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Political Science NCERT Solutions Class 9 Science Chapter 9 – Gravitation
Class 9 Science
Last Updated: 15 April 2026
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Important Points
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Practice MCQ Questions (29 questions)
Q1. What is the force called that keeps planets in orbit around the Sun?
✓ Correct Answer: A
Q2. Which scientist formulated the Universal Law of Gravitation?
✓ Correct Answer: A
Q3. What does the Universal Law of Gravitation state about the force between two objects?
✓ Correct Answer: B
Q4. What is the SI unit of the gravitational constant (G)?
✓ Correct Answer: B
Q5. If the distance between two objects is doubled, how does the gravitational force between them change?
✓ Correct Answer: C
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Frequently Asked Questions (FAQ)
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Tough Words (Glossary)
Gravitation
Meaning: Natural attraction between any two masses in the universe.
अर्थ: ब्रह्मांड में किसी भी दो द्रव्यमानों के बीच प्राकृतिक आकर्षण।
"Gravitation keeps the planets around the Sun."
Gravitational force
Meaning: The attractive force acting between two masses.
अर्थ: दो द्रव्यमानों के बीच लगने वाला आकर्षण बल।
"The gravitational force pulls a dropped stone toward Earth."
Universal law of gravitation
Meaning: Law stating that every mass attracts every other mass; proportional to product of masses and inversely proportional to square of distance.
अर्थ: नियम: हर द्रव्यमान दूसरे द्रव्यमान को आकर्षित करता है; बल द्रव्यमानों के गुणनफल के समानुपाती और दूरी के वर्ग के व्युत्क्रमानुपाती होता है।
"The universal law of gravitation explains tides and planetary motion."
Proportional
Meaning: Having a constant ratio with another quantity.
अर्थ: किसी दूसरी राशि के साथ नियत अनुपात में होना।
"The force is proportional to the product of masses."
Inversely proportional
Meaning: Varies such that the product with the other quantity is constant.
अर्थ: ऐसा बदलना कि दूसरी राशि के साथ गुणनफल स्थिर रहे।
"Gravitational force is inversely proportional to the square of distance."
Constant
Meaning: A fixed value that does not change.
अर्थ: एक नियत मान जो नहीं बदलता।
"G is a universal constant in gravitation."
Gravitational constant (G)
Meaning: A proportionality constant in Newton’s law of gravitation (6.673×10⁻¹¹ N·m²/kg²).
अर्थ: न्यूटन के गुरुत्वाकर्षण नियम का अनुपातांक (6.673×10⁻¹¹ N·m²/kg²)।
"Cavendish measured the value of G."
Centripetal force
Meaning: Force directed toward the center that keeps a body moving in a circular path.
अर्थ: वह बल जो केंद्र की ओर होता है और वस्तु को वृत्तीय पथ में बनाए रखता है।
"Earth’s gravity provides the centripetal force for the Moon."
Tangent
Meaning: A straight line that touches a circle at exactly one point.
अर्थ: ऐसी सीधी रेखा जो वृत्त को केवल एक बिंदु पर स्पर्श करे।
"A released stone moves along the tangent to the circle."
Orbit
Meaning: The curved path of a body around another due to gravity.
अर्थ: गुरुत्वाकर्षण के कारण किसी पिंड का दूसरे के चारों ओर वक्र पथ।
"Satellites move in an orbit around Earth."
Satellite
Meaning: A body that revolves around a planet.
अर्थ: एक पिंड जो किसी ग्रह की परिक्रमा करता है।
"The Moon is Earth’s natural satellite."
Free fall
Meaning: Motion of a body under the influence of gravity alone.
अर्थ: केवल गुरुत्वाकर्षण के प्रभाव में किसी पिंड की गति।
"In free fall, all objects accelerate equally near Earth."
Acceleration due to gravity (g)
Meaning: Acceleration of a body in free fall near Earth (≈9.8 m/s²).
अर्थ: पृथ्वी के पास मुक्त पतन में पिंड का त्वरण (≈9.8 m/s²)।
"We take g as 9.8 m/s² in calculations."
Mass
Meaning: Amount of matter in a body; measure of inertia.
अर्थ: किसी पिंड में पदार्थ की मात्रा; जड़त्व का माप।
"Mass remains the same on Earth and on the Moon."
