Class 9th Force Physics Notes Rebellion Classes
Balanced and
Unbalanced Forces
In everyday life, we observe many objects moving, stopping,
speeding up, slowing down, or changing direction. These changes happen because
of force.
Force can act in different directions and with different
strengths. Sometimes forces cancel each other, while sometimes one force
becomes greater and changes the motion of the object.
On the basis of their effect, forces are divided into:
- Balanced
Forces
- Unbalanced
Forces
Force
Definition
A force is a push or pull acting on an object.
Examples:
- Pushing
a trolley
- Pulling
a drawer
- Kicking
a football
- Stretching
a rubber band
Effects of Force
A force can:
- Start
motion
- Stop
motion
- Increase
speed
- Decrease
speed
- Change
direction
- Change
shape or size
Examples of Effects of Force
Push and Pull in Daily Life
State of Motion
The state of motion of an object depends on:
- Its
speed
- Its
direction
If either speed or direction changes, the state of motion
changes.
Example:
- A
car moving straight with constant speed has same state of motion.
- Turning
the car changes its state of motion.
Balanced Forces
Definition
When two or more forces acting on an object are equal in
magnitude and opposite in direction, the resultant force becomes zero.
Such forces are called balanced forces.
Visual of Balanced Forces
Explanation
Suppose a box is pulled with equal forces from opposite
directions.
10 N ← [ BOX ]
→ 10 N
Both forces cancel each other.
Resultant force:
10 N – 10 N = 0
Therefore:
- The
box does not move.
- There
is no change in state of motion.
Important Characteristics of Balanced Forces
- Net
force is zero
- No
change in state of motion
- Object
may remain at rest
- Moving
object may continue moving with same speed
- Shape
may change
Balanced Forces Can Change Shape
Balanced forces may not move an object, but they can change
its shape.
Example:
- Stretching
a spring
- Pressing
a rubber ball
Unbalanced Forces
Definition
When forces acting on an object are unequal, the resultant
force is not zero.
Such forces are called unbalanced forces.
Visual of Unbalanced Forces
Explanation
Suppose:
15 N → [ BOX ]
← 5 N
Resultant force:
15 N – 5 N = 10 N
The box moves in the direction of greater force.
Important Characteristics of Unbalanced Forces
- Net
force is not zero
- Changes
state of motion
- Can
start motion
- Can
stop motion
- Can
change speed
- Can
change direction
Friction Force
Definition
Friction is a force that opposes motion.
It always acts opposite to the direction of motion.
Friction Example
When children push a heavy box on a rough floor:
Case 1: Small Push
Push → 5 N
Friction ← 5 N
Net force = 0
The box does not move because forces are balanced.
Case 2: Greater Push
Push → 15 N
Friction ← 5 N
Net force:
15 N – 5 N = 10 N
Now the box starts moving.
Visual of Friction Acting on Box
Bicycle Example from NCERT
When we stop pedalling a bicycle, it gradually slows down and
stops.
Reason:
- Friction
force
- Air
resistance
oppose the motion of the bicycle.
Important Conclusion
An object changes its motion only when an unbalanced
external force acts on it.
Galileo’s Observation
Galileo observed:
- Objects
stop moving because of friction.
- Without
friction, a moving object would continue moving for a very long time.
This idea later became the basis of Newton’s laws of motion.
Difference Between Balanced and Unbalanced Forces
|
Balanced Forces |
Unbalanced Forces |
|
Net force = 0 |
Net force ≠ 0 |
|
No change in motion |
Motion changes |
|
Forces are equal |
Forces are unequal |
|
Object may stay at rest |
Object accelerates |
|
May change shape only |
Changes speed/direction |
Important Terms
Resultant Force
The single force representing the combined effect of all
forces acting on an object.
External Force
A force applied on an object by another object or person.
Quick Revision
- Force
is a push or pull.
- Balanced
force → No change in motion.
- Unbalanced
force → Changes motion.
- Friction
opposes motion.
- Motion
changes only when net force is not zero.
First Law of Motion
After understanding balanced and unbalanced forces, the next
important question is:
Why does a stationary
object remain at rest?
Why does a moving object continue moving
unless something stops it?
These questions are answered by Newton’s First Law of
Motion.
Newton’s First Law of Motion
Statement of the Law
An object remains:
- at
rest, or
- in
uniform motion along a straight line
unless acted upon by an unbalanced external force.
Meaning of the First Law
This law tells us:
- A
stationary object will remain stationary unless force acts on it.
- A
moving object will continue moving with same speed and direction unless
force changes its motion.
Force is required only
to change the state of motion, not to maintain motion.
Visual Understanding of First Law
Examples of Newton’s First Law
1. Book on Table
A book lying on a table remains at rest until someone pushes
it.
Reason:
- No
unbalanced force acts on it.
2. Moving Bicycle
A bicycle moving on a road continues moving for some distance
even after pedalling stops.
Reason:
- Bicycle
tends to continue its motion.
- It
finally stops because of friction and air resistance.
