Newton’s Laws of Motion
Introduction:
Newton’s Laws of Motion are among the most important topics in physics for UPSC, SSC, RRB, State PSC, railway, police, defence, and other competitive examinations. These laws explain how objects move, stop, accelerate, and react when force is applied. They are not only fundamental in physics but also appear in daily life, making them easier to understand through practical examples.
For students preparing for exams, this chapter is highly scoring because the concepts are clear, direct, and frequently asked in objective questions. Questions are often framed from definitions, examples, numerical relations, units, and common misconceptions. A strong understanding of these laws also helps in related topics such as force, momentum, friction, equilibrium, and motion.
Table of Contents
What Is Motion?
Motion means a change in the position of an object with time. If an object changes its place relative to a reference point, it is said to be in motion. If it does not change its position, it is said to be at rest.
Motion can
be straight, circular, rotational, oscillatory, or irregular. However, Newton’s
Laws mainly help us understand how and why motion changes when forces act on an
object.
What is Force?
Force is a push or pull that can change the state of rest or motion of an object. It may also change the shape, size, or direction of motion of a body. In simple language, force is the cause, and motion changes because of that cause.
The unit of
force in the SI system is newton, written as N. One newton is the force
required to produce an acceleration of 1 m/s² in a body of mass 1 kg.
Who Gave Newton’s Laws?
Sir Isaac Newton, one of the greatest scientists in history, proposed the three laws of motion. These laws were published in his famous work Principia Mathematica in 1687. They became the foundation of classical mechanics.
Newton’s laws describe the relationship among force, mass, and acceleration. They remain valid for everyday life and for most ordinary objects moving at speeds much lower than the speed of light.
Newton’s First Law of Motion
Newton’s First Law is also called the Law of Inertia. It states that an object remains at rest or continues to move with uniform velocity in a straight line unless acted upon by an external unbalanced force.
This means a body does not change its state by itself. A body at rest will stay at rest, and a moving body will keep moving unless something forces it to change. This natural resistance to change is called inertia.
Meaning of Inertia
Inertia is the tendency of a body to resist any change in its state of rest or motion. The greater the mass of a body, the greater its inertia. That is why a heavy truck is harder to start, stop, or turn than a bicycle.
Inertia is not a force. It is a property of matter. Many students confuse inertia with force, but they are different. Force causes change; inertia resists change.
Examples of First Law
· When a bus suddenly starts, passengers tend to lean backward because their bodies want to remain at rest.
· When a moving bus suddenly stops, passengers lean forward because their bodies try to continue moving.
· A book lying on a table remains there until someone pushes it.
· A rolling football stops only after friction or another force acts on it.
· A spinning top keeps rotating for some time because of inertia of motion.
Exam Point
The First Law explains why seat belts are important in vehicles. During sudden braking, the body of the passenger continues moving forward due to inertia. The seat belt applies the necessary force to stop the body safely.
Types of Inertia
There are three main types of inertia:
1. Inertia of Rest
A body at rest tends to remain at rest unless acted upon by an external force.
Example: A coin placed on a card falls into a glass when the card is pulled quickly, because the coin tends to remain at rest.
2. Inertia of Motion
A moving body tends to continue moving with the same speed and direction unless acted upon by an external force.
Example: When a running horse stops suddenly, the rider falls forward.
3. Inertia of Direction
A body tends to continue moving in the same direction unless a force changes its direction.
Example: When a car takes a turn, passengers
are pushed sideways due to inertia of direction.
Newton’s Second Law of Motion
Newton’s Second Law states that the rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of that force. In simple form, it can be written as:
F=ma
Here, F is Force, m is Mass & a is Acceleration
This law is one of the most important in physics because it gives a direct mathematical relation between force and motion. It tells us that if mass increases, more force is needed to produce the same acceleration. If force increases, acceleration also increases.
Momentum and the Second Law
Momentum is the product of mass and velocity. It is written as:
p=mv
Newton’s Second Law can also be stated as force equals the rate of change of momentum. This means force depends on how quickly the momentum of a body changes.
This is very useful in understanding collisions, sports, brakes, and machine motion. In competitive exams, the force-momentum relation is often asked along with the formula
Examples of Second Law
· It is easier to push an empty trolley than a loaded trolley.
· A cricket ball can be thrown farther with a stronger force.
· A lighter bicycle accelerates more easily than a heavy truck when the same force is applied.
