Force – TNPSC Study Material

Force is one of the important concepts in Physics and forms the basis for understanding many everyday physical phenomena. This TNPSC study material explains the fundamental concepts of force in a simple and structured manner, covering its effects, types of forces, balanced and unbalanced forces, Newton’s Laws of Motion, inertia, momentum, friction, gravitational force, mass and weight, and the everyday applications of force. This Free Notes also include important formulas, units, key facts, and points useful for TNPSC General Science preparation.

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👉🏻Force – TNPSC Physics Test Series

1. Introduction to Force

Force is a push or pull that acts on an object. It arises due to an interaction between objects and can change the state of rest or motion of an object. For example: * pushing a door, * pulling a box, * kicking a ball or *squeezing a lemon involves the application of force.

A force can produce different effects on an object. It can make a stationary object move, change the speed or direction of a moving object, or change the shape of an object. The effect of a force depends on its magnitude and direction.

The SI unit of force is the newton (N), named after Sir Isaac Newton. The magnitude of a force indicates its strength. The relationship between force, mass and acceleration is expressed by the formula:

F = ma , Where

• F = Force
• m = Mass
• a = Acceleration

Thus, force is related to the mass of an object and the acceleration produced in it.

Force is a vector quantity, which means it has both magnitude and direction. Therefore, both the strength and direction of a force are important when describing its effect on an object. For example, pushing an object towards the right and pushing it towards the left involve forces acting in different directions.

Key Points To Remember – Introduction to Force

• Force is a push or pull acting on an object.
• Force results from the interaction between objects.
• Force can change the state of rest or motion of an object.
• Force can change the speed, direction or shape of an object.
• The SI unit of force is the newton (N).
• Force is a vector quantity.
• The relationship between force, mass and acceleration is expressed as F = ma.

2. Effects of Force

Force plays an important role in changing the condition of an object. Depending on how and where it is applied, a force can produce changes in the motion, speed, direction or shape of an object. Here are some examples:

• ➡️ Changes in motion: Consider a football lying on the ground. When it is kicked, the applied force sets the ball in motion.
• ➡️ Changes in speed: A force applied to a moving object can slow it down or bring it to rest. Applying brakes to a moving bicycle is a familiar example of this effect.
• ➡️ Changes in direction: When a force acts on a moving object, its direction can change. A cricket ball travelling towards a batsman changes its direction when struck by the bat.
• ➡️ Changes in shape: Force can also alter the shape or size of an object. Stretching a rubber band, compressing a sponge and bending a wire are simple examples of this effect.

The important effects of force to remember:

• It can make a stationary object move.
• It can stop a moving object.
• It can increase or decrease the speed of an object.
• It can change the direction of motion.
• It can change the shape or size of an object.

The effects related to speed, direction and motion are closely connected with the concepts covered in our Motion – TNPSC Study Material, which can be referred to for a detailed explanation.

3. Types of Forces

Forces can be broadly classified according to whether physical contact between the objects is required for the force to act. On this basis, forces are commonly divided into two main types: contact forces and non-contact forces. This classification is useful for understanding how different forces act on objects in everyday situations.

• ➡️ Contact Forces: These forces act only when two objects are in physical contact with each other. The force applied while pushing a table or pulling a cart is a contact force. Muscular force and frictional force are common examples.
• ➡️ Non-Contact Forces: These forces can act between objects without direct physical contact. Gravitational force, magnetic force and electrostatic force are important examples. For instance, the Earth attracts objects towards it through gravitational force even though there is no direct contact between the Earth and the falling object.

The main types of forces relevant to basic science and TNPSC preparation can be summarized as follows:

Understanding the distinction between contact and non-contact forces is important because many questions in basic mechanics ask how a particular force acts and whether physical contact is necessary. TNPSC’s General Science syllabus includes Force within the broader area of mechanics and its everyday applications.

4. Balanced and Unbalanced Forces

When two or more forces act on an object, their combined effect depends on their magnitude and direction. Based on the net effect of the forces, they are classified as balanced forces or unbalanced forces.

