General Science MCQs – Page 9 (Q81-90)

The International System of Units recognizes seven base units: the meter for length, kilogram for mass, second for time, ampere for electric current, kelvin for temperature, mole for amount of substance, and candela for luminous intensity. All other physical quantities, such as force, energy, and pressure, are expressed as derived units built by combining these seven through multiplication or division, like the newton or joule. The SI system, maintained by the International Bureau of Weights and Measures, was formally established in 1960 and has since been progressively redefined using fundamental physical constants for greater precision and universality.

The joule is a derived SI unit rather than a base unit because it is expressed as a combination of the base units for mass, length, and time (kg·m squared/s squared), unlike the second, kilogram, and meter, which are themselves fundamental base units. Other common derived units include the newton for force, the watt for power, and the pascal for pressure, each built from combinations of the seven SI base units. Derived units allow scientists to express complex physical quantities consistently across all fields of science using a coherent, unified system rooted in just seven independent building blocks.

One kilometer equals 1,000 meters, following the standard metric prefix kilo-, which always denotes a multiplication by 1,000 in the SI system. This decimal-based structure makes unit conversions in the metric system far simpler than in older imperial systems, since moving between units like millimeters, meters, and kilometers only requires shifting a decimal point. Other common metric prefixes include centi- (one-hundredth), milli- (one-thousandth), and mega- (one million), used consistently across different physical quantities such as length, mass, and volume. Nearly every country except the United States, Liberia, and Myanmar uses the metric system officially.

One kilogram equals 1,000 grams, consistent with the metric kilo- prefix that multiplies a base unit by 1,000. The kilogram is the SI base unit of mass and, since 2019, has been defined using the fixed numerical value of the Planck constant rather than the physical platinum-iridium cylinder once kept near Paris. Everyday mass measurements often use grams for small quantities like food ingredients and kilograms for larger ones like body weight, while the tonne, equal to 1,000 kilograms, is used for very large masses such as vehicles or shipping cargo.

A vernier caliper is a precision instrument used to measure small lengths as well as internal and external diameters, using a main scale combined with a sliding vernier scale to achieve readings accurate to about 0.02 millimeters. Invented in its modern form by French mathematician Pierre Vernier in 1631, it typically features jaws for external measurements, smaller jaws for internal measurements, and a depth rod for measuring depths of holes or slots. It is widely used in engineering, manufacturing, and physics laboratories wherever more precision is needed than an ordinary ruler can provide. Digital calipers now often replace the manual vernier scale with an electronic display.

A screw gauge, also called a micrometer screw gauge, is used to measure very small thicknesses or diameters with high precision, typically down to about 0.01 millimeters, exceeding the precision of a vernier caliper. It works by using a calibrated screw thread, where a full rotation advances the spindle by a fixed, known distance, allowing fractional turns to be read precisely on a circular scale. Screw gauges are commonly used to measure the diameter of thin wires, the thickness of sheets of paper or metal, and small machined components in engineering and physics laboratories.

A stopwatch is the simple instrument commonly used to measure short time intervals in physics experiments, allowing a user to start, stop, and reset timing manually or electronically with precision often down to hundredths of a second. Mechanical stopwatches use spring-driven mechanisms, while modern digital stopwatches use quartz crystal oscillators for greater accuracy and are often built into smartphones and lab timers. Accurate timing is essential in experiments such as measuring a pendulum's period or an object's acceleration during free fall, where small timing errors can significantly affect calculated results.

A barometer is the instrument commonly used to measure atmospheric pressure, with the mercury barometer, invented by Evangelista Torricelli in 1643, being one of the earliest designs, using the height of a mercury column balanced against air pressure. Standard atmospheric pressure at sea level supports a mercury column about 760 millimeters high, a value still used as a reference unit. Modern aneroid barometers use a sealed, flexible metal capsule that expands or contracts with pressure changes instead of liquid mercury. Barometers are essential tools in weather forecasting, since falling pressure often signals approaching storms while rising pressure indicates fair weather.

The time period of a simple pendulum mainly depends on its length and the local acceleration due to gravity, following the formula T = 2 pi times the square root of length divided by gravitational acceleration. A longer pendulum takes more time to complete each swing, while a stronger gravitational field shortens the period, which is why a pendulum clock would run at a different rate on the Moon than on Earth. Christiaan Huygens applied this relationship to invent the pendulum clock in 1656, dramatically improving timekeeping accuracy for centuries afterward.

For small oscillations, the time period of a simple pendulum is independent of the mass of the bob, depending only on the pendulum's length and the local gravitational acceleration. This is because both the gravitational force pulling the bob and its inertial resistance to that force scale together with mass, so heavier and lighter bobs of the same length swing with identical periods, mirroring how all objects fall at the same rate under gravity alone. The period does, however, depend on amplitude for larger swing angles, a factor considered negligible in the small-angle approximation used in most pendulum calculations.