· A goalkeeper catches a fast ball by moving his hands backward, increasing the time of impact and reducing force.
· In car braking, greater momentum needs a larger stopping force.
Practical Understanding
Suppose two objects have different masses. If the same force is applied to both, the lighter object will accelerate more. That is why a small scooter picks up speed faster than a fully loaded truck.
This law also explains why force
must be greater when the body is heavier. It is one of the most common ideas
tested in MCQs.
Newton’s Third Law of Motion
Newton’s Third Law states that for every action, there is an equal and opposite reaction. This means forces always occur in pairs. If one body exerts a force on another body, the second body exerts an equal force back on the first body in the opposite direction.
The action and reaction forces act on different bodies, not on the same body. This is a very important concept because many students wrongly think that the two forces cancel each other. They do not cancel because they act on different objects.
Examples of Third Law
· When you walk, your foot pushes the ground backward, and the ground pushes you forward.
· When a gun is fired, the bullet moves forward and the gun recoils backward.
· When you jump from a boat, the boat moves backward.
· A rocket moves upward by pushing gases downward.
· A swimmer pushes water backward and moves forward.
Exam Point
The rocket example is very important
for exams. The rocket expels gases downward with high speed. As a reaction, the
gases push the rocket upward. This is why rockets can move in space without
air.
Comparison of the Three Laws
|
Law |
Main Idea |
Simple Formula/Statement |
Common Example |
|
First Law |
Inertia |
A body remains at rest or in uniform motion unless acted upon by an external force |
Passengers lurch in a moving bus |
|
Second Law |
Force and acceleration |
F=ma |
Pushing an empty trolley is easier |
|
Third Law |
Action and reaction |
For every action, there is an equal and opposite reaction |
Rocket propulsion |
This
table is useful for quick revision before exams. The three laws are connected
and together explain most everyday motion. The First Law introduces inertia,
the Second Law measures the effect of force, and the Third Law explains the
interaction between bodies.
MONEY & BANKING – COMPLETE EXAM NOTES WITH MCQs.
MCQs
1. Newton’s First Law is also called
A. Law of Gravitation
B. Law
of Momentum
C. Law of Inertia
D. Law
of Friction
Answer: C
Explanation: Newton’s First Law says a body remains at rest or in uniform motion unless acted upon by an external unbalanced force. This resistance to change is called inertia.
2. The inertia of a body depends on its
A. Speed
B.
Shape
C. Mass
D.
Temperature
Answer: C
Explanation:
Greater mass means greater inertia, so heavier bodies resist changes in motion
more strongly.
3. Newton’s Second Law is related to
A. Force and momentum
B. Force, mass, and acceleration
C.
Work and energy
D.
Pressure and density
Answer: B
Explanation:
Newton’s Second Law gives the relationship between force, mass, and
acceleration.
4. The SI unit of force is
A. Dyne
B.
Joule
C.
Watt
D. Newton
Answer: D
Explanation:
The SI unit of force is newton. One newton is the force needed to produce 1
m/s² acceleration in a 1 kg mass.
5. One newton is equal to
A. 1 kg m/s
B. 1 kg m/s²
C. 1
kg/s²
D. 1
m/s²
Answer: B
Explanation:
By definition,
6. Newton’s Third Law states that
A. Force is proportional to mass
B. A
body remains at rest forever
C. For every action, there is an equal
and opposite reaction
D.
Speed is constant always
Answer: C
Explanation:
Action and reaction forces are equal in magnitude and opposite in direction.
7. When a bus suddenly starts, passengers fall backward due to
A. Friction
B.
Gravity
C. Inertia of rest
D.
Reaction force
Answer: C
Explanation:
The lower part of the body moves with the bus, but the upper part tends to
remain at rest, so the passenger leans backward.
8. When a moving bus suddenly stops, passengers fall forward due to
A. Inertia of rest
B. Inertia of motion
C.
Gravity
D.
Balance of forces
Answer: B
Explanation:
The body tends to continue moving forward even when the bus stops suddenly.
9. The mathematical form of Newton’s Second Law is
A.
B.
C.
D.
Answer: B
Explanation:
For constant mass, force equals mass times acceleration.
10. If force remains constant and mass increases, acceleration
A. Increases
B.
Remains same
C. Decreases
D.
