All questions of Gravitation for UPSC CSE Exam

Understanding Gravitational Force
Gravitational force is a fundamental interaction that occurs between any two objects with mass, and it plays a crucial role in the dynamics of celestial bodies as well as everyday objects.
Characteristics of Gravitational Force
- Universal Attraction: Unlike other forces, gravitational force is always attractive. This means that two masses will always pull towards each other regardless of the distance separating them.
- Distance Independence: The force acts at all distances, meaning that even at vast distances, the gravitational force is present, albeit weaker.
- Inverse Square Law: The strength of the gravitational force decreases with the square of the distance between the two masses. This law means that as you double the distance, the gravitational force becomes four times weaker, but it never becomes zero.
Implications of Gravitational Force
- Cosmic Scale: On a cosmic scale, gravitational force governs the motions of planets, stars, and galaxies, keeping them in orbit and influencing their behaviors.
- Everyday Life: In everyday situations, gravity ensures that objects fall towards the Earth, providing stability and structure to our environment.
- No Repulsive Force: Unlike electromagnetic forces, which can be either attractive or repulsive, gravitational force lacks any repulsive component. This is why the correct answer is "attractive at all distances."
Conclusion
In summary, gravitational force is an ever-present attractive force between masses, operating at all distances without exception. This fundamental nature of gravity is why option 'C' is correct. Understanding this force is essential for comprehending both terrestrial and astronomical phenomena.

Understanding the Universal Gravitational Constant (G)
The Universal Gravitational Constant (G) is a fundamental constant in physics, crucial for understanding gravitational forces between masses. Its value is typically expressed in different units depending on the system used.
Value of G in C.G.S Units
In the C.G.S (centimeter-gram-second) system, the value of the Universal Gravitational Constant is represented as:
- G = 6.67 x 10^-8 cm³/g·s²
This value is derived from the standard value of G in SI units, which is approximately 6.674 x 10^-11 N·m²/kg². To convert it into the C.G.S system, we apply the necessary conversions:
- 1 N (Newton) = 10^5 dyne
- 1 m = 100 cm
- 1 kg = 1000 g
Using these conversions, the value of G in C.G.S units becomes:
- G = 6.674 x 10^-11 N·m²/kg² = 6.67 x 10^-8 cm³/g·s²
Explanation of Options
Now, let’s analyze the options provided:
- a) 6.67 * 10^-6 cgs unit (Incorrect)
- b) 6.67 * 10^-7 cgs unit (Incorrect)
- c) 6.67 * 10^-8 cgs unit (Correct)
- d) 6.67 * 10^-10 cgs unit (Incorrect)
The correct answer is option 'C' (6.67 * 10^-8 cgs unit), which aligns with the established value of G in the C.G.S system.
Conclusion
Understanding the value of G in different units is essential for solving problems in gravitational physics, especially in fields like astrophysics and engineering. The correct conversion and recognition of this constant are crucial for accurate calculations in these domains.

Understanding Weight Changes at High Altitudes
When an apple is taken to the mountain top, its weight is affected by the change in gravitational force. Here’s a detailed explanation:
Gravity and Weight
- Weight is the force exerted by gravity on an object.
- It is calculated as the product of mass and gravitational acceleration (Weight = Mass x Gravity).
- Gravity varies with altitude; it decreases as you go higher above sea level.
Effect of Altitude on Gravity
- At sea level, the gravitational force is strongest due to the Earth's mass being closer.
- As you ascend to a mountain top, the distance from the Earth's center increases, leading to a decrease in gravitational pull.
- This results in the apple weighing less than it would at sea level.
Conclusion
- Therefore, when the apple is taken to a mountain top, its weight is indeed decreased.
- This phenomenon is crucial in understanding how gravity works at different altitudes and impacts daily objects.
In summary, the correct answer is option 'A' because the decrease in gravitational force at higher altitudes leads to a reduction in the weight of the apple.