Explanation: When a man inside a moving train tosses a coin and it falls behind him, it means the train is moving forward with acceleration. The coin, after being tossed, continues to move with the train’s initial velocity, but since the train speeds up, the coin appears to fall backward relative to the man.

The purity of milk is tested using a Lactometer. It works on the principle of Archimedes’ principle and is used to measure the specific gravity (density) of milk. Pure milk has a specific gravity ranging between 1.026 and 1.032. If water or other substances are added, the density decreases, indicating adulteration. The lactometer thus helps determine whether the milk is pure or diluted.

Other options (for context)

• Electrometer: Measures electric potential or charge, not related to milk testing.
• Radiometer: Measures radiant energy or light intensity.
• Refractometer: Used to measure refractive index, commonly for sugar solutions, not milk purity.

Explanation: A CD-ROM (Compact Disc Read-Only Memory) is an optical disk used to store large amounts of data. It is read using a laser beam in a CD drive and is commonly used for distributing software, music, and multimedia content.

Explanation: The angle of friction and the angle of repose are equal to each other. The angle of friction is the angle made by the resultant of the limiting friction and normal reaction with the normal force, while the angle of repose is the maximum angle of an inclined plane at which an object just begins to slide. Both represent the same physical condition of impending motion.

Explanation: The intensity of sound is measured in decibels (dB). The decibel is a logarithmic unit that expresses the ratio of a sound’s intensity compared to a reference level (usually the faintest sound the human ear can detect, known as the threshold of hearing). The higher the decibel value, the louder the sound. For example, normal conversation is around 60 dB, traffic noise is about 80–90 dB, and sounds above 120 dB can cause pain or hearing damage.

Explanation: A rainbow is formed in the part of the sky opposite to the Sun. It occurs when sunlight is refracted, internally reflected, and dispersed by raindrops, splitting the light into its seven constituent colors. The observer must have the Sun behind them and rain in front to see the rainbow clearly.

Graphite is used in pencils. It is a soft form of carbon that leaves a dark mark on paper due to its layered structure, where carbon atoms are arranged in sheets that slide easily over each other. Though commonly called “lead,” pencils actually contain graphite mixed with clay to adjust hardness.

• Silicon: Used in electronics, not pencils.
• Charcoal: Used in art drawing but not regular pencils.
• Phosphorous: Used in safety matches and fertilizers, not pencils.

Explanation: The tusks of elephants are actually modified second upper incisors. They are made primarily of dentine, commonly known as ivory, and covered by a thin layer of enamel at the tip (which wears off with time). Tusks grow continuously throughout the elephant’s life and are used for digging, lifting objects, stripping bark, defense, and dominance displays.

Note: Both male and female African elephants have tusks, whereas in Asian elephants, tusks are generally present only in males.

Composition: Tusks are composed of dentine (ivory) with fine concentric growth lines, and they grow from sockets in the skull.