• ➡️ Balanced Forces: When two or more forces acting on an object are equal in magnitude and opposite in direction, they balance each other. The net force is zero, so there is no change in the state of motion of the object. For example, when two people push a box with equal forces in opposite directions, the box does not move.
• ➡️ Unbalanced Forces: When the forces acting on an object are not equal or do not cancel each other, the forces are unbalanced. The net force is not zero, and the object’s state of motion can change. It may start moving, stop, speed up, slow down or change its direction.

It is important to remember that balanced forces do not always mean that an object is at rest. An object already moving with constant speed in a straight line can continue moving in the same way when the forces acting on it are balanced.

Quick Recall on Balanced and Unbalanced Forces

• Balanced forces have a zero net force.
• Unbalanced forces have a non-zero net force.
• Balanced forces do not change the state of motion.
• Unbalanced forces can change the speed or direction of motion.
• A change in motion occurs when there is a net unbalanced force acting on an object.

5. Newton’s Laws of Motion

The motion of objects is governed by certain basic principles described by Sir Isaac Newton. These principles are known as Newton’s laws of motion. They explain how a force affects the motion of an object and form an important part of the study of mechanics. The three laws are Newton’s first law, second law and third law of motion.

Newton’s First Law of Motion

An object remains at rest, or continues to move with uniform velocity in a straight line, unless an external unbalanced force acts on it. This property of an object to resist any change in its state of rest or motion is called inertia.

For example, when a moving bus stops suddenly, passengers tend to move forward because their bodies tend to continue in their state of motion. Similarly, when a bus starts suddenly, passengers may tend to move backwards due to inertia.

Newton’s Second Law of Motion

The second law describes the relationship between the force acting on an object, its mass and the acceleration produced. The acceleration of an object depends on the net force acting on it and its mass. For a given mass, a greater force produces greater acceleration.

The mathematical expression commonly used at this level is: F = ma, where:

• F = Force
• m = Mass
• a = Acceleration

The SI unit of force is the newton (N). One newton is the force that produces an acceleration of 1 m/s² in a mass of 1 kg.

For example, it is easier to accelerate an empty cart than a heavily loaded cart with the same applied force because the loaded cart has greater mass.

Newton’s Third Law of Motion

For every action, there is an equal and opposite reaction. The two forces always act on different objects. Therefore, the action and reaction forces do not cancel each other because they act on different objects.

Walking is a simple example. When a person pushes the ground backwards with the feet, the ground exerts a force on the person in the opposite direction, helping the person move forward. Similarly, a swimmer pushes water backwards and the water provides a forward force on the swimmer.

Newton’s three laws together provide the basic framework for understanding the relationship between force and motion. They are also useful for explaining many events in everyday life and are therefore important for TNPSC preparation.

Laws of Motion TNPSC Quick Recall

• First law → explains inertia.
• Second law → relates force, mass and acceleration.
• Third law → explains action and reaction.
• The SI unit of force is newton (N).
• Action and reaction forces are equal in magnitude and opposite in direction, but they act on different objects.

Internal Link: The detailed concepts of motion, velocity and acceleration are covered in our Motion – TNPSC Study Material.

6. Inertia

Every object tends to resist a change in its state of rest or motion. This property of an object is called inertia. An object at rest tends to remain at rest, while a moving object tends to continue moving with the same velocity unless an external unbalanced force acts on it. Inertia is therefore closely related to Newton’s First Law of Motion.

There are three common types of inertia:

• ➡️ Inertia of Rest: An object at rest tends to remain at rest. For example, when a stationary bus starts suddenly, passengers may fall backwards because their bodies tend to remain in the state of rest.
• ➡️ Inertia of Motion: A moving object tends to continue moving. For example, when a moving bus stops suddenly, passengers may move forward because their bodies tend to continue in motion.
• ➡️ Inertia of Direction: A moving object tends to continue moving in the same direction. For example, passengers in a moving bus may lean towards one side when the bus takes a sudden turn.

