The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

You must have experienced that you need to apply force to the pedal whenever you want the cycle to move ahead. Here, in this case, force is required to set the cycle in motion. **Laws of motion** will help us in understanding why the object moves or remains still. Laws of motion also tells us why don’t we float through over bed or fall through the floor in our homes.

**Contact and Non-contact forces** are different in their fundamental nature. Let us talk about them in more detail.

A force that comes into action when two or more objects are in contact is called Contact forces. For example, you are pushing a book kept on the table, and here the book moves because of the applied force when it comes in contact with you.

Non-contact forces come into action even when the objects are not in contact with each other. For example, a ball is freely falling on the ground due to the Earth’s gravitational pull.

**Newton’s First Law of motion** states that,

- A body will continue to be in a state of rest if no external force is applied.
- A body will continue to be in a state of motion without changing its direction unless no external force is applied to it.

For example, if a book is kept on the table, it rests in the same position until we apply force. Similarly, the cycle continues to move on a frictionless road unless we use brakes or a type of force to stop it.

**Newton’s First Law of motion** is divided into the below parts.

**Inertia****Force**- In the previous examples, the brakes applied to the moving vehicle are applied to change the moving car’s state.

When we apply force on an object, its acceleration changes, and so does its velocity. It brings about a change in the momentum of the object.

**Newton’s Second Law of Motion** describes that the momentum changes rate is proportional to the quantity of force applied to the object. Also, the direction in which the change occurs is the same as the applied force’s direction.

**Newton’s Second Law of Motion **is mathematically written as,

It implies that momentum can change when the object’s mass changes or when its velocity changes, or both.

**Newton’s Third Law of Motion** describes that there is an equal and opposite reaction to every action.

For example, when a book is kept on the table, it exerts its weight(action)on the table. At the same time, the table applies an equal and opposite force(reaction). As a result of which, the book remains on the table.

The attractive force present between two particles owing to their mass is called the Gravitational force of attraction.

The **Universal Law of Gravitation** describes that the gravitational force acting between the two particles is proportional to the product of their mass. It is inversely proportional to the square of the distance between them.

Consider two particles of mass m1 and mass m2 separated by a distance r. Let F be the attractive force acting between them.

According to the **Universal Law of Gravitation**,

&

where G is called the Universal constant of **Gravitation=**6.67 10-11N m2kg-2

The three **Laws of Motion** will help us understand how an object moves and stops by applying force. This section has also helped us understand the attractive force between any two particles at a finite distance.

- Derive
**Newton’s First Law of Motion**from the Second Law of Motion - From
**Newton’s Second Law of Motion**, -
**F = ma** - a = 0
- It means if no force is applied, the object will continue to be at rest, i.e., a=0.
- Explain how the gun follows
**Newton’s Third Law of Motion**. - When we fire a bullet from a weapon-like gun, in this case, the gun applies a Force F on the shell (action), and at the same time, our shoulder recoils because of the weapon’s reactive force.
- What is the S.I and C.G.S unit for force?
- The S.I unit for Force is Newton(N), and the C.G.S. unit is dyne.
- Derive an expression for the rate of change of momentum?
- Let the initial velocity of the object be “u.” Let F be the force applied on it due to which its velocity changes to “v.” Let “m” be used to denote the mass of an object.
- Rate of change of momentum =
- Rate of change of momentum =
- Rate of change of momentum =
- We know, acceleration =
- Rate of change of momentum=ma
- Why does the bicycle stop only after covering a longer distance, even after we stop pedalling, on a smooth road with zero friction?
- According to
**Newton’s** **First Law of motion**, the cycle will continue to be in the state of motion unless an external force(frictional force=0 in this case)is applied. So it covers some distance before it stops after we stop pedalling.