Physical Science: Matter, Forces, Energy, and Waves
What to know
Matter is anything with mass and volume. It has measurable properties - density, melting point, conductivity, solubility - that help identify substances. Density equals mass divided by volume, so a bigger object is not necessarily denser. Physical changes alter form, size, or state without creating a new substance (ice melting, salt dissolving); chemical changes produce new substances with different properties (rusting, burning, baking).
Forces are pushes or pulls described by strength and direction. Unbalanced forces change motion; balanced forces do not. An object at rest stays at rest unless a net force acts on it. Friction opposes motion and converts kinetic energy into heat. Speed is distance divided by time; velocity adds direction. Energy appears as motion, light, sound, thermal, electrical, and stored chemical or gravitational potential energy. It transfers and transforms but is not created or destroyed.
Light travels in straight lines, reflects at equal angles, and bends (refracts) when it moves between materials. Sound needs a medium and travels faster in solids than in gases. A circuit needs a closed loop for current to flow; bulbs in series share the voltage and dim as more are added, while parallel branches stay bright.
Traps to watch
Heat and temperature are not the same thing, and mass differs from weight (weight depends on gravity). "Chemical" is not a synonym for "dangerous," and dissolving is a physical change, not a chemical one.
Worked example
A car travels 150 m in 10 s, so its speed is 150 ÷ 10 = 15 m/s. Rule of thumb: if the substance's identity changes, the change is chemical; if only its form or state changes, it is physical.