UPSC CSE Prelims
Mechanics and Motion Previous Year Questions (PYQs)

Detailed Explanation:

Answer: Option 3 — All three

An accelerometer is a sensor that measures acceleration forces, including changes in velocity, sudden movements, and changes in orientation (tilt). All three actions described in the question require the detection of acceleration changes or orientation shifts, making the accelerometer essential for their functioning.

✅ Action 1 – Car Crash Detection: Accelerometers detect the rapid deceleration during a collision, triggering immediate airbag deployment through the crash detection system.

✅ Action 2 – Laptop Free Fall Detection: Accelerometers sense the characteristic acceleration pattern of free fall (near-zero resistance), immediately parking the hard drive heads to prevent damage upon impact.

✅ Action 3 – Smartphone Tilt Detection: Accelerometers continuously monitor the device's orientation relative to gravity, enabling automatic display rotation between portrait and landscape modes.

📝 Short Notes: Accelerometer Applications

• Definition: An accelerometer is an electromechanical device that measures acceleration forces, both static (gravity) and dynamic (movement, vibration, shock).
• Working Principle: It detects changes in velocity, tilt, rotation, and vibration by measuring the displacement of a proof mass on springs when subjected to acceleration.
• Types: MEMS (Micro-Electro-Mechanical Systems) accelerometers are most commonly used in consumer electronics due to their small size and low power consumption.
• Automotive Safety: Used in airbag deployment systems, electronic stability control (ESC), anti-lock braking systems (ABS), and rollover detection.
• Consumer Electronics: Smartphones (screen rotation, step counting, gesture recognition), laptops (hard drive protection), gaming controllers (motion sensing), and fitness trackers.
• Industrial Applications: Vibration monitoring in machinery, structural health monitoring of buildings and bridges, and seismic activity detection.
• Aviation and Navigation: Inertial navigation systems, flight stabilization, and drone orientation control.

Detailed Explanation:

Answer: Option 4 — Speed of light is always same.

Astronomical distances are measured in light-years because the speed of light in a vacuum is a universal constant (approximately 299,792 km/s). This constant nature makes it a reliable and standardized unit for measuring the vast distances in space. A light-year represents the distance light travels in one year, providing a practical scale for cosmic distances.

❌ Statement A – Incorrect: Distances between stellar bodies do change due to orbital motion, stellar evolution, and galactic movements; this is not why light-years are used as a unit.

❌ Statement B – Incorrect: While gravity is fundamental, the gravitational effects of stellar bodies vary with mass and distance changes; this has no bearing on the choice of light-year as a distance unit.

❌ Statement C – Incorrect: Light does not always travel in a straight line; it bends around massive objects due to gravitational lensing, as predicted by Einstein's General Theory of Relativity.

✅ Statement D – Correct: The speed of light in a vacuum is constant at approximately 3 × 10⁸ m/s, making it an ideal standard for measuring astronomical distances.

📝 Short Notes: Astronomical Distance Measurements

• Light-Year: The distance light travels in one year in a vacuum (approximately 9.46 trillion kilometers or 5.88 trillion miles).
• Astronomical Unit (AU): The average distance from Earth to the Sun (approximately 150 million km); used for measuring distances within the solar system.
• Parsec: Another unit for astronomical distances, equal to about 3.26 light-years; commonly used by professional astronomers.
• Speed of Light (c): A fundamental physical constant equal to 299,792,458 meters per second in vacuum; remains constant regardless of the observer's motion or the source's motion.
• Gravitational Lensing: The bending of light around massive objects due to gravity, predicted by Einstein's General Theory of Relativity and observed around galaxies and black holes.

Detailed Explanation:

Answer: Option 3 — 1 and 3 Only

In a pressure cooker, the cooking temperature depends on the internal pressure, which determines the boiling point of water. The area of the hole in the lid and the weight of the lid regulate how much pressure builds up inside the cooker, thereby controlling the cooking temperature. The flame temperature only affects the rate of heating, not the maximum temperature achieved.

✅ Statement 1 – Correct: The size of the hole in the lid regulates steam release; a larger hole allows more steam to escape, reducing internal pressure and thus lowering the cooking temperature.

❌ Statement 2 – Incorrect: The temperature of the flame affects only the rate of heating, not the final cooking temperature, which is determined by the internal pressure of the cooker.

✅ Statement 3 – Correct: A heavier weight on the lid requires higher internal pressure to lift it, allowing steam to escape only when higher pressure is reached, which increases the boiling point and thus the cooking temperature.

📝 Short Notes: Pressure Cooker Mechanism

• Principle: Pressure cookers work on the principle that increasing pressure raises the boiling point of water above 100°C.
• Normal Boiling Point: At atmospheric pressure (1 atm), water boils at 100°C.
• In Pressure Cooker: Pressure can reach 15 psi (about 2 atm), raising the boiling point to approximately 121°C.
• Pressure Regulation: The weight (regulator) on the vent controls the maximum pressure by allowing steam to escape when pressure exceeds a threshold.
• Vent Hole Size: The area of the vent hole determines how easily steam can escape, affecting pressure buildup.
• Cooking Time: Higher temperature due to increased pressure reduces cooking time significantly.
• Flame Role: The flame intensity affects how quickly the cooker reaches operating pressure, not the final cooking temperature.

Detailed Explanation:

Ball bearings reduce the effective area of contact between the wheel and axle, replacing sliding friction with rolling friction.

Reduced contact area means lower friction and higher efficiency, as rolling balls have a much lower coefficient of friction than flat surfaces sliding against each other.

Detailed Explanation:

❌ Statement 1 – Incorrect: Gravity is the weakest of the four fundamental forces, not the strongest.
✅ Statement 2 – Correct: Electromagnetism acts only on particles with electric charge (positive or negative).
✅ Statement 3 – Correct: Weak nuclear force is responsible for radioactivity and beta decay.
✅ Statement 4 – Correct: Strong nuclear force holds protons and neutrons together inside the atomic nucleus.