Frequently Asked Questions About Physics Fundamentals

Galileo demonstrated around 1590 that all objects accelerate at 9.8 m/s² near Earth's surface regardless of mass, contradicting Aristotelian physics that dominated for 2000 years. The reason involves two offsetting factors: heavier objects experience proportionally stronger gravitational force (F = mg), but they also resist acceleration proportionally more due to greater inertia (F = ma). Since both force and resistance scale with mass identically, mass cancels out in the equation, leaving a = g. A 1 kg ball and a 100 kg ball dropped from 10 meters both hit the ground in 1.43 seconds. Air resistance complicates this for low-density objects like feathers, but in vacuum, even feathers and hammers fall together, as Apollo 15 astronaut David Scott demonstrated on the Moon in 1971.

Energy represents the capacity to perform work or cause change, measured in joules. It exists in forms including kinetic (motion), potential (position in a field), thermal (random molecular motion), chemical (bond arrangements), and electromagnetic (field configurations). Conservation of energy emerges from time symmetry in physical laws, proven mathematically by Emmy Noether's theorem in 1915. When you lift a 2 kg book 1.5 meters, you transfer 29.4 joules of chemical energy from your muscles into gravitational potential energy. Dropping the book converts this to kinetic energy, then to thermal energy and sound when it hits the floor. The total energy remains constant at every stage, though it becomes less concentrated and harder to use, which relates to entropy increasing.

This apparent paradox confused students for centuries. The horse pushes backward on the ground with its hooves, and the ground pushes forward on the horse with equal force (third law pair). Simultaneously, the horse pulls backward on the cart, and the cart pulls backward on the horse with equal force (another third law pair). The cart moves forward because the ground's forward push on the horse exceeds the friction between cart wheels and ground. The net force on the cart equals the tension in the harness minus wheel friction. If the horse exerts 500 N backward on the ground and wheel friction is 100 N, the cart experiences 400 N net forward force, producing acceleration according to F = ma. The key insight: third law pairs act on different objects, so they don't cancel when analyzing motion of a single object.

Water expands approximately 9% when freezing due to its molecular structure forming hexagonal crystals with empty space between molecules. Liquid water at 4°C has density 1000 kg/m³, while ice at 0°C has density 917 kg/m³. Since ice is less dense, buoyant force (equal to the weight of displaced water) exceeds ice weight, causing it to float with about 92% submerged. This unusual property occurs because water molecules are V-shaped with oxygen attracting electrons more strongly than hydrogen, creating partial charges. In liquid form, molecules pack efficiently despite random motion. Upon freezing, hydrogen bonds lock molecules into rigid hexagonal lattices with gaps, reducing density. This anomaly proves ecologically crucial: lakes freeze from the top down, allowing aquatic life to survive underneath. Most substances contract when solidifying because ordered crystalline packing occupies less volume than random liquid arrangements.

Temperature measures average kinetic energy per particle, while heat measures total energy transferred between systems due to temperature difference. A bathtub of water at 40°C contains far more thermal energy than a cup of water at 90°C, even though the cup has higher temperature. Adding 4,186 joules of heat raises 1 kg of water by 1°C. A 150 kg bathtub at 40°C contains roughly 25 million joules of thermal energy above 0°C, while a 0.25 kg cup at 90°C contains only 94,000 joules. Temperature determines the direction of heat flow (hot to cold), but the amount of heat transferred depends on mass, specific heat capacity, and temperature change according to Q = mcΔT. This distinction matters practically: touching a 200°C metal spoon briefly causes minor burns because metal's low mass transfers limited total heat, while 80°C water causes severe burns because large mass transfers enormous heat energy.

Light exhibits wave properties (interference, diffraction) in some experiments and particle properties (photoelectric effect, discrete energy packets) in others. This wave-particle duality emerged from experiments between 1801 and 1923. Thomas Young's double-slit experiment in 1801 showed light creating interference patterns characteristic of waves. Albert Einstein's 1905 photoelectric effect explanation demonstrated light delivers energy in discrete packets (photons) with energy E = hf, where frequency f determines energy. A photon of 500 nm green light carries 3.97×10⁻¹⁹ joules. Modern quantum mechanics resolves this apparent contradiction: light fundamentally behaves as neither classical wave nor classical particle, but as a quantum field. Which aspect we observe depends on how we measure it. Wave models predict behavior when we don't track individual photons; particle models apply when we detect discrete impacts. Both descriptions are approximations of the underlying quantum reality.