pull. 
• Pressure is defined as the force acting on a unit area of a surface. 
• Atmospheric Pressure: The pressure exerted by the atmosphere is known as atmospheric pressure.

Download the NCERT Class 8 Science Chapter 8 Solutions PDF Here!

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NCERT Solutions of Class 8 Science Chapter 8: Force and Pressure

Let us now explore NCERT Solutions of Class 8 Science Chapter 8 to get answers to all writing

questions in the lesson Force and Pressure.

Exercise Questions

1. Give two examples each of situations in which you push or pull to change the state of motion of objects.

Soln:

Examples of situations where push or pull is involved to change the state of motion of objects are:- 

1. Pull situations

a) we pull the handle of a drawer to open it. This changes the state of motion of the drawer.

b) we pull the rope fixed to a bucket to draw water from the well. This changes the state of motion of the bucket.

2. Push Situations

a) When a football players kick a football. It changes the state of motion of the ball. 

b) A heavy box needs to be pushed to move it around. This changes the state of motion of the box. 

2. Give two examples of situations in which applied force causes a change in the shape of an object.

Soln:

Two examples where the force applied changes the shape of an object are mentioned below:

i) By pressing a rubber ball between the hands, it deforms.

ii) when we squeeze a plastic bottle, its shape changes.

3. Fill in the blanks in the following statements.

(a) To draw water from a well, we have to __________ at the rope.

(b) A charged body __________ an uncharged body towards it.

(c) To move a loaded trolley, we have to __________ it.

(d) The north pole of a magnet __________the north pole of another magnet.

Soln:

• To draw water from a well, we have to pull at the rope.
• A charged body attracts an uncharged body towards it.
• To move a loaded trolley, we have to pull or push it.
• The north pole of a magnet repels the north pole of another magnet.

4. An archer stretches her bow while taking aim at the target. She then releases the arrow, which begins to move towards the target. Based on this information, fill up the gaps in the following statements using the following terms.

muscular, contact, non-contact, gravity, friction, shape, attraction

(a) To stretch the bow, the archer applies a force that causes a change in its __________.

(b) The force applied by the archer to stretch the bow is an example of __________ force.

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a __________ force.

(d) While the arrow moves towards its target, the forces acting on it are due to __________ and that due to __________ of air

Soln:

(a) To stretch the bow, the archer applies a force that causes a change in its shape.

(b) The force applied by the archer to stretch the bow is an example of muscular force.

(c) The type of force responsible for a change in the state of motion of the arrow is an example of a contact force.

(d) While the arrow moves towards its target, the forces acting on it are due to gravity and that due to friction of air.

5. In the following situations, identify the agent exerting the force and the object on which it acts. State the effect of the force in each case.

(a) Squeezing a piece of lemon between the fingers to extract its juice.

(b) Taking out paste from a toothpaste tube.

(c) A load suspended from a spring while its other end is on a hook fixed to a wall.

(d) An athlete making a high jump to clear the bar at a certain height

Soln:

a) The force being exerted is the muscular force on the lemon, as a result, its shape will change and lemon juice will be extracted

b) The force being exerted in the muscular force on the toothpaste tube which will lead to the toothpaste coming out of it and changing the shape of the toothpaste tube.

c) The load attached to the spring will exert force on it, making the spring stretch and hence, changing its shape. 

d) In this case, the feet of the athlete will exert force on the ground pushing it, which will allow the athlete to jump over the bar. This will cause a change in the state of motion of the athlete. 

6. A blacksmith hammers a hot piece of iron while making a tool. How does the force due to hammering affect the piece of iron?

Soln:

When a blacksmith hammers a piece of iron, he uses muscular force for that. This results in the change of shape of the iron so that it can be given the required shape.

7. An inflated balloon was pressed against a wall after it had been rubbed with a piece of synthetic cloth. It was found that the balloon stuck to the wall. What force might be responsible for the attraction between the balloon and the wall?

Soln:

A balloon when rubbed with a piece of synthetic cloth acquires charge. And an object that is charged attracts an uncharged object. Hence the balloon sticks to the wall because of the electrostatic force.

8. Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.

Soln:

When we hold the bucket containing water above the ground level, the gravitational force acts on it pulling it towards the ground. That force is countered by the muscular force that our hand applies. Since both these forces are equal and opposite in magnitude, the net resultant force is zero. Hence, the state of motion of the bucket remains unchanged.

9. A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.

Soln:

Immediately after leaving the launch pad, gravitational force acts on the rocket pulling it to the ground. The force of friction because of the air acts on the rocket opposing its motion. 

10. When we press the bulb of a dropper with its nozzle kept in water, the air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to

(a) pressure of water

(b) gravity of the earth

(c) shape of rubber bulb

(d) atmospheric pressure

Soln:

d) Atmospheric pressure

When we push the bulb of the dropper, we release the inside atmospheric pressure that was countering the outside atmospheric pressure. This leads to low pressure created inside the dropper. Now the atmospheric pressure outside being comparatively high, pushes the water inside the dropper leading to a rise of water in the dropper. 

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