The chapter “Force and Laws of Motion” in Class 9 Physics is one of the most important topics of the NCERT Science curriculum. It explains how objects move, stop, or change direction when forces act on them. This chapter introduces the fundamental concepts of force, inertia, momentum, and Newton’s three laws of motion, which are the foundation of mechanics in physics.
Students will learn about:
Balanced and Unbalanced Forces – when forces cancel out or produce motion.
Newton’s First Law (Law of Inertia) – an object remains at rest or continues in motion unless an external force acts on it.
Newton’s Second Law – the relationship between force, mass, and acceleration (F = ma).
Newton’s Third Law – every action has an equal and opposite reaction.
Inertia and its Types – inertia of rest, motion, and direction.
Momentum and its Conservation – how momentum is transferred and conserved during interactions.
Practical Applications – examples from daily life such as a ball hitting a bat, rocket propulsion, or pushing objects.
This chapter not only builds strong conceptual understanding but also forms the base for higher classes in physics. With numerical problems, examples, diagrams, and real-life applications, it helps students develop problem-solving and analytical skills.
By mastering this chapter, students will gain clarity on how forces govern motion in the physical world, making it easier to score high in CBSE exams, school tests, and competitive assessments.
Trigonometric Functions form a crucial foundation of higher mathematics and play a vital role in physics, engineering, astronomy, and real-life proble...
Trigonometric Functions form a crucial foundation of higher mathematics and play a vital role in physics, engineering, astronomy, and real-life proble...
A force is an effort that changes the state of an object at rest or at motion. It can change an object’s
direction and velocity. Force can also change the shape of an object.
Effects of Force
Some effects of force include the following:
Force moves stationary objects
Force stops objects from moving
Force changes the shape of a body
Force changes the direction of motion
Push:
Push is defined as an action of force which causes an object to move from its
place. Example:
Opening and closing the door
Pushing the table
Pushing a car
Pushing of thumb pins
Walking
Pull:
Pull is defined as an action to make something move by either tugging or dragging. Example:
Plucking the string of a guitar
Pulling ropes while playing tug of war
Opening the drawer
Pulling the window curtain
Opening and closing the doors
Hit:
A hit is an action in which one body exerts a force on another body for a very
short
duration of time, often resulting in a change in the motion of the second body.
It is a type of contact force, because the bodies must touch each other.
During a hit, the force applied is usually large and acts over a very short
time. Example:
A ball striking a bat
A hammer hitting a nail
A cue striking a billiard ball
Hit is an example of impulse in physics
Balanced and Unbalanced Force
Balanced forces:
Balanced forces cancel each other out, so they do not change the state of motion of an object.
Examples of balanced forces include a person standing on the floor or a book resting on a table.
When two people push a refrigerator in opposite directions with equal force, the forces are
balanced.
Tug of war (Unbalanced Force)
Unbalanced forces
Unbalanced forces cause an object to change its state of motion.
Examples of unbalanced forces include an object sinking in water, a car accelerating, braking,
or turning, or a fruit dropping from a tree.
When unbalanced forces are applied to an object that is sitting still, the object will move in
The direction pushed or pulled by the stronger force.
First Law of Motion
A body continues to be in the state of rest or uniform motion in a straight line unless acted upon by an
external unbalanced force. The First Law is also called the Law of Inertia.
Inertia:
Basically, all objects have a tendency to resist a change in the
state of motion
or rest. This tendency is called inertia. All bodies do not have the same inertia. Inertia depends
on the mass of a body. The mass of an object is the measure of its inertia. More the mass \(\Rightarrow\) more the inertia and vice
versa. Object in motion - remains in motion
Inertia of Rest:
An object stays at rest, and it remains at rest until an
external force
affects it. Example: When a car accelerates, passengers may feel as though their bodies are moving
backwards. In reality, inertia is making their bodies stay in place as the car moves forward.
Inertia of Motion
An object will continue to be in motion until a force acts on it. Example:
A hockey puck will continue to slide across the ice until acted upon by an outside force.
Second Law of Motion
In order to understand the Second Law, we need to first understand momentum.
Momentum
Impacts produced by objects depend on their mass and velocity. The momentum of an object is defined as
the product of its mass and velocity.
\(p=mv\)
Vector quantity has direction and magnitude. An example of momentum is a baseball flying through the
air and a bullet fired from a gun.
Second Law of Motion (Mathematical Formulation)
The rate of change of momentum of an object is directly proportional to the applied unbalanced force in
the direction of the force.
