Natural Sciences (Physics, Chemistry, Biology, Geography)
Physics
Speed is distance divided by time; acceleration is the change of speed per second. Newton's three laws explain inertia, F=ma and action–reaction pairs. Pressure is p=F/S, work is A=Fs, power is work per unit time, and energy is conserved. In electricity, Ohm's law gives I=U/R; resistances add in series and combine as 1/R=1/R₁+1/R₂ in parallel.
Chemistry
Use the periodic table for group, period and valence. Balance equations so every element has the same number of atoms on both sides. The mole links mass and particle number: n=m/M, and one mole contains 6.02×10²³ particles. Acid plus base gives salt plus water, and pH below 7 is acidic.
Biology
Know cell parts (nucleus, membrane, cytoplasm, mitochondria, and chloroplasts in plants), the photosynthesis equation (carbon dioxide plus water, using light, gives glucose and oxygen), the difference between mitosis and meiosis, and how energy passes along a food chain.
Geography
Read maps with the scale: on a 1:100 000 map, 1 cm represents 1 km. Know coordinates, climate belts and local relief - Mount Aragats at 4090 m and Lake Sevan.
Traps
Unit conversion is the biggest killer: 150 km in 2.5 h is 60 km/h, not 150 divided by 2.5 in other units. In chemistry an unbalanced equation ruins every later step, and moles are not molecules. In biology, students reverse the gases in photosynthesis.
Worked example
A 12 V source is connected to a 4 Ω resistor. I=U/R=12/4=3 A. If the resistance doubles to 8 Ω, the current halves to 1.5 A, because at constant voltage current and resistance are inversely proportional.