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The foundation every physics student needs — SI units, prefixes, significant figures and how to check any equation using dimensions.
Physics starts with measuring things correctly. Get this right and everything after it is easier.
THE 7 SI BASE UNITS
• Length — metre (m)
• Mass — kilogram (kg)
• Time — second (s)
• Electric current — ampere (A)
• Temperature — kelvin (K)
• Amount of substance — mole (mol)
• Luminous intensity — candela (cd)
Everything else (force, energy, speed…) is derived from these.
COMMON PREFIXES
• kilo (k) = 10³ , mega (M) = 10⁶ , giga (G) = 10⁹
• milli (m) = 10⁻³ , micro (µ) = 10⁻⁶ , nano (n) = 10⁻⁹
SIGNIFICANT FIGURES (quick rules)
• All non-zero digits count.
• Zeros between non-zeros count.
• Trailing zeros after a decimal count.
• In multiplication/division, the answer takes the fewest significant figures of the inputs.
DIMENSIONAL ANALYSIS (a superpower)
Every quantity has dimensions, e.g. speed = [L T⁻¹], force = [M L T⁻²].
Use it to CHECK equations: both sides must have the same dimensions. If they don't, the equation is wrong — no matter how neat it looks.
Example: is v = u + at correct? [L T⁻¹] = [L T⁻¹] + [L T⁻²][T] = [L T⁻¹]. ✓ Dimensions match.
Remember: a number without a unit is meaningless in physics.