\(\begin{equation}\Delta p=m\cdot \Delta v\\\tag{1}\end{equation}\)
Dividing both side of eqn(1) by \(\quad\Delta t\\\)
\(\\\\\qquad\dfrac{\Delta p}{\Delta t}=m\cdot\dfrac{\Delta v}{\Delta t}\\\\
\color{blue}\Rightarrow\left(\dfrac{\Delta p}{\Delta t} =\color{blue}F,\quad \dfrac{\Delta v}{\Delta t}=\color{blue}a\right)\\\\\qquad\qquad\color{blue}F=\color{blue}ma\\\)
Examples
Ex.1 A body of mass 5kg is moving with a velocity 2m/s. Calculate the linear momentum
Solution:
mass of body \(m=5kg\) Velocity \(v = 2m/s\)
$$\begin{aligned}m&=5kg\\
v&=2m/s\\
p&=mv\\
&=5\times 2\\
&=10kgm/s\end{aligned}$$
Ex.2 A force of 10N acts on a body of mass 2kg for 3 seconds initially at rest. Calculate
(1) the velocity acquired by the body
(2) change in momentum of the body
Solution:
(1) the velocity acquired by the body
$$\begin{aligned}F&=10N\\
m&=2kg\\
t&=3s\\
F&=ma\\
&=m\cdot \left( \dfrac{v-u}{t}\right) \\
10&=2\dfrac{\left( v-0\right) }{3}\\
v&=\dfrac{10\times 3}{2}\\
&=15m/s\end{aligned}$$
(2) change in momentum of the body
$$\begin{aligned}p&=mv\\
\Delta p&=m\Delta v\\
\Delta p&=m\left( v-u\right)&\quad\quad\Delta v = v-u \\
\Delta p&=2\left( 15-0\right)\\
&=30kgm/s\end{aligned}$$
Ex.3 In the figure given, the velocity-time graph of a particle of mass 100g moving in a straight
line.
Calculate the force acting upon it
Ex.4 A car of mass 480 kg moving at a speed of 54km/h, is stopped by applying the brakes in 10s. Calculate
the force applied by the brakes.
Solution:
$$\begin{aligned}\scriptsize\text{Mass of the car} (m)&\scriptsize=480kg\\\scriptsize\text{Time to stop }t&\scriptsize=10s\\\scriptsize\text{Initial Velocity }u&\scriptsize=54km/h\\\\
\scriptsize u&\scriptsize=\dfrac{54\times 1000}{60\times 60}m/s\\\\
&\scriptsize=15ms^-1
\end{aligned}$$
Final Velocity \(v=0\), therefore, acceleration \(a\) can be given as
\[\begin{aligned}
a&\scriptsize=\dfrac{v-u}{t}\\
&\scriptsize=\dfrac{0-15}{10}\\
&\scriptsize=-1.5m/s^{2}\end{aligned}\]
(-ve sign shows -ve acceleration \(\Rightarrow\) retardation
Force applied by breaks \[\begin{aligned}\scriptsize\Rightarrow F&\scriptsize=ma\\
&\scriptsize=480\times 1.5\\
&\scriptsize=720.0 N\end{aligned}\]
Ex.5 A bullet of mass 50 g moving with an initial velocity of 100 m/s strikes a wooden block and comes to
rest
after penetrating a distance of 2 cm into it. Calculate
a. initial momentum of the bullet
b. final momentum of the bullet
c. retardation caused by the wooden block
d. resistive force exerted by the wooden block
Solution:
a. initial momentum of the bullet
$$\begin{aligned}
m&=50g\\
u&=100m| s\\
p&=mu\\
&=50\times 100\\
&=5kgms^{-1}\\\end{aligned}$$
b. final momentum of the bullet
$$\begin{aligned}
v&=0\\
p&=m\cdot v\\
&=\dfrac{50\times 0}{1000}\\
&=0\end{aligned}$$
c. retardation caused by the wooden block
$$\begin{aligned}
v&=0\\
u&=100ms^{-1}\\
s&=2cm\\
v^{2}-u^{2}&=2as\\\\
0^{2}-\left( 100\right) ^{2}&=\dfrac{2\cdot as}{100}\\\\
-100\times (100)^2&=4a\\
\Rightarrow a&=-\dfrac{10000\times 100}{4}\\
a&=-2.5\times 10^{5}ms^{-2}\end{aligned}$$
d. resistive force exerted by the wooden block
$$\begin{aligned}
F&=ma\\
&=50\times 10\times 2.5\times 10^{5}\\
&=1.25\times 10^{4}\\
&=12500N\end{aligned}$$
Conservation of Momentum
Concept of System
The part of the universe chosen for analysis is called a system.
Everything outside the system is called an environment.
For example, a car moving with constant velocity can be considered a system. All the forces
within the car are internal forces, and all forces acting on the car from the environment are
external forces like friction.
Conservation of Momentum
The total momentum of an isolated system is conserved.
Isolated system \(\Rightarrow\) net external force on the system is zero.