The amount of inertia depends on the mass of an object. An object with greater mass has greater inertia and therefore offers greater resistance to a change in its state of motion. For example, it is more difficult to change the motion of a loaded truck than that of a bicycle.

Quick Recall of Inertia

• Inertia is the tendency of an object to resist a change in its state of rest or motion.
• Inertia is explained by Newton’s First Law of Motion.
• The three types are inertia of rest, inertia of motion and inertia of direction.
• Mass is a measure of the inertia of an object.
• Greater mass means greater inertia.

7. Momentum

Momentum is a measure of the motion possessed by a moving object. It depends on the mass of the object and its velocity. A heavier object moving at the same velocity as a lighter object has greater momentum. Similarly, an object moving at a higher velocity has greater momentum when its mass remains the same.

The momentum of an object is calculated using the formula: p = mv, where:

• p = Momentum
• m = Mass
• v = Velocity

The SI unit of momentum is kilogram metre per second (kg m/s). Momentum is a vector quantity because velocity is a vector quantity, so momentum has both magnitude and direction.

For example, a moving truck has considerable momentum because of its large mass. A small ball moving at high speed can also have significant momentum because of its velocity. Changing the mass or velocity of an object changes its momentum.

Momentum is closely related to Newton’s Second Law of Motion and is also important in understanding the conservation of momentum, which we will discuss in the next section.

Quick Recall on Momentum

• Momentum is the product of mass and velocity.
• Formula: p = mv
• SI unit: kg m/s
• Momentum is a vector quantity.
• Greater mass or greater velocity results in greater momentum, when the other quantity remains constant.
• Momentum is an important concept in understanding collisions and conservation of momentum.

8. Conservation of Momentum

When two or more objects interact with each other, their individual momenta may change, but the total momentum of the system remains unchanged if no external force acts on it. This principle is known as the law of conservation of momentum.

For example, when two billiard balls collide, the momentum of each ball may change because of the collision. However, the total momentum of both balls before the collision is equal to their total momentum after the collision, provided no external force significantly affects the system.

For two objects, the law can be expressed as:

Total momentum before collision = Total momentum after collision

If the masses and velocities of two objects before and after a collision are represented by m₁, v₁, m₂, v₂ and m₁, v₁′, m₂, v₂′ respectively, then:

m₁v₁ + m₂v₂ = m₁v₁′ + m₂v₂′

The law of conservation of momentum is useful in understanding collisions and other interactions between objects. It is also an important application of Newton’s laws of motion.

Examples in everyday life:

• Recoil of a gun when a bullet is fired.
• Movement of a rocket as gases are expelled backwards.
• Collision between two moving vehicles or balls.
• A person jumping from a boat causing the boat to move in the opposite direction.

Remember These Points :

• The total momentum of an isolated system remains constant.
• Momentum before interaction = Momentum after interaction.
• Conservation of momentum applies when no external force acts on the system.
• It is commonly used to explain collisions, recoil and rocket motion.
• Momentum is conserved even though the momentum of individual objects may change.

9. Force, Mass and Acceleration

The relationship between force, mass and acceleration is explained by Newton’s Second Law of Motion. When a net force acts on an object, it produces acceleration in the direction of the net force. The amount of acceleration depends on both the applied net force and the mass of the object.

The relationship is expressed by the formula: F = ma, Where

• F = Net force
• m = Mass of the object
• a = Acceleration produced

For a given mass, increasing the net force increases the acceleration. For example, pushing a trolley with greater force makes it accelerate more rapidly. On the other hand, when the same force acts on objects of different masses, the object with greater mass produces less acceleration.

The formula can also be rearranged to calculate mass or acceleration:

a = F/m

m = F/a

The SI unit of force is the newton (N). One newton is the force required to produce an acceleration of 1 m/s² in an object of mass 1 kg.

