 # NCERT Solutions for Class 9 Physics Chapter 9 Free PDF Download

## NCERT Solutions for Class 9 Physics Chapter 9 – Force and Laws of Motion

Physics is the most essential part of science whose aim is to explain the behavior of the universe. And, it is an important part of class 9 Science. In this NCERT Solutions for Class 9 Physics Chapter 9, we are discussing the various aspects of Force and Law of Motion. Apart from this, we have explained each and every topic in a detailed easy language for easier understanding.

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### CBSE Class 9 Physics Chapter 9 – Force and Law of Motion NCERT Solutions

Generally, force refers to strength or energy. But, in Physics terms, force refers to the contact of an object that causes a movement change in its velocity. And, the law of motion refers to the three laws defined by Sir Isaac Newton.

Introduction- This topic tells what we have discussed in the previous chapter and what we are going to discuss in this chapter. Apart from that, it describes what is a force and how it works.

### 9.1 Balanced and Unbalanced force

This topic describes balanced and unbalanced forces.

• Balanced Force- When we apply an equal force on an object that does not cause any movement in it is a balanced force.
• Unbalanced Force- When an object move in a direction on applying unequal force from a different direction then it is an unbalanced force.

#### 9.2 First Law of Motion

Galileo first proposed the first law of motion but Sir Isaac Newton corrected it. According to Newton, an object remains in the state of rest or uniform motion in a straight line until it is compelled with an external force.

In simple words, all objects resist a change in their state of motion.

#### 9.3 Inertia and Mass

Inertia refers to the resistance that an object offers to remain in its current state of movement or rest. While on the other hand, mass refers to the weight of an object which decides its inertia.

#### 9.4 Second Law of Motion

The second law of motion states that the change of momentum of an object is proportional to the applied unbalanced force in the direction of the force.

9.4.1 Mathematical Formula of Second Law of Motion- This topic describes the mathematical formula of the second law of motion after defining different things related to it.

Change in momentum ∝ p2-p1
∝ mv – mu
∝ m × (v – u)
The rate of change of momentum = $$\frac{m×(v – u)}{t}$$
Or the applied force,
Force ∝ $$\frac{m×(v – u)}{t}$$
F = $$\frac{km×(v – u)}{t}$$
F = kma

#### 9.5 Third Law of Motion

According to this law, every action has an opposite and equal reaction. Also, the action and reaction are equal in magnitude.

#### 9.6 Conservation of momentum

This law tells that the momentum of two objects before and after collision remains the same. But the objects will move in the direction in which the heavier object was moving.

Along with every topic, there are several activities in the chapter for you to try.

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