States of Matter and Forces
Matter exists in three main states - solid, liquid, and gas - with particles that have different amounts of kinetic energy and different forces between them. Heating a system gives particles more kinetic energy, making them move faster until they have enough energy to break bonds and change state.
Density calculations ρ=m/V help you work out whether objects will float or sink. For gases, there's a really useful relationship: pressure and volume are inversely proportional when temperature stays constant PV=constant. This explains why a balloon shrinks in the fridge.
Forces come in two types: contact forces (like friction and air resistance) and non-contact forces (gravity, magnetism, electrostatics). Weight is different from mass - mass is how much stuff you have, whilst weight depends on gravitational field strength. On the Moon, your mass stays the same but your weight becomes much less.
Moments and equilibrium are about turning effects. A moment equals force × distance from the pivot M=Fd. Levers work by increasing the distance from the pivot where you apply force, so you need less effort to get the same turning effect.
Real-World Connection: Understanding pressure P=F/A explains why drawing pins have sharp points and why you sink into sand but not concrete!