Table of Contents

## Force

A body needs a** ‘push’ or ‘pull’** to move or to bring a moving body to rest or change its velocity. Hence this push or pull is called force. Force has both magnitude and direction. Therefore force is a **vector** quantity.

### Definition of force in physics

### Types of Forces

Force is classified into types as:

**Like parallel force**– Two or more forces of equal or unequal magnitude acting along the same direction, parallel to each other.**Unlike parallel force**– Two or more forces either equal or unequal forces act in opposite directions parallel to each other.

**Unit of Force: **SI unit of force is newton(N) and in the C.G.S system its unit is ** dyne**.

**Definition of 1 newton (N)** – The amount of force required for a body of mass 1 kg to produce an acceleration of *1* kg⋅m/s^{2}.

1 N = 1 kg m/s^{2}

**Dyne**

One dyne is the amount of force required for a body of mass 1 gram produces an acceleration of 1 cm/s^{-2}.

1 dyne = 1 g cm/s^{-2} ; also 1 N = 10^{5 }dyne.

**Unit Force**

The amount of force required to produce an acceleration of 1 ms^{-2}

in a body of mass 1 kg is called ‘**unit force**’.

**Gravitational unit of force**

The gravitational unit of force is kilogram-force, represented by kg f in SI units. In the C.G.S system, its unit is gram force, represented by gf.

1 kg f = 1 kg × 9.8 ms^{-2} = 9.8 N;

1 g f = 1 g × 980 ms^{-2} = 980 dyne

Issac Newton formulated three laws of motion.

- Newtons 1st Law
- Newtons 2nd Law
- Newtons 3nd Law

These three are discussed later in the notes.

### Mechanics

Mechanics is a branch of physics that studies the effect of force on bodies. It is divided into **Statics** and **dynamics**.

- Static – Static deals with the bodies that deal with the bodies that are at rest under the action of force.
- Dynamics – Dynamics deals with moving bodies under the action of forces. Dynamics is further divided into
**Kinematics**and**Kinetics**- Kinematics – Kinematics studies the motion of bodies without considering the cause of movement.
- Kinetics – Kinetics studies the motion of bodies considering the cause of movement.

## Inertia

When a force is applied to bodies, they resist change in their state. This property is called Inertia.

Example: We tend to move forward when the bus suddenly stops. Or We move backwards when the bike starts suddenly.

**Inertia** is defined as the interesting property of a body to resist the state of rest state of uniform motion unless it is influenced by an external unbalanced force.

### Types of Inertia

**Inertia of rest**– The resistance of a body to change its state of rest is known as**Inertia of rest**.**Inertia of motion**– The resistance of a body to change its state of motion is called**Inertia of motion**.**Inertia of direction**– It is the resistance of a body to change its direction of motion is called**Inertia of direction**.

## Linear Momentum

Linear momentum measures the impact of force on a body. The impact of a force is more when the velocity and the mass of the body are more.

*The product of mass and velocity of a moving body provides the magnitude of linear momentum. It acts in the direction of the velocity of the object.*

*Linear Momentum = Mass x Velocity*

That is: ** p = mv**. It helps to measure the magnitude of a force.

The unit of momentum in the SI system is** kg ms ^{-1}**and in the C.G.S system, its unit is

**g cms**.

^{-1}## Newton’s Laws of Motion

### Newton’s First Law of Motion

This law states that “*a body continues to be in its state of rest or state of motion along a straight line unless an external force at act on it”.*

This gives the definition of force as well as inertia.

### Newton’s Second Law of Motion

According to this law,” *The force acting on a body is directly proportional to the rate of change of linear momentum of the body. And the change in momentum takes place in the direction of the force.”*

This law is used to measure the amount of force. So this is also called as ‘** law of force**‘.

**F = m x a**

**Force = mass x acceleration**

### Newton’s Third Law of Motion

This law states that ‘*for every action,* there is an equal and opposite reaction ‘*.*

Example: Recoil of Gun, Swiming, Birds flying etc.

## Principal of Conservation of Linear Momentum

As per this principle, there is no change in the linear momentum of a system of bodies as long as no net external force acts on it.

## Moment of Force

The rotating effect of a force about a fixed point or fixed axis is called the moment of the force about that point or torque (τ). Torque is a vector quantity and its SI unit is N m.

The torque is measured by-product of the force (F) and the perpendicular distance (d) between the fixed point or axis and the line of action of the force.

**τ = F × d**

### Couple

When two equal and unlike parallel forces are applied simultaneously at two distinct points, is a **couple**.

The line of action of the two forces does not coincide and does not produce translator motion. This is because the resultant is zero. But a **couple** causes the rotation of the body.

This rotating effect of a **couple **is called the **moment of a couple**. The unit of the moment of the couple is Newton metre (N m) in the SI system and dyne cm in the C.G.S system.

Example: Turning of Tap, winding a Screw, Spinning of top

**Moment of a couple** = Force × perpendicular distance between the

line of action of forces

M = F x S

## Conclusion

This article is for Tnpsc, for the topic ‘**Force**‘. This article provides a brief description of basic concepts of force in Physics. For detailed notes please refer to the Samacheer Kalvi book, Science 10th Std, Unit-1, Laws of Motion.