Force and Laws of Motion Class 9th Notes - Free NCERT Class 9 Science Chapter 9 Notes - Download PDF

Ever wondered why you get pushed back when a bus suddenly starts, or why a ball stops after rolling for some time? These everyday experiences are beautifully explained in Class 9 Science Chapter 8: Force and Laws of Motion. These NCERT Notes introduces you to the basic principles of motion and the laws given by Sir Isaac Newton . This chapter is important not just for your exams but also for building a strong foundation for JEE and NEET.

These NCERT Notes for Class 9 Chapter 8 Force and Laws of Motion explain the chapter in a simple and easy-to-understand way. You will learn important topics like how forces work, what inertia is, what causes things to move or stop, and Newton’s three laws of motion. These NCERT Notes for class 9 science include clear examples from daily life, helpful diagrams and all the important points based on the latest CBSE syllabus.

Also Read

• NCERT Solutions for Class 9 Science Chapter 9 Laws of Motion
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NCERT Class 9 Science Chapter 8 Notes

Force

Force is a push or pull on an object, it can change the velocity of an object, change its direction of motion or change the size or shape of the object.

Effect of Force

(i) It may change its state of rest or of uniform motion.
(ii) It may change its direction of motion.
(iii) It may change its shape.

Balanced and unbalanced forces

Balanced Force

When a number of forces acting simultaneously on a body do not bring about any change in its state of rest or of uniform motion along a straight line, then the forces acting on the body are said to be balanced forces. In this case different forces acting on a body gives zero resultant. Balanced forces do not produce any acceleration.

For example - When two opposite forces having the same magnitude $F$ act on a block placed on a smooth horizontal table, they fail to move the block.

This is because the net force is equal to zero. Similarly, two opposite forces having the same magnitude cannot change the speed of a moving body.

Unbalanced Force

When a number of forces acting simultaneously on a body bring about a change in its state of rest or of uniform motion along a straight line, then these forces acting on the body are said to be unbalanced forces. In this case, different forces acting on body do not give zero resultant. If an unbalanced force is applied on the object, there will be a change either in its speed/velocity or in the direction of its motion. Thus, to accelerate an object, an unbalanced force is required.

When net force on the body is not equal to zero, then the body at rest starts moving in the direction of resultant force.

If $F_2>F_1$, then $F_2-F_1>0$ as a result the car accelerates in the direction of $F_2$.

First law of motion

Galileo observed the motion of objects on an inclined plane and concluded that no net force is needed to sustain an object’s uniform motion.

The first law of motion states that until driven to change by an applied force, an object will stay in a condition of rest or uniform motion in a straight path.

The first law of motion states that when an object is subjected to an unbalanced external force, its velocity changes, or the object accelerates. Newton's first law of motion, often known as the law of inertia, emphasises objects' ability to resist change in velocity.

Inertia and mass

Inertia is the tendency of a body to maintain its state of rest or of uniform motion on its own or we can say that it is the inability of a body to change its state of rest or of uniform motion on its own.

Inertia and mass : Consider two bodies of unequal masses, say a table tennis ball and a cricket ball. If both balls are pushed with equal force for the same time, a cricket ball will have much smaller velocity as compared to the tennis ball. Cricket ball has resistance more than table tennis ball. Or, cricket ball has larger inertia than table tennis ball. So, we can generalise, a heavier body has a larger inertia than a lighter body. Same is true for objects in motion also. If you have to stop a lighter body and a heavy body moving with equal velocities, you may stop the lighter body with your hands (depending on its velocity), while the heavier one may slow down only a little (depending on its velocity).

The larger the mass, the larger is the inertia. Hence, we may say mass is a measure of inertia.

Second law of motion

Linear Momentum

It is defined as the quantity of motion contained in a body. It is measured as the product of mass of the body and its velocity and has the same direction as that of the velocity. It is a vector quantity. It is represented by $p$.
Linear momentum $(p)=$ mass $(m)\times$ velocity $(v)$
or

$p=mv$

The SI unit of linear momentum is $\mathrm{kg}\mathrm{m}/\mathrm{s}$.

Second law of motion: The rate of change of linear momentum of a body is directly proportional to the applied unbalanced external force and it is in the direction of the resultant force.

Consider a body of mass $m$ moving with some initial velocity $u$. If an unbalanced constant force is applied and it changes its velocity from $u$ to $v$ in time $t$, then the acceleration of the body

$a=\frac{\text{ change in velocity }}{\text{ time taken }}=\frac{v-u}{t}$

The initial linear momentum of the body, is

$p_i=mu$

and final linear momentum, $p_f=mv$

The change in linear momentum $=p_f-p_i$

$\begin{array}{rl}& =mv-mu\\ & =m(v-u)\end{array}$

Therefore, the rate of change of linear momentum $\frac{m(v-u)}{t}\propto$Applied unbalanced force(F)

$F\propto \frac{m[v-u]}{t}$

or $F\propto ma$
or $F=kma$
where $k$ is a constant of proportionality. The unit of force is so chosen that here the value of the constant $k$ becomes one.

The SI unit of force is newton represented by ' $N$ '.

$\therefore F=ma$

Application of Newton's Second Law of Motion

1. Cricket player lowers his hand while catching the ball: The player increases the time during which the high velocity of moving ball reduces to zero.
$F\times \mathrm{\Delta }t=m(v-u)$
$u$ is very large, $v=0$,
$m(v-u)$ is fixed.
If we increase $\mathrm{\Delta }t,F$ decreases, so the force of impact on the palm of the fielder reduces. Hence, he is able to catch the ball without injuring himself.

2. A karate player can break a pile of tiles with a single blow of his hand: Because he strikes the pile of tiles with his hand very fast, during which the entire linear momentum of the fast moving hand is reduced to zero in very short interval of time. This exerts a very large force on the pile of tiles which is sufficient to break them, by a single blow of his hand.

Third law of motion

When one item puts a force on another, the second object reacts by exerting a force on the first, according to the third law of motion. These two forces have the same magnitude however operate in opposing directions and on separate objects.

Applications of Third Law

1. Recoiling of a gun : When a bullet is fired from a gun, it exerts a forward force on the bullet and the bullet exerts an equal (in magnitude) and opposite (in direction) force on the gun.

2. To walk, we press the ground in backward direction with feet : When we walk on the ground, our feet push the ground backward and in return the ground pushes our feet forward.

3. Jet aeroplanes and rockets : In jet engines and rockets, the fuel is burnt to produce a large quantity of hot gases. These hot gases comes out of a nozzle with a great force (this is action). According to the third law of motion, the equal (in magnitude) and opposite (in direction) reaction pushes the jet planes and rockets upward with a great speed (this is reaction).

NCERT Solutions Of Class 9 Subject-Wise

• NCERT solutions for class 9 Maths
• NCERT solutions for class 9 Science

NCERT Class 9 Exemplar Solutions for Other Subjects:

• NCERT Exemplar Class 9 Maths solutions
• NCERT Exemplar Class 9 Science solutions

Class 10 Chapter Wise Notes