Becomes zero
Answer: C
Explanation:
From , if is fixed and increases, must decrease.
11. Momentum is defined as
A. Force × time
B.
Mass × acceleration
C. Mass × velocity
D.
Weight × height
Answer: C
Explanation:
Momentum is the product of mass and velocity.
12. Which law explains rocket propulsion?
A. First Law
B.
Second Law
C. Third Law
D.
Zeroth Law
Answer: C
Explanation:
A rocket pushes gases downward, and the gases push the rocket upward with equal
and opposite reaction.
13. A gun recoils when a bullet is fired because of
A. Friction
B. Newton’s Third Law
C.
Newton’s First Law
D.
Centripetal force
Answer: B
Explanation:
The bullet moves forward and the gun moves backward as an action-reaction pair.
14. A swimmer moves forward by
A. Pulling water forward
B. Pushing water backward
C.
Pulling air backward
D.
Increasing body mass
Answer: B
Explanation:
The swimmer pushes water backward; the water pushes the swimmer forward with
equal and opposite reaction.
15. A body at rest will remain at rest unless acted on by
A. Balanced force
B. Unbalanced external force
C.
Weight
D.
Momentum
Answer: B
Explanation:
This is the statement of Newton’s First Law.
16. A rolling ball stops on the ground because of
A. Gravity only
B. Friction
C.
Inertia
D.
Pressure
Answer: B
Explanation:
Friction opposes motion and gradually brings the ball to rest.
17. A heavy truck is harder to stop because it has more
A. Speed
B.
Force
C. Inertia
D.
Weightlessness
Answer: C
Explanation:
Greater mass means greater inertia, so more force is needed to stop the truck.
18. In Newton’s Third Law, action and reaction forces act on
A. Same body in same direction
B.
Same body in opposite directions
C. Different bodies in opposite
directions
D.
Different bodies in same direction
Answer: C
Explanation:
This is why action and reaction do not cancel each other.
19. Which of the following is an example of inertia of motion?
A. Dust falling from a carpet
B. A
book lying on a table
C. A passenger moving forward when a
bus stops
D. A
coin at rest on a card
Answer: C
Explanation:
A moving body tends to continue in motion unless an external force stops it.
20. The force required to change momentum is given by
A.
B.
C.
D.
Answer: B
Explanation:
Newton’s Second Law can also be written as force equals the rate of change of
momentum.
21. If no external force acts on a body, its velocity
A. Must become zero
B.
Must increase
C. Remains constant
D.
Must decrease
Answer: C
Explanation:
Without external unbalanced force, the body maintains uniform motion.
22. The reaction force to a book placed on a table is
A. Friction force
B.
Weight of the book
C. Normal force by the table
D. Air
resistance
Answer: C
Explanation:
The book pushes down on the table, and the table pushes up on the book with
equal and opposite force.
23. Which law is most directly used in car seat belts?
A. First Law
B.
Second Law
C. First Law of Motion
D.
Third Law only
Answer: C
Explanation:
Seat belts protect passengers from the inertia of motion during sudden braking.
24. A force of 10 N acting on a mass of 2 kg produces acceleration of
A. 2 m/s²
B. 3
m/s²
C. 5 m/s²
D. 10
m/s²
Answer: C
Explanation:
Using ,
25. Which of the following best describes inertia?
A. Ability to move
B.
Ability to stop
C. Resistance to
change in state of motion
D. Force applied per unit area
Answer: C
Explanation:
Inertia is the property of matter by which it resists any change in its state
of rest or motion.
FAQs
1. What are Newton’s Laws of Motion?
Newton’s Laws of Motion are three basic laws that explain the relationship between force and motion.
2. What is Newton’s First Law called?
Newton’s First Law is called the Law of Inertia.
3. What is the formula of Newton’s Second Law?
The formula is .
4. What does Newton’s Third Law say?
It says that for every action, there is an equal and opposite reaction.
Conclusion:
Newton’s Laws of Motion form the backbone of mechanics. They explain how and why objects move, accelerate, stop, and react to forces. For competitive exams, this topic is very important because it combines theory, formula-based questions, and real-life examples. A student who understands these three laws clearly can solve many physics questions with confidence.
The easiest way to remember this chapter is simple: the First Law is about inertia, the Second Law is about force and acceleration, and the Third Law is about action and reaction. Once these three ideas are clear, the chapter becomes straightforward and scoring.
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