Weight
Meaning: Force with which Earth attracts a body (W = m×g).
अर्थ: बल जिससे पृथ्वी किसी पिंड को आकर्षित करती है (W = m×g)।
"Your weight is less on the Moon than on Earth."
Inertia
Meaning: Tendency of an object to resist change in its state of motion or rest.
अर्थ: वस्तु की अपनी अवस्था (गति/विश्राम) में परिवर्तन का विरोध करने की प्रवृत्ति।
"Heavier objects have more inertia."
Thrust
Meaning: Force acting on a surface in a direction perpendicular to it.
अर्थ: किसी सतह पर लम्बवत दिशा में लगने वाला बल।
"The thrust on the sand is higher when you stand."
Pressure
Meaning: Force per unit area.
अर्थ: प्रति इकाई क्षेत्रफल बल।
"Wide tyres reduce pressure on the road."
Fluid
Meaning: A substance that can flow, like a liquid or gas.
अर्थ: बहने वाला द्रव्य, जैसे तरल या गैस।
"Pressure in a fluid is transmitted equally in all directions."
Buoyancy
Meaning: The tendency of a fluid to exert an upward force on an immersed object.
अर्थ: द्रव का डूबे हुए पिंड पर ऊपर की ओर बल लगाने की प्रवृत्ति।
"Buoyancy makes you feel lighter in a pool."
Upthrust (Buoyant force)
Meaning: The upward force exerted by a fluid on an immersed object.
अर्थ: डूबे हुए पिंड पर द्रव द्वारा लगाया गया ऊपर की ओर बल।
"The upthrust on cork is greater than its weight, so it floats."
Density
Meaning: Mass per unit volume.
अर्थ: प्रति इकाई आयतन द्रव्यमान।
"Objects with density less than water float."
Displacement (of fluid)
Meaning: Volume of fluid pushed aside by an immersed object.
अर्थ: डूबे पिंड द्वारा हटाया गया द्रव का आयतन।
"A floating boat displaces its own weight of water."
Archimedes’ principle
Meaning: Upward buoyant force equals the weight of the displaced fluid.
अर्थ: उत्प्लावक बल विस्थापित द्रव के भार के बराबर होता है।
"Hydrometers work on Archimedes’ principle."
Lactometer
Meaning: Instrument to test the purity of milk using density.
अर्थ: दूध की शुद्धता घनत्व से जाँचने का यंत्र।
"A lactometer showed the milk was diluted."
Hydrometer
Meaning: Instrument used to measure the density of liquids.
अर्थ: तरल पदार्थों का घनत्व मापने का यंत्र।
"The mechanic checked battery acid with a hydrometer."
Equator
Meaning: An imaginary line around Earth equidistant from the poles.
अर्थ: पृथ्वी पर ध्रुवों से समान दूरी पर खींची गई काल्पनिक रेखा (भूमध्य रेखा)।
"The value of g is slightly less at the equator."
Poles
Meaning: The two points on Earth’s surface: North Pole and South Pole.
अर्थ: पृथ्वी की सतह के दो छोर: उत्तर ध्रुव और दक्षिण ध्रुव।
"g is slightly greater at the poles."
Radius (of Earth)
Meaning: Distance from Earth’s center to its surface.
अर्थ: पृथ्वी के केंद्र से उसकी सतह तक की दूरी (त्रिज्या)।
"Earth’s radius affects the value of g."
Cavendish experiment
Meaning: An experiment that measured the gravitational constant G.
अर्थ: G का मान मापने वाला प्रयोग।
"The Cavendish experiment used a torsion balance."
Vacuum
Meaning: A space with no matter (no air).
अर्थ: ऐसी जगह जहाँ पदार्थ (हवा) न हो।
"In a vacuum, a feather and a coin fall together."
Atmosphere
Meaning: Layer of gases surrounding Earth; also a unit of pressure.
अर्थ: पृथ्वी को घेरे गैसों की परत; दाब की एक इकाई भी।
"Air resistance in the atmosphere slows falling objects."
Equilibrium
Meaning: State in which net force is zero.
अर्थ: स्थिति जिसमें परिणामी बल शून्य हो।
"A floating object is in equilibrium of forces."
Vector
Meaning: A quantity with magnitude and direction.
