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Political Science NCERT Solutions Class 9 Science Chapter 8 – Force and Laws of Motion
Class 9 Science
Last Updated: 15 April 2026
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हिंदी में (Hindi)
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Important Points
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Practice MCQ Questions (29 questions)
Q1. What is the SI unit of force?
✓ Correct Answer: A
Q2. Which law of motion is also known as the law of inertia?
✓ Correct Answer: A
Q3. What is required to change the motion of an object?
✓ Correct Answer: B
Q4. What does the second law of motion relate?
✓ Correct Answer: C
Q5. Inertia of an object is measured by its?
✓ Correct Answer: B
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Frequently Asked Questions (FAQ)
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Tough Words (Glossary)
force
Meaning: A push or pull that can change an object's motion
अर्थ: धक्का या खींच जो वस्तु की गति बदल सकता है
"A force is needed to start the cart moving."
balanced force
Meaning: Equal forces acting in opposite directions that cancel out
अर्थ: विपरीत दिशाओं में समान बल, जो एक-दूसरे को काट देते हैं
"Balanced forces keep the box at rest."
unbalanced force
Meaning: Forces that do not cancel, causing a change in motion
अर्थ: ऐसे बल जो नहीं कटते और गति बदलते हैं
"An unbalanced force made the trolley speed up."
friction
Meaning: A force that opposes motion between surfaces in contact
अर्थ: स्पर्श करती सतहों के बीच गति का विरोध करने वाला बल
"Friction slows the bicycle when we stop pedalling."
inertia
Meaning: Tendency of an object to resist change in its state of motion
अर्थ: वस्तु की अपनी अवस्था में परिवर्तन का विरोध करने की प्रवृत्ति
"Your body lurches forward due to inertia when the bus stops."
mass
Meaning: Amount of matter in an object; a measure of inertia
अर्थ: वस्तु में पदार्थ की मात्रा; जड़ता का माप
"A truck has more mass than a car."
momentum
Meaning: Product of mass and velocity (p = m×v)
अर्थ: द्रव्यमान और वेग का गुणनफल (p = m×v)
"A fast ball has high momentum."
acceleration
Meaning: Rate of change of velocity with time
अर्थ: समय के साथ वेग में परिवर्तन की दर
"The car’s acceleration was 2 m/s²."
velocity
Meaning: Speed in a given direction
अर्थ: दिशा सहित चाल
"The velocity of the train is 60 km/h north."
displacement
Meaning: Shortest distance from initial to final position with direction
अर्थ: प्रारम्भिक से अंतिम स्थिति तक का लघुत्तम दूरी व दिशा सहित
"Her displacement was 3 km east."
Newton’s first law
Meaning: An object remains at rest or in uniform motion unless acted upon by a net force
अर्थ: यदि शुद्ध बल न लगे तो वस्तु स्थिर रहती है या समान गति से चलती रहती है
"Seat belts protect us per Newton’s first law."
Newton’s second law
Meaning: Force equals mass times acceleration (F = m×a)
अर्थ: बल = द्रव्यमान × त्वरण (F = m×a)
"Doubling the force doubles the acceleration."
Newton’s third law
Meaning: For every action, there is an equal and opposite reaction
अर्थ: हर क्रिया के बराबर व विपरीत प्रतिक्रिया होती है
"The gun recoils when the bullet is fired."
recoil
Meaning: A backward movement after a forward force is applied
अर्थ: आगे बल लगाने पर पीछे की ओर झटका
"The rifle’s recoil pushes the shoulder back."
collision
Meaning: A situation when two bodies strike each other
अर्थ: दो पिंडों का आपस में टकराना
"A soft ball causes less injury in a collision."
proportional
Meaning: Having a constant ratio to something else
अर्थ: किसी अन्य मात्रा के अनुपाती
"Acceleration is proportional to force."
rate
Meaning: How fast a quantity changes per unit time
अर्थ: प्रति इकाई समय किसी मात्रा के बदलने की तीव्रता
"Acceleration is the rate of change of velocity."
magnitude
Meaning: Size or amount of a quantity
अर्थ: किसी राशि का परिमाण या आकार
"Only the magnitude of speed was given."
direction
Meaning: The line or course along which something moves
अर्थ: वह दिशा जिसमें कोई वस्तु चलती है
"Velocity needs both speed and direction."
apply (a force)
Meaning: To put a force on an object
अर्थ: किसी वस्तु पर बल लगाना
"We apply a force to push the box."
lubricant
Meaning: A substance that reduces friction
अर्थ: घर्षण कम करने वाला पदार्थ
"Oil acts as a lubricant on axles."
