Extra 30 short-answer questions from Chapter:10 Gravitation (Class 9 CBSE Science):-
1. State Newton’s Universal Law of Gravitation.
Answer: Newton’s Universal Law of Gravitation states that every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
Formula:
F = G m1m2/r2
= Gravitational force
= Universal Gravitational Constant
= Masses of objects
= Distance between objects
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2. What is the value of the Universal Gravitational Constant (G)? State its SI unit.
Answer: The value of G is 6.67 x 10-¹¹. Nm²/kg².
SI Unit: Newton meter² per kilogram² (Nm²/kg²).
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3. Why is the gravitational force not noticeable in everyday life?
Answer: The gravitational force is weak unless the objects involved have large masses, like planets or stars. The force between small objects is negligible, making it unnoticeable in daily life.
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4. Why does an object fall freely towards the Earth?
Answer: An object falls freely towards the Earth due to the force of gravity, which attracts all objects toward its center. The acceleration due to gravity () acts downward, causing free fall.
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5. Define acceleration due to gravity. What is its value on Earth?
Answer:
Definition: The acceleration experienced by a freely falling object due to Earth's gravitational pull is called acceleration due to gravity (g).
Value on Earth: .9.8, m/s²
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6. Does the value of ‘g’ remain constant everywhere on Earth? Explain.
Answer: No, the value of g varies due to:
1. Shape of Earth – It is higher at poles and lower at the equator.
2. Altitude – It decreases as we move above the Earth's surface.
3. Depth – It decreases inside the Earth.
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7. How is mass different from weight?
Answer:
Difference Between Mass and Weight
1. Definition:
Mass is the amount of matter in an object.
Weight is the force with which an object is attracted toward the Earth due to gravity.
2. Symbol:
Mass is denoted by .M
Weight is denoted by .W
3. SI Unit:
The SI unit of mass is kilogram (kg).
The SI unit of weight is Newton (N).
4. Formula:
Mass: m
Weight: W=mg
g= 9.8m/s² (where on Earth).
5. Constant or Variable?
Mass remains constant everywhere.
Weight changes with gravity (e.g., lower on the Moon, higher on Jupiter).
6. Measured Using:
Mass is measured using a beam balance.
Weight is measured using a spring balance.
7. Example:
A 10 kg object has the same mass (10 kg) on Earth and the Moon.
The same object has different weights: 98 N on Earth, 16.3 N on the Moon.
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8. A stone and a paper are dropped from the same height. Why does the stone reach the ground first?
Answer: The stone reaches the ground first due to air resistance. If there were no air, both would fall at the same rate due to gravity.
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9. What happens to the weight of an object when taken to the Moon? Why?
Answer: The weight of an object decreases to 1/6th of its weight on Earth because the Moon's gravity is six times weaker than Earth's.
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10. Derive the formula for weight of an object.
Answer:
Weight is given by:
W = mg
= mass of object
= acceleration due to gravity
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11. Why do astronauts feel weightless in space?
Answer: Astronauts experience weightlessness because they are in free fall inside the spacecraft, falling at the same rate as the spacecraft due to gravity.
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12. How does the gravitational force depend on mass and distance?
Answer:
F = G m1m2/r2
Force decreases if distance increases.
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13. Define free fall.
Answer: A freely falling object is one that moves under the influence of gravity without air resistance. Its acceleration is equal to g (9.8 m/s²).
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14. What is the motion of an object thrown upwards?
Answer:
It moves upward with decreasing speed.
At maximum height, speed = 0.
It falls back with increasing speed due to gravity.
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15. What happens to the gravitational force if the distance between two objects is doubled?
Answer:
If distance is doubled, the force decreases by times.
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16. What happens to the value of ‘g’ as we move away from Earth’s surface?
Answer: It decreases as the gravitational pull becomes weaker with increasing altitude.
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17. Why does a satellite not fall back to Earth?
Answer: A satellite moves at a high speed (orbital velocity), balancing gravitational pull, which keeps it in orbit instead of falling.
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18. How does gravity help in maintaining planetary motion?
Answer: The Sun's gravity pulls planets inward, while their motion prevents them from falling in, maintaining orbits.
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19. Why does a parachute slow down the fall of a person?
Answer: It increases air resistance, balancing gravity and slowing descent.
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20. How does weight change on different planets?
Answer: Weight depends on planetary gravity. A stronger gravity increases weight, while a weaker gravity decreases it.
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21. Why does a ball thrown up return to the ground?
Answer: Gravity pulls it downward, slowing it up, stopping it, then accelerating it down.
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22. Define the term ‘escape velocity’.
Answer: Escape velocity is the minimum speed required to escape Earth's gravity without falling back. It is 11.2 km/s for Earth.
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23. What is the weight of an object in a freely falling elevator?
Answer: The weight is zero, as both the object and elevator fall at the same rate, creating weightlessness.
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24. Why does the Moon not have an atmosphere?
Answer: The Moon's gravity is too weak to hold atmospheric gases.
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25. What is the importance of gravitational force in the universe?
Answer:
1. Keeps planets and moons in orbit.
2. Holds Earth’s atmosphere.
3. Causes tides in oceans.
4. Helps in star formation.
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26. Why do deep mines experience lower gravity?
Answer: Gravity decreases inside Earth as the effective mass pulling downward is reduced.
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27. Why does a rolling ball eventually stop?
Answer: Due to friction and air resistance counteracting gravity.
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28. Why do planets not move in a straight line?
Answer: The Sun’s gravity pulls them, causing curved orbits.
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29. What is a geostationary satellite?
Answer: A satellite that orbits Earth in 24 hours, appearing stationary over a location.
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30. Why do we weigh more at the poles than at the equator?
Answer: The Earth is slightly flattened at poles, making g stronger there, increasing weight.
