Earth and Space Science

Weathering breaks rock apart exactly where it sits — nothing moves anywhere. Erosion is the transport of that broken material by wind, water, ice or gravity, with deposition marking where it finally lands. There are three weathering mechanisms: physical (pure force, no chemical change), chemical (a reaction alters the minerals), and biological (a living organism does the breaking). A single agent, like a river, can do both jobs at once — dissolving rock chemically while carrying the debris downstream.

For a cube of side $s$, surface area is $SA=6s^2$ and volume is $V=s^3$, so the ratio $\frac{SA}{V}=\frac{6}{s}$ — this gets bigger as $s$ gets smaller. More exposed surface means more area for chemical reactions like acid rain to attack, so fragmented rock weathers faster overall, even though the reaction rate per square centimetre stays constant. The classic exam trap is comparing raw mass-loss figures between samples of different sizes and concluding 'smaller pieces weather faster' without doing the SA/V calculation first.

In investigations like identical rocks placed in water, vinegar and lemon juice, the independent variable is the thing you deliberately changed — here, the liquid — not the rocks themselves. The dependent variable is what you measured at the end, usually mass loss. Everything else (rock type, starting mass, temperature, time) must be kept identical as controlled variables, precisely so the only thing affecting mass loss is the liquid being tested.

The Sun holds about 99.8% of the Solar System's mass, with everything else — Mercury, Venus, Earth, Mars, the asteroid belt, then Jupiter, Saturn, Uranus, Neptune — orbiting it under gravity. The inner four are terrestrial planets (small, dense, rocky, solid surfaces); the outer four are gas or ice giants (huge, low density, no solid surface, ring systems). Dwarf planets like Pluto are round and orbit the Sun, but haven't cleared other debris from their orbital path — the exact reason Pluto lost full-planet status in 2006.

The Moon produces no light of its own — we only ever see the half currently lit by the Sun. As the Moon orbits Earth over about 27.3 days, the fraction of its sunlit half facing us keeps changing, producing the sequence from new Moon through crescent, quarter, gibbous, to full Moon, then back down (waning). Eclipses need a near-perfect straight-line alignment of Sun, Earth and Moon, and because the Moon's orbit is tilted about 5° relative to Earth's orbit, it usually passes above or below that line — which is why eclipses are rare rather than monthly.