TNPSC Points to Remeber:

• Newton’s Second Law gives the relationship between force, mass and acceleration.
• Formula: F = ma
• For the same mass, greater force produces greater acceleration.
• For the same force, greater mass produces smaller acceleration.
• The acceleration is in the direction of the net force.
• SI unit of force: newton (N).

10. Action and Reaction

When two objects interact, they exert forces on each other. According to Newton’s Third Law of Motion, for every action, there is an equal and opposite reaction. The two forces are equal in magnitude and opposite in direction, but they act on different objects.

For example, when a person pushes a wall, the person applies a force on the wall. At the same time, the wall exerts an equal force in the opposite direction on the person.

Similarly, while walking, a person pushes the ground backwards with the feet, and the ground exerts a force on the person in the forward direction.

The action and reaction forces do not cancel each other because they act on different objects. This is an important point to remember when applying Newton’s Third Law.

Examples of Action and Reaction

• ➡️ Walking: The foot pushes the ground backwards, and the ground pushes the person forwards.
• ➡️ Swimming: A swimmer pushes water backwards, and the water exerts a forward force on the swimmer.
• ➡️ Rocket motion: The rocket pushes gases downwards, and the gases exert an upward force on the rocket.
• ➡️ Jumping: A person pushes the ground downwards, and the ground pushes the person upwards.

11. Frictional Force

Friction is a force that opposes the relative motion, or the tendency of relative motion, between two surfaces in contact. It acts along the surfaces in contact and generally in the direction opposite to the motion.

Friction is useful in many everyday activities. It provides the grip needed for walking and helps vehicles move, turn and stop safely. We can also write with a pencil because of friction between the pencil and paper.

However, friction can also have undesirable effects. It causes wear and tear of surfaces and produces heat when surfaces rub against each other. In machines, lubricants such as oil and grease are used to reduce friction between moving parts.

Depending on the type of motion between the surfaces, friction is commonly classified into static friction, sliding friction and rolling friction. Static friction acts when there is no relative motion between the surfaces, while sliding friction acts when one surface slides over another. Rolling friction acts when an object rolls over a surface and is generally less than sliding friction.

Examples of Friction in Daily Life

• ➡️ Friction between our feet and the ground helps us walk.
• ➡️ Friction between vehicle tyres and the road helps vehicles move and stop.
• ➡️ Friction between a pencil and paper helps us write.
• ➡️ Friction in machine parts can produce heat and cause wear.
• ➡️ Oil and grease are used to reduce friction in moving machine parts.

Quick Recall of Friction

• Friction opposes relative motion or the tendency of relative motion between surfaces in contact.
• It acts along the surfaces in contact.
• Friction is useful for walking, writing and vehicle movement.
• Excessive friction causes heat, wear and tear.
• The common types are static, sliding and rolling friction.
• Rolling friction is generally less than sliding friction.

12. Gravitational Force, Mass and Weight

Gravitational force is the force of attraction between objects that have mass. The Earth attracts objects towards its centre because of gravity. This force is responsible for keeping objects on the ground and causing objects to fall towards the Earth when they are released.

The gravitational force between two objects depends on their masses and the distance between them. Greater masses produce a greater gravitational attraction, while increasing the distance between the objects reduces the gravitational force.

Near the surface of the Earth, the gravitational force acting on an object is called its weight.

Weight can be calculated using the formula:

Weight = Mass × Acceleration due to gravity

W = mg

where W is weight, m is mass and g is the acceleration due to gravity. The approximate value of g near the Earth’s surface is 9.8 m/s².

Mass and weight are different physical quantities. Mass is the amount of matter in an object and remains the same when the object is moved from one place to another. Weight depends on the gravitational field and can therefore change from one place to another. For example, an object weighs less on the Moon because the Moon’s gravitational acceleration is much lower than that of the Earth.

