2. Recap: Physical Science Essentials

Physical Science is about matter and energy -- the rules everything else obeys.

The must-knows:

• Atoms: protons are positive, electrons negative, neutrons neutral. The atomic number = the number of protons, which identifies the element.
• States of matter: solid → liquid → gas as particles gain energy and move faster. These are physical changes (no new substance).
• Chemical change: a new substance forms (burning, rusting). Conservation of mass: matter is not created or destroyed, so mass is the same before and after.
• Energy: potential energy is stored (position/height); kinetic energy is motion. Energy transforms but is conserved.
• Forces & motion: Newton's second law, \(F = ma\); speed \(= \dfrac{\text{distance}}{\text{time}}\).
• Waves: higher frequency means shorter wavelength.

Common mix-up: a physical change (ice melting) is reversible and makes no new substance; a chemical change (burning) does.