Chapter 9: Gravitation Long Questions | science Class 9 CBSE

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Chapter 9: Gravitation Long Questions | science Class 9 CBSE | ByteNotes.in

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Question 1

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Q1. Define gravitation. How did Newton explain it through the example of a falling apple and the motion of the moon?

• Gravitation is the force by which every object in the universe attracts every other object.
• Newton observed that a falling apple was pulled by the Earth, suggesting a force of attraction.
• He reasoned that the same force also holds the moon in orbit around the Earth.
• The moon’s path is curved due to this continuous pull by the Earth.
• Thus, both the falling apple and moon’s orbit are due to gravitational force.
• This led Newton to propose the universal law of gravitation.

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Question 2

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Q2. State the Universal Law of Gravitation. Write its mathematical expression and explain each term.

• Every object attracts every other object with a force directly proportional to the product of their masses.
• The force is inversely proportional to the square of the distance between their centres.
• Formula: F = G × (M × m) / d²
• F is the gravitational force, G is the gravitational constant.
• M and m are the masses of the two objects.
• d is the distance between the centres of the two objects.
• The force acts along the line joining the two centres.

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Question 3

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Q3. What are the applications and significance of the Universal Law of Gravitation?

• Explains why objects fall to the ground.
• Describes the motion of the moon around the Earth.
• Accounts for the motion of planets around the Sun.
• Helps understand tides due to moon and Sun's gravitational pull.
• Explains why the Earth remains in orbit around the Sun.
• Basis for calculating satellite orbits.
• Universal in nature—applies to all objects regardless of size or location.

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Q4. What is free fall? How does acceleration due to gravity influence it?

• Free fall refers to the motion of objects falling under the influence of gravity alone.
• No other forces like air resistance act during free fall.
• During free fall, objects experience acceleration towards the Earth.
• This acceleration is known as acceleration due to gravity (g).
• Near Earth’s surface, g is approximately 9.8 m/s².
• All objects fall with the same acceleration g, regardless of mass.
• Galileo demonstrated this using objects dropped from the Leaning Tower of Pisa.

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Question 5

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Q5. Derive the formula for the gravitational force between Earth and an object on its surface.

• The universal law states: F = G × (M × m) / R²
• M is Earth's mass, m is object's mass, R is Earth's radius.
• g = G × M / R², where g is acceleration due to gravity.
• Substituting g in the equation gives F = m × g.
• This shows gravitational force is proportional to the object's mass.
• The value of g is taken as 9.8 m/s² near Earth's surface.
• This relation simplifies calculations involving weight.

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Question 6

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Q6. Explain how the value of g is calculated and what factors affect it.

• g is calculated using the formula: g = G × M / R²
• G is gravitational constant: 6.7 × 10⁻¹¹ N·m²/kg².
• M is Earth’s mass: 6 × 10²⁴ kg.
• R is Earth’s radius: 6.4 × 10⁶ m.
• Substituting values gives g ≈ 9.8 m/s².
• g decreases with altitude and varies with latitude.
• g is greater at poles than at the equator due to Earth’s shape.

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Q7. What is the difference between mass and weight?

• Mass is the amount of matter in a body; weight is the force with which Earth attracts it.
• Mass is scalar; weight is vector (has direction).
• Mass remains constant everywhere; weight varies with gravity.
• Weight = mass × g (W = m × g).
• SI unit of mass is kg; weight is newton (N).
• On the Moon, weight is 1/6th of that on Earth.
• A spring balance measures weight, not mass.

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Question 8

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Q8. Explain why astronauts feel weightlessness in an orbiting spacecraft.

• In orbit, both the astronaut and spacecraft fall towards Earth with the same acceleration.
• No normal reaction force acts on the astronaut.
• There is no contact force to feel weight.
• Apparent weight becomes zero; real weight still exists due to mass and g.
• This condition is called microgravity or weightlessness.
• Weightlessness is due to free fall condition in orbit.
• It makes movement and orientation difficult for astronauts.

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Question 9

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Q9. Derive the three equations of motion for freely falling bodies under gravity.

• First equation: v = u + g × t
• Second equation: h = ut + ½ × g × t²
• Third equation: v² – u² = 2 × g × h
• u = initial velocity, v = final velocity, g = acceleration due to gravity, t = time, h = height.
• Equations apply when object falls freely under gravity.
• If object is thrown upward, g becomes negative in equations.
• Useful for solving vertical motion problems.

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Question 10

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Q10. Why does a feather fall slower than a stone in air but both fall together in vacuum?

• In air, feather experiences more air resistance due to its shape.
• Air drag slows its fall; stone faces negligible resistance.
• In vacuum, there is no air resistance.
• All objects fall at same rate under gravity in absence of air.
• Galileo proved this using experiments and logic.
• Shows acceleration due to gravity is independent of mass.
• Air resistance affects motion, not gravity itself.

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Chapter 9: Gravitation | Long Questions

Q1. Define gravitation. How did Newton explain it through the example of a falling apple and the motion of the moon?

Q2. State the Universal Law of Gravitation. Write its mathematical expression and explain each term.

Q3. What are the applications and significance of the Universal Law of Gravitation?

Q4. What is free fall? How does acceleration due to gravity influence it?

Q5. Derive the formula for the gravitational force between Earth and an object on its surface.

Q6. Explain how the value of g is calculated and what factors affect it.

Q7. What is the difference between mass and weight?

Q8. Explain why astronauts feel weightlessness in an orbiting spacecraft.

Q9. Derive the three equations of motion for freely falling bodies under gravity.

Q10. Why does a feather fall slower than a stone in air but both fall together in vacuum?

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