States of Matter and Density

Ever wondered why you can't squash a rock but you can compress air in a bike pump? It's all about how particles are arranged and behave in solids, liquids, and gases.

In solids, particles are tightly packed and only vibrate in fixed positions - that's why your desk doesn't randomly change shape! Liquids have weaker forces between particles, so they can flow and take the shape of containers whilst staying the same volume. Gases have particles that move freely with loads of space between them, which is why you can compress air.

Density is simply how much mass is crammed into a space - think of it as how "heavy" something feels for its size. The formula is: Density = mass ÷ volume. When measuring irregular objects, you can use water displacement methods like an eureka can to find the volume.

Quick Tip: Remember that denser liquids sink below less dense ones - that's why oil floats on water!

Heat Transfer and Energy

Heat always moves from hot to cold objects - never the other way around. There are three ways this happens: conduction (through solids), convection (in liquids and gases), and radiation (through empty space, like heat from the Sun).

Conduction works because heated particles vibrate more vigorously and bump into their neighbours, passing energy along. This is why metal spoons get hot when you leave them in hot soup.

Specific heat capacity tells you how much energy is needed to heat up 1kg of a material by 1°C. You can measure this using a heater, thermometer, and some careful timing. Internal energy is the total kinetic and potential energy of all particles in a system.

When substances change state (like ice melting), they need specific latent heat - energy that breaks bonds without changing temperature. The formula is E = mL, where L is the specific latent heat.

Remember: During state changes, temperature stays constant even though you're adding energy!

Forces, Moments and Pressure

Your centre of mass is where all your weight seems to be concentrated - it's why you spread your legs wider on a wobbly bus to stay balanced! You can find it for irregular shapes by hanging them from different points and seeing where the lines cross.

Stability depends on two things: having a wide base and keeping your centre of mass low. Think about why sports cars are low and wide whilst double-decker buses have heavy engines at the bottom.

Moments create turning effects around a pivot point. The formula is Moment = Force × distance from pivot. For balance, clockwise moments must equal anticlockwise moments - that's why spanners have long handles for extra turning power.

In fluids (liquids and gases), pressure increases with depth because the weight of fluid above pushes down. The formula is Pressure = density × height × gravity. This explains why your ears pop when diving deep or going up mountains.

Real-world connection: Hydraulic car jacks use fluid pressure to lift heavy cars with relatively little force!

Upthrust and Atmospheric Pressure

Upthrust explains why some things float whilst others sink - it's the upward force that fluids exert on objects. If an object's weight equals the upthrust, it floats. If the weight is greater than upthrust, it sinks.

Here's the key rule: denser objects sink, less dense objects float. A steel ship floats because its overall density (including all the air inside) is less than water, even though steel itself is denser than water.

Atmospheric pressure decreases as you go higher up mountains because there are fewer air molecules above you pressing down. Gravity pulls most air molecules closer to Earth's surface, leaving less air at higher altitudes.

This pressure difference is why your crisp packets puff up on aeroplanes and why mountain climbers need oxygen masks at extreme heights.

Fun fact: The pressure at the bottom of the ocean is over 1000 times greater than at sea level - enough to crush a submarine!