GK (Science)

The ionosphere, located between 60-1000 km altitude, contains ionized particles that reflect radio waves back to Earth, enabling long-distance radio communication. The exosphere (A) is too thin, stratosphere (C) contains ozone but doesn't reflect radio waves well, and troposphere (D) is where weather occurs but doesn't reflect radio waves.

A physical change doesn't produce new substances and is often reversible. Magnetizing a needle only aligns magnetic domains - the material remains the same and can be demagnetized. Radioactive decay (A) changes elements, photosynthesis (B) produces glucose from CO₂ and water, and rusting (D) creates iron oxide - all are chemical changes.

Graphite and diamond are both allotropes of carbon, meaning they have the same atomic composition and thus the same relative atomic mass (12). They differ in electrical conductivity (graphite conducts, diamond doesn't), crystal structure (hexagonal vs cubic), and density (diamond is denser). The only property they share is their atomic weight.

Dry ice is the solid form of carbon dioxide (CO₂). It's called 'dry' because it sublimates (transitions directly from solid to gas) without becoming liquid, unlike water ice. Options B, C, and D are incorrect - dry ice is not made of hydrocarbons, water, or chlorine.

The SI system has many derived units formed by combining base units. Common examples include newton (force), joule (energy), watt (power), pascal (pressure), and volt (voltage). The options 5, 3, and 1 are incorrect - there are far more derived units than these small numbers.

Hertz (Hz) is the SI unit for frequency, measuring cycles per second. While pitch is a perceptual quality of sound, the physical frequency that determines pitch is measured in Hz. Decibel measures intensity level, coulomb measures electric charge, and 'cycles' is not a standard unit.

A fathom is a unit of depth equal to 6 feet (approximately 1.83 meters), traditionally used for measuring water depth in navigation and maritime contexts. It is not used for measuring sound, general distances, or frequency.

Linear momentum is always conserved in any collision (elastic or inelastic) when no external forces act. Temperature remaining the same is a condition stated in the problem, not a conserved quantity. Velocity and kinetic energy are NOT conserved - in inelastic collisions, kinetic energy is lost to deformation/heat, and velocities of individual balls change.

From Newton's second law: F = ma, rearranged gives m = F/a. Mass equals applied force divided by resulting acceleration. Option D (F/Δp) is incorrect because Δp = F·Δt, so F/Δp = 1/Δt, not mass. Options A and B incorrectly use velocity instead of acceleration.

Graphite rods in nuclear reactors serve as moderators - they slow down fast neutrons produced during fission to thermal energies, making them more likely to cause further fission reactions. Graphite is NOT used as fuel (uranium/plutonium are fuel), not as lubricant in this context, and it is a conductor, not an insulator.

Newton's second law (F = ma) explains this - the loaded truck has more mass (m), so at the same velocity, it has more momentum. Stopping it requires more force to produce the deceleration. Newton's first law describes inertia, third law describes action-reaction pairs, and gravitational law governs attraction between masses.

Nautical mile is a unit of distance equal to 1.852 km, specifically used in maritime and aviation navigation. It is NOT used in astronomy (parsecs and light years are used), not for road distances (km or miles are used), and not specifically for measuring national boundaries.

When lifting by hand, the arm functions as a second-class lever: the elbow joint is the fulcrum, the effort (muscle force) is applied between the fulcrum and the load (weight in hand). In a first-class lever, the fulcrum is between effort and load (like a seesaw). In a third-class lever, the load is between fulcrum and effort (like tweezers).

A light year is the distance light travels in one vacuum year (approximately 9.46 trillion km). It measures stellar and interstellar distances, NOT the speed of light (which is constant at about 300,000 km/s). It is not used for measuring speeds of rockets or airplanes.

At the lowest point of vertical circular motion, tension is maximum because: (1) the string must support the full weight of the ball (mg downward), and (2) provide centripetal force (mv²/r) directed upward toward the center. Total tension T = mg + mv²/r. At the highest point, tension is minimum (T = mv²/r - mg, if the ball completes the circle).

When a body moves uniformly in a straight line, its velocity is constant (both speed and direction unchanged). By Newton's first law, this means zero net force acts on it. Zero force is neither maximum, minimum, nor average - it's simply zero. Any acceleration would require a net force, causing non-uniform motion.

Pressure is defined as force per unit area (P = F/A). When lying flat, the body's weight is distributed over the largest possible surface area, resulting in minimum pressure. Standing or sitting concentrates weight on smaller areas (feet or buttocks), increasing pressure.

Among the four fundamental forces of nature, gravitational force is the weakest by far - approximately 10^36 times weaker than the strong nuclear force and 10^25 times weaker than electromagnetic force. Nuclear force is strongest, followed by electromagnetic, then weak nuclear, with gravity being weakest.

Momentum (p) is a vector quantity defined as the product of an object's mass and its velocity (p = mv). It represents the quantity of motion and is conserved in closed systems. Mass × acceleration is force, not momentum.

The SI (International System of Units) base unit for temperature is kelvin (K). While Celsius and Fahrenheit are commonly used in daily life, kelvin is the scientific standard, particularly in thermodynamics, where absolute temperature is measured from absolute zero.

The kilogram (kg) is the SI base unit of mass. Gram is a metric unit but not the base SI unit. Pound is imperial, and quintal is a non-SI metric unit. The kilogram is defined by the international prototype kilogram standard.

One horsepower (HP) is equivalent to approximately 746 watts (specifically 745.7 W). This conversion factor was established by James Watt and relates to the power needed to lift 550 pounds by one foot in one second. The other options (550W, 3300W, 33000W) are incorrect.

Electric field intensity is a vector quantity because it has both magnitude and direction - it points in the direction a positive test charge would move. Electric charge, potential, and resistance are all scalar quantities with magnitude but no direction.