General Science and Physics Fundamentals

The critical resistance of a DC generator is the resistance of the field circuit. It is the maximum field circuit resistance at which the generator will just build up voltage and self-excite. Above this resistance, the generator fails to excite.

The cat survives falls from higher floors because air resistance (drag) increases with speed, leading to a terminal velocity. From the seventh floor, the cat reaches terminal velocity and can orient itself to land properly. From the third floor, it hasn't reached terminal velocity and has less time to right itself.

In DC generators, the generated emf and armature current flow in the same direction - both leave the positive terminal during generation. This is the fundamental principle of generator operation. In motors, the emf (back emf) opposes the current flow. Series and shunt motors consume electrical power, so their emf opposes current.

Cricket ball swing is caused by pressure difference on the two sides of the ball due to asymmetric airflow over the seam. Bernoulli's equation explains how faster-moving air creates lower pressure, causing the ball to swing toward the side with higher pressure. Navier-Stokes and Euler equations govern fluid flow but are unnecessarily complex for this phenomenon.

When starting resistance of a series motor is short-circuited, the motor receives full voltage immediately, causing very high current and torque. This excessive torque makes the motor run at dangerously high speeds, which can damage the motor. The starting resistance is meant to limit initial current and prevent this unsafe condition.

This question tests physics concepts related to motion on surfaces. A leather cricket ball rolls farther and faster than a tennis ball due to multiple factors: (A) Tennis balls experience more friction due to their fuzzy surface and greater deformation, (C) Tennis balls are softer and deform more, increasing rolling resistance, and (D) Leather balls are harder and maintain their shape better, reducing energy loss. Option B is incorrect because shininess doesn't significantly affect rolling distance.

The conical shape of loudspeakers serves two key purposes: it allows sound waves to transition gradually from the speaker to the air (impedance matching), and it ensures maximum power transfer between the speaker cone and the surrounding air. Echoing would distort sound rather than help, and climate protection is not the primary purpose.

The chemical symbol for Aluminium is Al, derived from its Latin name 'Aluminum' or 'Aluminium'. Au is the symbol for Gold, Am represents Americium, and Sb is the symbol for Antimony. These symbols are standardized internationally by IUPAC.

The moon's mass is calculated by analyzing the orbital motion of satellites (both natural and artificial) around the moon using Newton's law of gravitation. By measuring how objects orbit the moon, scientists can determine its gravitational influence and thus its mass. While we have lunar samples, they're not used for mass calculation this way, and the moon's revolution speed around Earth gives Earth's mass, not the moon's.

On the moon's surface with near-vacuum conditions, water would rapidly evaporate (sublime) due to extremely low atmospheric pressure. The water wouldn't simply float (A), freeze immediately (B), or form slow droplets (C). Evaporation into the vacuum happens quickly.

Friction is a non-conservative force because the work done by friction depends on the path taken, not just endpoints. Energy is lost as heat during friction, making it irreversible. Conservative forces like gravity are path-independent.

Helium's chemical symbol is He, derived from its Greek name 'Helios.' H is hydrogen, Ho is holmium, and Au is gold. The first letter is capitalized, second is lowercase.

Magnesium's chemical symbol is Mg, using the first letter and a distinguishing second letter. Mn is manganese, Ma and Mu are not element symbols. Elements use 1-2 letters only.

Zinc's chemical symbol is Zn, derived from its German name 'Zink.' Z is not used alone, and Zi, Zc are not valid element symbols. The symbol follows standard two-letter convention.

Thorium's chemical symbol is Th, directly from its name. To and Tho are not element symbols. Ti is titanium. Thorium is an actinide radioactive element.

Uranium's chemical symbol is U, a single-letter symbol. Ua and Ur are not valid element symbols. Tm is thulium. Uranium is a radioactive actinide element used as nuclear fuel.

Titanium's chemical symbol is Ti, derived from its Latin name 'Titanum'. Option A (Tt) looks logical but is incorrect. Tm is the symbol for Thulium, and Um is not a valid element symbol.

Sodium's symbol is Na from its Latin name 'Natrium'. Option A (So) is a common incorrect guess based on the English name. Nu is not a valid symbol, and Sm represents Samarium.

Silicon's symbol is Si. Option A (Sl) is a reasonable but incorrect guess. Ag is Silver, and Sn is Tin (from Latin 'Stannum'). Chemical symbols are standardized internationally.