3. Hockey Ball Example
A hockey ball keeps moving on a smooth surface for a longer
time.
Reason:
- Friction
is less.
- Opposing
force is smaller.
Galileo’s Observation
Before Newton, people believed:
👉 Continuous force is needed to keep an object moving.
Galileo proved this idea wrong.
He observed:
- In
absence of friction, moving objects continue moving.
- Friction
is responsible for stopping objects.
This became the foundation of Newton’s First Law.
Inertia
Definition
The natural tendency of an object to resist any change in its
state of rest or motion is called inertia.
Important Meaning of Inertia
Because of inertia:
- Objects
at rest resist motion.
- Moving
objects resist stopping.
- Objects
resist change in direction.
Visual Understanding of Inertia
Types of Inertia
1. Inertia of Rest
The tendency of an object at rest to remain at rest.
Examples
- Passenger
falls backward when bus starts suddenly.
- Coin
falls into glass when card is flicked quickly.
2. Inertia of Motion
The tendency of a moving object to continue moving.
Examples
- Passenger
falls forward when bus stops suddenly.
- Moving
fan continues rotating for some time after switch is turned off.
3. Inertia of Direction
The tendency of an object to resist change in direction.
Examples
- Passengers
move sideways when bus takes sharp turn.
- Stone
tied to string flies tangentially if string breaks.
Coin and Card Activity (NCERT)
A coin is placed on a card kept over a glass.
When the card is flicked quickly:
- Card
moves away
- Coin
falls vertically into glass
Reason:
- Coin
tends to remain at rest due to inertia.
Visual of Coin and Card Experiment
Relation Between Inertia and Mass
Mass is the measure of inertia.
👉 Greater the mass → greater the inertia
Examples:
- Truck
has greater inertia than bicycle.
- Loaded
cart is harder to move than empty cart.
Important Points About First Law
- First
law is also called Law of Inertia
- It
explains why seat belts are important
- It
explains why passengers jerk in buses
- It
introduces the concept of force
Seat Belt Example
When a car stops suddenly:
- Passenger’s
body tends to keep moving due to inertia of motion.
Seat belt:
- Prevents
forward motion
- Reduces
injury
Key Difference Between Rest and Motion
|
State of Rest |
State of Motion |
|
Object does not change position |
Object changes position |
|
No motion |
Motion present |
|
Inertia resists movement |
Inertia resists stopping |
Important Terms
External Force
A force applied by another object/person from outside.
Uniform Motion
Motion with constant speed in straight line.
Quick Revision
- Newton’s
First Law is also called Law of Inertia.
- Force
is needed to change motion.
- Inertia
is resistance to change in motion.
- Greater
mass means greater inertia.
- Friction
opposes motion.
- Without
friction, objects continue moving.
8.4 Second Law of Motion
Newton’s First Law explains when an object changes its state
of motion. But another important question still remains:
👉 How much does the motion change when force acts?
👉 Why does a small force move a football easily but not
a truck?
👉 Why does a fast cricket ball hurt more than a slow
ball?
These questions are answered by Newton’s Second Law of
Motion.
The second law gives the mathematical relation between:
- Force
- Mass
- Acceleration
- Momentum
It is one of the most important laws in physics.
Momentum
Before understanding the second law, we must understand momentum.
Definition of Momentum
Momentum is the quantity of motion possessed by an object.
It depends on:
- Mass
of object
- Velocity
of object
Formula of Momentum
Where:
- p =
momentum
- m =
mass
- v =
velocity
SI Unit of Momentum
kg m s⁻¹
Important Points About Momentum
- Greater
mass → greater momentum
- Greater
velocity → greater momentum
- Momentum
has both magnitude and direction
- Direction
of momentum is same as velocity
Visual Understanding of Momentum
Examples of Momentum
Example 1: Truck vs Bicycle
A moving truck is harder to stop than a bicycle because:
- Truck
has larger mass
- Therefore,
greater momentum
Example 2: Fast Cricket Ball
A fast-moving cricket ball hurts more because:
- Greater
velocity
- Greater
momentum
Newton’s Second Law of Motion
Statement of the Law
The rate of change of momentum of an object is directly
proportional to the applied unbalanced force and takes place in the direction
of force.
Meaning of the Law
The law tells us:
- Greater
force → greater change in motion
- Greater
mass → more force needed
- Greater
acceleration → greater force required
Mathematical Formulation of Second Law
Suppose:
- Mass
of object = m
- Initial
velocity = u
- Final
velocity = v
- Time
taken = t
Step 1: Initial Momentum
p_1 = mu
Step 2: Final Momentum
p_2 = mv
Step 3: Change in Momentum
p_2 - p_1 = mv - mu
Taking common factor:
= m(v-u)
Step 4: Rate of Change of Momentum
Meaning of Formula F = ma
Force depends on:
- Mass
- Acceleration
More mass → more force
needed
More acceleration → more force needed
SI Unit of Force
newton (N)
Definition of 1 Newton
1 newton is the force that produces an acceleration of 1 m
s⁻² in a body of mass 1 kg.