Example: Collision of 2 balls, A and B.
From Newtons 3rd law \(F_{AB} = -F_{BA}\)
Third Law of Motion
Newton’s \(3^{rd}\) law states that every action has an equal and opposite reaction. Action and reaction
forces
are equal, opposite and acting on different bodies
Inertial and Non-Inertial Frames
A non-inertial frame of reference is a frame of reference in which Newton’s laws of motion do not
hold. A non-inertial reference frame is a frame of reference that is undergoing acceleration with respect to an inertial frame. An accelerometer at rest in a non-inertial frame will, in general,
detect a non-zero acceleration.
A frame of reference where Newton’s Laws hold is known as an inertial frame of reference.
Points to remember
First law of motion: An object continues to be in a state of rest or of uniform motion along a
straight line unless acted upon by an unbalanced force.
The natural tendency of objects to resist a change in their state of rest or of uniform motion is
called inertia.
The mass of an object is a measure of its inertia. Its SI unit is kilogram (kg).
The Force of friction always opposes the motion of objects.
Second law of motion: The rate of change of momentum of an object is proportional to the applied force.
unbalanced force in the direction of the force. The SI unit of force is kg m s–2. This is also known as Newton and represented by the symbol N. A force of one newton produces an acceleration of 1 m s^2
on an object of mass 1 kg.
The momentum of an object is the product of its mass and velocity and has the same direction as that of the velocity. Its SI unit is \(kgms^{-1}\).
Third law of motion: To every action, there is an equal and opposite reaction, and they act on two
different bodies.
Frequently Asked Questions
Force is a push or pull on an object that can change its state of motion or shape.
Balanced forces are equal in magnitude and opposite in direction, producing no change in motion.
Unbalanced forces cause a change in the state of motion or shape of an object.
Sir Isaac Newton formulated the three laws of motion.
An object remains at rest or in uniform motion unless acted upon by an unbalanced external force.
Inertia is the tendency of an object to resist any change in its state of motion or rest.
The rate of change of momentum is directly proportional to the applied force and occurs in its direction.
For every action, there is an equal and opposite reaction.
Momentum is the product of an object’s mass and velocity, given byp=m×vp = m \times vp=m×v.
The SI unit of force is the newton (N).
The SI unit of momentum is kg·m/s.
One newton is the force that produces an acceleration of 1 m/s² in an object of mass 1 kg.
F=m×aF = m \times aF=m×a— whereFFFis force,mmmis mass, andaaais acceleration.
Acceleration depends directly on the applied force and inversely on the object’s mass.
Due to inertia, the passenger's body resists the forward motion.
Due to inertia of motion, the upper part of the passenger’s body continues moving forward.
Running increases momentum, helping them cover a greater distance.
Force is directly proportional to acceleration (F?aF ? aF?a).
The forces balance each other, and the object remains in uniform motion or rest.
The total momentum of a system remains constant when no external force acts on it.
When a gun is fired, the bullet moves forward, and the gun recoils backward with equal momentum.
Total momentum before and after collision remains equal if no external force acts.
Action and reaction are equal in magnitude, opposite in direction, and act on different bodies.
Mass is the amount of matter in an object and a measure of its inertia.
The SI unit of mass is kilogram (kg).
It is the rate of change of velocity when a force is applied to a body.
A heavy object has greater mass, so it resists changes in motion more.
F=m×aF = m \times aF=m×a, showing that force causes acceleration depending on mass.
Due to conservation of momentum; bullet and gun move in opposite directions.
When we walk, our foot pushes the ground backward, and the ground pushes us forward.
Momentum depends on mass and velocity of the object.
A large force acting for a short time, such as in collisions or explosions.
Impulse is the product of force and time, equal to the change in momentum.
The SI unit of impulse is newton-second (N·s).
Momentum is zero when either mass or velocity is zero.
By increasing its velocity or mass.
A lighter body has less mass and hence less inertia.
Motion in which an object covers equal distances in equal time intervals in a straight line.
Acceleration decreases as mass increases (a?1/ma ? 1/ma?1/m).
It is also known as the Law of Acceleration.
Zero, as velocity is zero.
Sand increases the time of impact, reducing the effect of the force on landing.
By expelling gases downward, which push the rocket upward due to action-reaction pairs.
To reduce pressure and distribute weight evenly for stability.
Kicking a football causes it to move due to an unbalanced force.
Airbags increase the time of impact during collisions, reducing force and injuries.
Retardation or negative acceleration occurs when velocity decreases with time.
Due to opposing frictional force between the object and the surface.
Force equals the rate of change of momentum (F=?p?tF = \frac{?p}{?t}F=?t?p).
They are known as the fundamental laws describing motion and dynamics of objects.
