General Science MCQs – Page 6 (Q51-60)

Work is said to be done on an object only when an applied force causes a displacement, meaning the object must actually move some distance in the direction of the force for physics-defined work to occur. Holding a heavy box stationary, however tiring, involves no work in the scientific sense because there is no displacement, even though muscular energy is still being expended as heat. This precise definition differs from the everyday meaning of work and is essential for correctly applying the work-energy theorem, which relates the work done on an object to its change in kinetic energy.

The unit of work, the joule, is the same as the unit of energy because work and energy are fundamentally linked concepts: doing work on an object transfers energy to or from it. This is why lifting a book against gravity does work equal to the gravitational potential energy gained, and why a moving object's kinetic energy equals the work needed to accelerate it from rest. The work-energy theorem formalizes this relationship, stating that the net work done on an object equals its change in kinetic energy, tying together mechanics and energy conservation.

Power is defined as the rate at which work is done, or equivalently the rate at which energy is transferred, calculated by dividing work by the time taken to do it. This means two people lifting the same weight to the same height do the same amount of work, but the one who does it faster exerts more power. Power is measured in watts, and in everyday contexts is also expressed in horsepower for engines or kilowatts for household appliances. High power output is crucial in applications like sprinting, rocket launches, and electrical grid design.

One watt is defined as one joule of energy transferred or work done per second, making it a compact way to express the rate of energy use or production. This definition, honoring James Watt, lets us compare the power of vastly different devices, from a 5-watt LED bulb to a 100,000-watt wind turbine. Larger units like the kilowatt (1,000 watts) and megawatt (one million watts) are used for household appliances and power plants respectively. The unit also underlies the kilowatt-hour, the standard measure of energy consumption on electricity bills.

The gravitational force between two bodies depends on their masses and the distance between them, as described by Newton's law of universal gravitation: force increases with greater mass and decreases sharply as distance increases. This explains why the Moon, despite its much smaller mass than the Sun, exerts a stronger tidal pull on Earth's oceans because it is so much closer. The law applies universally, governing everything from apples falling to the ground to the orbits of planets and galaxies, making it one of the most far-reaching principles in physics.

Newton's law of gravitation states that gravitational force is inversely proportional to the square of the distance between two bodies, meaning that if the distance doubles, the force weakens to one-quarter of its original strength. This inverse-square relationship also appears in other physical phenomena, such as the intensity of light or sound spreading from a point source. Combined with the direct proportionality to the product of the two masses, the full law is expressed as F = G(m1m2)/r squared, where G is the universal gravitational constant.

The universal gravitational constant is represented by the symbol G, a very small number (approximately 6.674 times ten to the negative 11th cubic meters per kilogram per second squared) that reflects how comparatively weak gravity is compared to other fundamental forces like electromagnetism. English scientist Henry Cavendish famously measured G experimentally in 1798 using a sensitive torsion balance, allowing scientists to later calculate the mass of the Earth for the first time. G should not be confused with lowercase g, which represents the local acceleration due to gravity, approximately 9.8 m/s squared on Earth's surface.

Earth's escape velocity, the minimum speed needed for an object to break free of Earth's gravitational pull without further propulsion, is approximately 11.2 kilometers per second, or about 40,300 kilometers per hour. This is significantly higher than the roughly 7.9 km/s orbital velocity needed to maintain a stable low Earth orbit, since escaping requires overcoming gravity entirely rather than just balancing it with centripetal motion. Escape velocity depends on a body's mass and radius, which is why the Moon's escape velocity is much lower at about 2.4 km/s, making lunar launches far less demanding than Earth launches.

An object thrown straight upward slows down on its way up mainly due to the force of gravity, which continuously decelerates it at about 9.8 m/s squared until its upward velocity reaches zero at the peak of its trajectory. After reaching maximum height, gravity then accelerates the object back downward, and in the absence of air resistance, it would return to its starting height with the same speed it was launched, just directed downward. This symmetry is a classic feature of projectile motion problems in introductory physics, illustrating the constant, unchanging nature of gravitational acceleration near Earth's surface.

The curved path followed by a projectile under the influence of gravity alone, such as a thrown ball or a launched artillery shell, is called a parabola, resulting from the combination of constant horizontal velocity and constantly accelerating vertical motion due to gravity. Galileo was among the first to mathematically describe this parabolic trajectory in the seventeenth century. This principle is essential in fields ranging from sports, like calculating the optimal angle for a basketball shot, to engineering, such as designing the trajectories of missiles and space capsule reentries.