inclined plane
Meaning: A sloping surface used to raise objects
अर्थ: ढलानदार सतह जिससे वस्तु ऊपर चढ़ाई जाती है
"A marble speeds up on an inclined plane."
frictionless
Meaning: Having almost no friction
अर्थ: लगभग शून्य घर्षण वाला
"On a frictionless surface, the block keeps moving."
net force
Meaning: Overall force after combining all forces
अर्थ: सभी बलों के संयोजन के बाद कुल बल
"If net force is zero, velocity stays constant."
uniform motion
Meaning: Motion with constant speed in a straight line
अर्थ: सीधी रेखा में समान चाल वाली गति
"The trolley showed uniform motion on smooth rails."
non-uniform motion
Meaning: Motion with changing speed or direction
अर्थ: बदलती चाल या दिशा वाली गति
"City traffic causes non-uniform motion."
rest
Meaning: State of no motion relative to a reference
अर्थ: संदर्भ के सापेक्ष न चलने की अवस्था
"The ball is at rest on the ground."
reference point
Meaning: A fixed point used to describe position
अर्थ: स्थिति बताने के लिए लिया गया स्थिर बिंदु
"The station is our reference point."
vector
Meaning: A quantity having magnitude and direction
अर्थ: परिमाण और दिशा वाली राशि
"Velocity is a vector quantity."
scalar
Meaning: A quantity having only magnitude
अर्थ: केवल परिमाण वाली राशि
"Speed is a scalar quantity."
resistance (to change)
Meaning: Opposition to a change in motion
अर्थ: गति में परिवर्तन का विरोध
"Inertia is resistance to change in motion."
linear
Meaning: In a straight line
अर्थ: सीधी रेखा में
"Uniform linear motion is simplest to study."
impact
Meaning: The effect or force of one object hitting another
अर्थ: टकराने से उत्पन्न प्रभाव/बल
"The impact was reduced by soft padding."
exert
Meaning: To use or apply (force or effort)
अर्थ: बल/प्रयास लगाना
"The engine exerts a force on the wheels."
impulse
Meaning: Change in momentum (force × time)
अर्थ: संवेग-परिवर्तन (बल × समय)
"A longer catch time reduces impulse force."
gravity
Meaning: Attractive force between masses (Earth’s pull)
अर्थ: द्रव्यमानों के बीच आकर्षण बल (पृथ्वी का आकर्षण)
"Gravity makes the marble roll down."
accelerate
Meaning: To increase speed; to gain velocity
अर्थ: गति बढ़ाना; वेग बढ़ाना
"The scooter began to accelerate."
decelerate
Meaning: To reduce speed; negative acceleration
अर्थ: गति घटाना; ऋणात्मक त्वरण
"He decelerated before the turn."
opposite
Meaning: Facing or going in a contrary direction
अर्थ: विपरीत दिशा में
"Reaction force is opposite to action."
equal
Meaning: Having the same value or amount
अर्थ: समान मात्रा/मान वाला
"Action and reaction are equal in magnitude."
rigid support
Meaning: A firm support that does not deform easily
अर्थ: ऐसा ठोस सहारा जो आसानी से न झुके/मुड़े
"Tie the spring balance to a rigid support."
scales (instrument)
Meaning: Graduated markings to measure quantity
अर्थ: मापक यंत्र पर अंकित पैमाना
"Both spring scales showed 10 N."
muscular effort
Meaning: Force produced by muscles
अर्थ: मांसपेशियों द्वारा उत्पन्न बल
"Pushing the car needs muscular effort."
stationary
Meaning: Not moving; at rest
अर्थ: अचल; स्थिर
"A stationary block can start moving if pushed."
equilibrium
Meaning: State where net force is zero
अर्थ: वह अवस्था जहाँ शुद्ध बल शून्य हो
"The box stayed in equilibrium on the table."
push
Meaning: To apply force away from oneself
अर्थ: धक्का देना
"Push the crate gently."
pull
Meaning: To apply force towards oneself
अर्थ: खींचना
"Pull the rope to move the block."
opposes
Meaning: Acts against; resists
अर्थ: विरोध करता है
"Friction opposes motion."
constant
Meaning: Not changing with time
अर्थ: समय के साथ न बदलने वाला
"A constant velocity means no acceleration."
cushioned
Meaning: Softly padded to absorb impact
अर्थ: गद्देदार, जो आघात सोख ले
"High jumpers land on a cushioned mat."
pendulum
Meaning: A weight that swings from a fixed point
अर्थ: लोलक—टंगे वज़न का झूलना
"Galileo studied time using a pendulum."
telescope
Meaning: Instrument to see distant objects
अर्थ: दूर की वस्तुएँ देखने का यंत्र
"Galileo improved the telescope."