General Science MCQs – Page 4 (Q31-40)

One horsepower is approximately equal to 746 watts, a unit originally devised by James Watt in the late eighteenth century to compare the power output of his steam engines to that of working horses. Watt calculated the figure based on how much work a horse could do turning a mill wheel over a set time. Horsepower remains widely used today to rate the power of car and motorcycle engines, even though the watt is the standard SI unit of power. Metric horsepower and mechanical horsepower differ slightly, with common usage in the US relying on the latter.

Speed is defined as the distance covered by a moving object divided by the time taken to cover it, expressed in units such as meters per second or kilometers per hour. It is a scalar quantity, meaning it only has magnitude and no specified direction, unlike velocity which includes direction. Average speed considers total distance over total time, while instantaneous speed refers to the speed at a specific moment, such as what a car's speedometer displays. The distinction between speed and velocity becomes important in physics problems involving circular or changing motion.

Velocity is a vector quantity because it specifies both magnitude, or how fast an object moves, and direction, or which way it moves, distinguishing it from the scalar quantity of speed. An object moving in a circle at constant speed still has changing velocity because its direction constantly changes, which is why it experiences centripetal acceleration. Vector quantities like velocity, force, and displacement must be added using vector rules that account for direction, unlike scalars such as mass, time, and temperature. This distinction is fundamental throughout classical mechanics.

Speed is a scalar quantity because it only describes magnitude, or how fast something is moving, without specifying a direction, unlike velocity, which is speed combined with direction. A car's speedometer, for example, shows only speed, while a GPS tracking its path also captures velocity and direction changes. Other common scalar quantities include mass, distance, time, and energy, all of which are fully described by a single numerical value. Recognizing whether a quantity is scalar or vector is essential for correctly applying physics formulas, especially in problems involving motion in more than one dimension.

Acceleration is defined as the rate of change of velocity with respect to time, meaning it measures how quickly an object speeds up, slows down, or changes direction. It is a vector quantity measured in meters per second squared, and can result from a change in speed, direction, or both, such as when a car goes around a curve at constant speed. Newton's second law directly links acceleration to force and mass through F = ma, making acceleration central to understanding how forces affect motion. Deceleration, or negative acceleration, occurs when an object slows down.

An object moving with uniform velocity travels equal distances in equal intervals of time along a straight line path, meaning both its speed and direction remain constant with zero acceleration. This is distinct from uniform circular motion, where speed may stay constant but the changing direction means velocity is not uniform and acceleration is present. Graphically, uniform velocity produces a straight-line distance-time graph, and Newton's first law explains why an object continues in such motion unless acted upon by an external force. Real-world examples are rare due to friction and air resistance, but the concept is foundational in kinematics.

The recoil of a gun when fired is best explained by Newton's third law of motion, which states that for every action there is an equal and opposite reaction: the explosive force propelling the bullet forward exerts an equal force pushing the gun backward into the shooter's shoulder. Heavier guns recoil less than lighter ones firing the same ammunition because the backward momentum is spread across more mass, following the principle that momentum is conserved in the system. This same action-reaction principle explains rocket propulsion, where expelled exhaust gases push the rocket in the opposite direction.

Weight is a measure of the gravitational force acting on an object's mass, calculated as mass multiplied by the local gravitational acceleration (W = mg), which is why weight is technically a force measured in newtons. This means an object's weight changes depending on the strength of gravity where it is located, becoming lighter on the Moon, where gravity is about one-sixth that of Earth, and heavier on more massive planets like Jupiter. Mass, in contrast, is an intrinsic property representing the amount of matter in an object and does not change with location.

Unlike weight, the mass of an object remains constant regardless of its location, because mass measures the fixed amount of matter within it, while weight depends on the local strength of gravity. An astronaut has the same mass on the Moon as on Earth, but weighs far less on the Moon due to its weaker gravitational pull. This distinction is often confused in everyday language, where people say they weigh a certain number of kilograms, though kilograms are technically a unit of mass, not force. Scales calibrated for Earth's gravity would give inaccurate readings elsewhere in the solar system.

Weight, being a force resulting from gravity acting on mass, is measured in newtons in the SI system, even though everyday scales often display readings in kilograms, which technically measure mass. A kilogram of mass on Earth weighs about 9.8 newtons due to Earth's gravitational acceleration. This distinction matters in physics and engineering, where precise calculations of force, such as those needed for structural loads or spacecraft launches, require newtons rather than the colloquial use of kilograms for weight. Spring scales measure force directly, while balance scales compare mass against known reference masses.