Visual Understanding of Force and Acceleration
Important Formulae from Second Law
Force Formula
F = ma
Momentum Form
Impulse Form
Ft = mv - mu
Impulse
Definition
The product of force and time is called impulse.
Formula of Impulse
Impulse = Ft
Impulse is equal to change in momentum.
Applications of Impulse
1. Catching a Ball
Fielders pull hands backward while catching a cricket ball.
Reason:
- Time
of stopping increases
- Force
decreases
- Injury
is reduced
2. Sand Beds in High Jump
Athletes fall on soft sand because:
- Stopping
time increases
- Force
decreases
3. Seat Belts
Seat belts increase stopping time during accident.
Therefore:
- Force
decreases
- Injuries
reduce
Visual Understanding of Impulse
Velocity-Time Graph and Acceleration
A straight sloping line in velocity-time graph shows:
- Uniform
acceleration
A downward slope shows:
- Retardation
(negative acceleration)
Important Conclusions
- Force
changes momentum
- Greater
mass requires greater force
- Greater
acceleration requires greater force
- Impulse
changes momentum
- Second
law gives mathematical definition of force
Key Difference Between Momentum and Force
|
Momentum |
Force |
|
Quantity of motion |
Cause of change in motion |
|
Depends on mass & velocity |
Depends on mass & acceleration |
|
Symbol = p |
Symbol = F |
Quick Revision
- Momentum
= mass × velocity
- Newton’s
Second Law relates force and acceleration
- Formula:
F = ma
- Impulse
= Ft
- Greater
stopping time reduces force
- Force
changes momentum
8.5 Third Law of Motion
Newton’s First Law explains why objects resist change in
motion, and the Second Law explains how force changes motion. But another
important question still remains:
How does a person walk
forward?
Why does a gun move backward after
firing?
How does a rocket move upward in space?
All these situations are explained by Newton’s Third Law
of Motion.
This law explains that forces always act in pairs.
Newton’s Third Law of Motion
Statement of the Law
To every action, there is an equal and opposite reaction.
Meaning of the Law
Whenever one object exerts a force on another object:
- the
second object also exerts a force back
- both
forces are equal in magnitude
- both
forces act in opposite directions
Important Point
Action and reaction:
- act
on different objects
- occur
simultaneously
- are
always equal and opposite
Visual Understanding of Third Law
Action and Reaction Forces
Suppose a boy pushes a wall.
Boy pushes wall → →
Wall pushes boy ← ←
The wall also pushes the boy backward with equal force.
Why We Can Walk
Walking is one of the best examples of Newton’s Third Law.
What Happens While Walking?
- Our
foot pushes the ground backward.
- The
ground applies equal and opposite force forward.
- This
forward reaction force moves us ahead.
Gun Recoil
When a bullet is fired:
- Gun
pushes bullet forward.
- Bullet
pushes gun backward.
As a result:
- Gun
recoils backward.
This backward motion is called recoil.
Visual of Gun Recoil
Rocket Motion
A rocket works on Newton’s Third Law.
What Happens?
- Rocket
throws gases downward at very high speed.
- Gases
push rocket upward with equal and opposite force.
Therefore:
- Rocket
rises upward.
Visual of Rocket Launch
Boat Example
When a sailor jumps from a boat onto the shore:
- Sailor
pushes boat backward.
- Boat
pushes sailor forward.
As a result:
- Boat
moves backward.
Swimmer Example
When a swimmer swims:
- Swimmer
pushes water backward.
- Water
pushes swimmer forward.
Spring Balance Activity (NCERT)
Two spring balances are connected together and pulled.
Observation:
- Both
spring balances show equal readings.
Reason:
- Action
and reaction forces are equal.
Visual of Spring Balance Activity
Important Characteristics of Action and Reaction
|
Action Force |
Reaction Force |
|
Acts on one object |
Acts on another object |
|
Equal in magnitude |
Equal in magnitude |
|
Opposite in direction |
Opposite in direction |
|
Occurs simultaneously |
Occurs simultaneously |
Important Examples of Third Law
|
Action |
Reaction |
|
Foot pushes ground backward |
Ground pushes foot forward |
|
Gun pushes bullet forward |
Bullet pushes gun backward |
|
Rocket pushes gases downward |
Gases push rocket upward |
|
Swimmer pushes water backward |
Water pushes swimmer forward |
Common Misconception
Some students think action and reaction cancel each other.
This is wrong because:
- they
act on different objects
- cancellation
occurs only when forces act on same object
Why Action and Reaction Do Not Cancel
Example:
- While
walking:
- Foot
force acts on ground
- Ground
force acts on person
Since forces act on different bodies:
- they
do not cancel
Applications of Third Law
- Walking
- Swimming
- Flying
of birds
- Rocket
launching
- Gun
recoil
- Rowing
boats
Important Conclusions
- Forces
always occur in pairs.
- No
force exists alone.
- Action
and reaction are equal and opposite.
- Forces
act on different objects.
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