Remember These Points

• Gravitational force is a force of attraction between objects having mass.
• The Earth attracts objects towards its centre.
• Weight is the gravitational force acting on an object.
• Formula: W = mg
• SI unit of mass: kilogram (kg)
• SI unit of weight: newton (N)
• Mass remains constant, while weight can change with the gravitational field.
• The approximate value of g on Earth is 9.8 m/s².

13. Everyday Applications of Force

Force is involved in many activities we perform in our daily lives. We use force whenever we push, pull, lift, stretch, bend, stop or move an object. Many common actions can be understood using the basic principles of force.

Some everyday applications of force include:

• ➡️ Opening and closing a door: Pushing or pulling the door applies force and changes its position.
• ➡️ Walking: We push the ground backwards with our feet, and the frictional force from the ground helps us move forward.
• ➡️ Riding a bicycle: Force applied to the pedals helps turn the wheels and move the bicycle forward.
• ➡️ Lifting an object: Muscular force is used to lift an object against the force of gravity.
• ➡️ Stopping a vehicle: Brakes produce friction that opposes the motion of the wheels and helps slow down or stop the vehicle.
• ➡️ Playing sports: Kicking a football or hitting a cricket ball with a bat changes its motion, speed or direction.
• ➡️ Using a hammer: The force applied by a hammer can drive a nail into wood.
• ➡️ Stretching a rubber band: Applying force changes the shape and length of the rubber band.

These examples show that force is closely connected with movement and many other physical changes around us. Understanding the effects and types of force helps us explain several simple activities in everyday life.

14. Important Formulae, Units and Facts

The following formulae, units and facts are useful for quick revision of the important concepts related to force. These are particularly helpful for TNPSC examinations, where direct questions may be asked from basic Physics concepts.

Important Facts

• Force is a vector quantity and has both magnitude and direction.
• Momentum is the product of mass and velocity.
• Weight is the gravitational force acting on an object.
• Mass is measured in kilogram (kg), while weight is measured in newton (N).
• Newton’s First Law of Motion is associated with inertia.
• Newton’s Second Law gives the relationship F = ma.
• Newton’s Third Law states that every action has an equal and opposite reaction.
• The total momentum of an isolated system remains constant when no external force acts on it.
• Friction opposes relative motion between surfaces in contact.
• Gravitational force is a non-contact force.

15. Important Points for TNPSC – Quick Revision for Force

• Force is a push or pull that can change the state of rest or motion of an object.
• Force is a vector quantity because it has both magnitude and direction.
• The SI unit of force is the newton (N).
• Newton’s First Law of Motion explains the concept of inertia.
• Inertia is the tendency of an object to resist a change in its state of rest or motion.
• The inertia of an object depends on its mass. Greater mass means greater inertia. Newton’s Second Law gives the relationship between force, mass and acceleration: F = ma.
• Newton’s Third Law states that for every action, there is an equal and opposite reaction.
• Action and reaction forces act on different objects.
• Momentum is the product of mass and velocity: p = mv.
• The SI unit of momentum is kg m/s.
• The law of conservation of momentum states that the total momentum of an isolated system remains constant when no external force acts on it.
• Friction opposes relative motion between surfaces in contact.
• Gravitational force is a non-contact force that acts between objects having mass.
• Weight is the gravitational force acting on an object and is given by W = mg.
• Mass is measured in kilogram (kg), whereas weight is measured in newton (N).
• Contact forces require physical contact between objects, while non-contact forces can act without direct contact.
• Balanced forces have zero net force, while unbalanced forces have a non-zero net force.
• An unbalanced force can change the speed or direction of an object’s motion.
• Force has many applications in everyday activities such as walking, cycling, lifting objects and stopping vehicles.

Further Reference – Force

Students who want to study the concepts of force and Newton’s Laws of Motion in greater detail can refer to the official NCERT textbook chapter.

👉 NCERT Textbook – How Forces Affect Motion (Chapter 6) – Read the official NCERT chapter

This is especially useful for understanding force, balanced and unbalanced forces, friction, and Newton’s Laws of Motion with examples and activities.