अर्थ: मात्रा जिसमें परिमाण और दिशा दोनों हों।
"Weight is a vector; it acts downward."
Scalar
Meaning: A quantity with only magnitude.
अर्थ: मात्रा जिसमें केवल परिमाण हो।
"Mass is a scalar quantity."
Magnitude
Meaning: Size or numerical value of a quantity.
अर्थ: किसी मात्रा का परिमाण/संख्यात्मक मान।
"The magnitude of force doubles when mass doubles."
Direction
Meaning: The line or course along which something moves or acts.
अर्थ: दिशा जिसमें कुछ चलता या कार्य करता है।
"Gravity acts in the downward direction."
Net force
Meaning: Overall force after combining all forces acting on an object.
अर्थ: वस्तु पर लगने वाले सभी बलों का कुल (परिणामी) प्रभाव।
"With zero net force, velocity stays constant."
Acceleration
Meaning: Rate of change of velocity with time.
अर्थ: समय के साथ वेग का परिवर्तन दर।
"Free fall gives an acceleration equal to g."
Gravity
Meaning: The natural force that attracts bodies toward each other, especially toward Earth.
अर्थ: प्राकृतिक बल जो पिंडों को एक-दूसरे की ओर, विशेषकर पृथ्वी की ओर, आकर्षित करता है।
"Gravity keeps us grounded on Earth."
Weightlessness
Meaning: Condition when a body experiences no apparent weight.
अर्थ: स्थिति जब शरीर को भार महसूस नहीं होता।
"Astronauts feel weightlessness in orbit."
Microgravity
Meaning: Very weak effective gravity experienced in orbit.
अर्थ: कक्षा में अनुभव होने वाला बहुत कमजोर प्रभावी गुरुत्व।
"Plants grow differently in microgravity."
Tides
Meaning: Periodic rise and fall of sea levels caused mainly by the Moon’s gravity.
अर्थ: समुद्र के जलस्तर का चक्रीय बढ़ना-घटना, मुख्यतः चंद्रमा के गुरुत्व के कारण।
"High tides occur due to the Moon’s gravitational pull."
Apparent weight
Meaning: The weight a body seems to have due to additional forces (e.g., in an elevator).
अर्थ: अतिरिक्त बलों के कारण दिखाई देने वाला भार (जैसे लिफ्ट में)।
"Your apparent weight increases in a rising lift."
Pascal (unit)
Meaning: SI unit of pressure equal to one newton per square metre.
अर्थ: दाब की एसआई इकाई, एक न्यूटन प्रति वर्ग मीटर।
"Tyre pressure is not measured in pascals at the pump."
Pascal’s law
Meaning: Pressure applied to an enclosed fluid is transmitted undiminished in all directions.
अर्थ: बंद द्रव पर लगाया गया दाब बिना घटे सभी दिशाओं में प्रसारित होता है।
"Hydraulic brakes work on Pascal’s law."
Newton (unit)
Meaning: SI unit of force.
अर्थ: बल की एसआई इकाई।
"Weight is measured in newtons."
Metre per second squared
Meaning: SI unit of acceleration (m/s²).
अर्थ: त्वरण की एसआई इकाई (मी/से²)।
"The acceleration due to gravity is about 9.8 metre per second squared."
Tidal force
Meaning: The difference in gravitational pull that stretches objects.
अर्थ: गुरुत्व खिंचाव के अंतर से वस्तुओं का फैलना (ज्वारीय बल)।
"Tidal forces cause ocean tides on Earth."
Center of mass
Meaning: Point representing the mean position of the mass of a body.
अर्थ: पिंड के द्रव्यमान के औसत स्थान का प्रतिनिधि बिंदु।
"Gravity seems to act at the center of mass."
Terminal velocity
Meaning: The constant speed a falling object reaches when drag balances weight.
अर्थ: वह स्थिर वेग जो गिरती वस्तु को तब मिलता है जब घर्षण बल भार के बराबर हो जाए।
"Raindrops fall at terminal velocity."
Escape velocity
Meaning: Minimum speed needed to leave a body’s gravitational field without falling back.
अर्थ: किसी पिंड के गुरुत्व क्षेत्र से बिना लौटे निकलने का न्यूनतम वेग।
"Rockets must exceed Earth’s escape velocity."