FORCE AND LAWS OF MOTION, CBSE CLASS IX, PHYSICS NOTES PART I


CBSE CLASS IX, FORCE AND LAWS OF MOTION, SCIENCE (PHYSICS) NOTES-(PART I)

       FORCE AND LAWS OF MOTION

According to the CBSE Syllabus 2025-26

CBSE Class 9 Science Chapter 9 Force and Laws of Motion Notes

In Class 9 Science Chapter 9 Force and Laws of Motion.

TOPICS IN THE CHAPTER

(a) Introduction. 

(b) Effect of Force. 

(c) Balanced Force. 

(d) Unbalanced Force. 

(e) Laws of Motion. 

(f) Newton's Laws of Motion.                                 

• First Law of Motion. 

(g) Mass and Inertia. 

(h) Momentum.                                          

• Momentum and Mass.                                               

• The momentum of an object that is in a state of rest.      

• Unit of momentum.                                                            

• Numericals based on momentum.

(i) Second Law of Motion.                                                   

• Proof of Newton's First law of motion from the Second Law. 

(j) Third Law of Motion                                                         

• Law of conservation of Momentum

Introduction to Force

A force is an effort that changes the state of an object at rest or in motion. It can change an object’s direction and velocity. Force can also change the shape of an object. For example, to open a door, we push or pull. A drawer is pulled open and pushed closed.

Effect of Force

1. Force can move a stationary body or object.

For example, a football can be set to move by kicking it i.e., by applying a force.

2. Force can stop a moving body.                                                                       

For example, applying the brakes can stop a running cycle or a vehicle in motion.

3. Force can change the direction of a moving object.

For example, by applying force, i.e., by moving the handle, the direction of a running bicycle can be changed. Similarly, by moving the steering, the direction of a running vehicle is changed.

4. Force can change the speed of a moving body.

For example, by accelerating, the speed of a running vehicle can be increased, or by applying brakes, the speed of a running vehicle can be decreased.

5. Force can change the shape and size of an object.                                      

For example, by hammering, a block of metal can be turned into a thin sheet. By hammering, a stone can be broken into pieces.

6. Push is defined as an action of force that causes an object to move from its place.                                                                                                           

Examples of push include opening and closing the door, pushing the table, pushing a car, pushing thumb pins, and walking.

7. Pull is defined as an action to make something move by either tugging or dragging.                                                                                                     

Examples of pull: Plucking the string of a guitar, pulling ropes while playing tug of war, opening the drawer, pulling the window curtain, and Opening and closing the doors.

Forces are mainly of two types:

(i) Balanced forces

(ii) Unbalanced forces

Balanced Forces

If the resultant of applied forces is equal to zero, it is called balanced forces.

Example: In the tug of war, if both teams apply similar magnitudes of forces in opposite directions, the rope does not move on either side.

This happens because the balanced forces in which the resultant of applied forces becomes zero.

For example, when forces are applied from both sides to a balloon, the size and shape of the balloon are changed.

Balanced forces do not cause any change in the state of an object. Balanced forces are equal in magnitude and opposite in direction.                          

Balanced forces can change the shape and size of an object.

Unbalanced Forces

If the resultant of applied forces is greater than zero, the forces are called unbalanced forces.

Unbalanced forces can do the following:                                                                  

1. Move a stationary object.                                                                                      

2. Increase the speed of a moving object.                                                                

3. Decrease the speed of a moving object.                                                               

4. Stop a moving object.                                                                                            

5. Change the shape and size of an object

Net Force

When multiple forces act on a body, they can be resolved into one component known as the net force acting on the object. The net force decides the direction of motion.

Frictional Force

The force that opposes relative motion is called friction. It arises between the surfaces in contact.                                                                                       

Example: When we try to push a table and it does not move is because it is balanced by the frictional force.

Laws of Motion

Galileo Galilei: Galileo first of all said that objects move with a constant speed when no forces act on them. This means if an object is moving on a frictionless path and no other force is acting upon it, the object would be moving forever. That is, there is no unbalanced force working on the object. But practically, it is not possible for any object. To attain the condition of zero, an unbalanced force is impossible. The force of friction, the force of air, and many other forces are always acting upon an object.

 Newton’s Laws of Motion

Newton studied the ideas of Galileo and gave the three laws of motion. These laws are known as Newton’s laws of motion.

Newton’s First Law of Motion (Law of Inertia)

Any object remains in the state of rest or in uniform motion along a straight line, until it is compelled to change the state by applying an external force.

Inertia

Basically, all objects have a tendency to resist change in their state of motion or rest. This tendency is called inertia. All bodies do not have the same inertia. Inertia depends on the mass of a body. The mass of an object is the measure of its inertia.

The more mass → the more inertia and vice versa.

Newton’s First Law of Motion in Everyday Life

(i) A person standing in a bus falls backward when the bus starts moving suddenly.
 → This happens because the person and bus are both at rest while the bus is not moving, but as the bus starts moving, the legs of the person start moving along with the bus, but the rest of his body has the tendency to remain at rest. Because of this, the person falls backward if he is not alert.

(ii) A person standing in a moving bus falls forward if the driver suddenly applies the brakes.
 → This happens because when the bus is moving, the person standing in it is also in motion along with the bus. But when the driver applies the brakes, the speed of the bus decreases suddenly, or the bus comes to a state of rest suddenly. In this condition, the legs of the person who is in contact with the bus come to rest while the rest of his body has the tendency to remain in motion. Because of this person falls forward if he is not alert.
 → This happens because when clothes are suddenly made in motion by giving jerks, the water droplets in it have the tendency to remain at rest and they are separated from the clothes and fall on the ground.
 → This happens because when the pile is struck with a striker, the coin at the bottom comes into motion while the rest of the coin in the pile has the tendency to remain in the rest and they vertically fall onto the carom-board and remain in the same place.

(iii) Before hanging the wet clothes over the laundry line, usually many jerks are given to the clothes to get them dry quickly. Because of the jerks, droplets of water from the pores of the cloth fall on the ground and reduce the amount of water in clothes, drying them quickly.

(iv) When the pile of coins on the carom board is hit by a striker, only the bottom moves away, leaving the rest of the pile of coins in the same place.

Inertia of Rest

An object stays at rest, and it remains at rest until an external force affects it. Example: When a car accelerates, passengers may feel as though their bodies are moving backwards. In reality, inertia is making their bodies stay in place as the car moves forward.

 Inertia of Motion

An object will continue to be in motion until a force acts on it. Example: A hockey puck will continue to slide across the ice until acted upon by an outside force.

Mass and Inertia

→ The property of an object because of which it resists being disturbed in its state is called inertia.
 → The inertia of an object is measured by its mass. Inertia is directly proportional to the mass. This means inertia increases with an increase in mass and decreases with a decrease in mass.
 → A heavy object will have more inertia than the lighter one. In other words, the natural tendency of an object that resists a change in state of motion or rest of the object is called inertia.
 → Since a heavy object has more inertia, it is difficult to push or pull a heavy box over the ground than a lighter one.
 



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