Physics is the natural science that studies matter, energy, and the fundamental forces that govern the universe. Physicists use the scientific method — a systematic process for building reliable knowledge through observation and experiment.

The steps of the scientific method are:

1. Observe a phenomenon in the natural world.
2. Hypothesize — propose a testable explanation.
3. Test the hypothesis through controlled experiments.
4. Conclude — analyze results and accept, reject, or refine the hypothesis.

This cycle may repeat many times before a reliable scientific theory is established.

Scientific Notation and Significant Figures

Scientists often work with very large or very small numbers. Scientific notation expresses any number as a coefficient between 1 and 10, multiplied by a power of ten. For example, Avogadro's number is $6.022 \times 10^{23}$ particles per mole, and the mass of a proton is approximately $1.673 \times 10^{-27}\ \text{kg}$.

Significant figures express the precision of a measurement. The rules are:

• All non-zero digits are significant.
• Zeros between non-zero digits are significant (e.g., 1007 has 4 sig figs).
• Leading zeros are not significant (e.g., 0.0052 has 2 sig figs).
• Trailing zeros after a decimal point are significant (e.g., 3.50 has 3 sig figs).

When multiplying or dividing, round your answer to the same number of significant figures as the measurement with the fewest sig figs.