China 八年级（初二） Physics

Chapters: 13

1. Introduction – starting scientific exploration

What physics studies; scientific inquiry

The Scientific Method – The scientific method is the careful way scientists find out how the world works. Observe something, ask a testable question, make a hypothesis (a clear, testable guess), test it with a fair experiment (change one variable, measure one, keep the rest the same), repeat and record data, analyse it, draw a conclusion and share it so others can check. Results that fail the test are useful too: they send you back to a new hypothesis.

2. Ch.1 Mechanical motion

Measuring length and time · Describing motion; relativity of motion · Speed · Measuring speed

Measurement and Units: How We Measure Anything – To measure something is to compare it with a fixed amount called a unit. Every measurement has a number and a unit. Scientists everywhere use the SI system, with seven base units such as the metre, kilogram and second. Prefixes like kilo (×1000), centi (÷100) and milli (÷1000) make units bigger or smaller. A good measurement starts at zero, is read with the eye straight above the mark, and is only as accurate as the smallest division (least count). Rounded values hide a small range, given by upper and lower bounds.
Motion: Distance, Speed, Velocity, Acceleration and Graphs – An object is in motion when its position changes with time. Distance is the full path length (a scalar); displacement is the straight gap from start to finish with a direction (a vector). Speed = distance ÷ time; velocity = displacement ÷ time. Acceleration = change in velocity ÷ time. The slope of an s–t graph gives velocity, the slope of a v–t graph gives acceleration, and the area under a v–t graph gives the distance. For uniform acceleration: v = u + at, s = ut + ½at², v² = u² + 2as.

3. Ch.2 Sound

Production and travel of sound · Pitch, loudness, timbre · Uses of sound · Noise and its control · Interdisciplinary: sound-proof room model

Sound – Sound is made by vibrating objects. It travels through a medium (air, water, solids) as a longitudinal wave: particles move back and forth, making crowded parts (compressions) and spread-out parts (rarefactions). Frequency (Hz) sets the pitch, amplitude sets the loudness, and speed v = f × λ. Sound cannot travel in vacuum. Humans hear 20 Hz to 20,000 Hz; below is infrasound, above is ultrasound. Reflected sound gives echoes, used and controlled in buildings.
Build a Sound-Proof Room Model – A sound-proof room keeps noise out (or in). Sound is a vibration, so we stop it with heavy, dense walls that reflect it, soft porous materials that soak it up, and layers with air between them. A tiny gap lets sound leak, so every crack must be sealed. In this project you test different materials with a fair test, record the noise level in decibels, then build a small room model and judge it against clear goals such as noise level, cost and thickness.

4. Ch.3 Changes of state

Temperature · Melting and solidification · Vaporisation and liquefaction · Sublimation and deposition · Interdisciplinary: kitchen state changes · Water cycle and water resources

Temperature – Temperature tells us how hot or cold something is. Inside every object, tiny particles are always moving; temperature measures their average kinetic energy (how fast they jiggle on average). We measure it with a thermometer, in degrees Celsius (°C), kelvin (K, the SI unit) or degrees Fahrenheit (°F). K = °C + 273 and °F = 9/5 × °C + 32. The lowest possible temperature is absolute zero, 0 K = −273 °C. When a hot and a cold object touch, heat flows from hot to cold until both have the same temperature (thermal equilibrium).
Changes of State: Melting, Boiling, Evaporation and More – A substance can change between solid, liquid and gas when we heat it or cool it. Melting, boiling, evaporation and sublimation need heat. Freezing, condensation and deposition give heat out. While the state is changing, the temperature stays the same, because the heat is used to break (or is released by making) the pull between particles. This hidden heat is called latent heat. Evaporation happens at any temperature, only from the surface, and it cools things down. Changes of state are physical changes: no new substance forms and the mass stays the same.
State Changes in the Kitchen – The kitchen is full of changes of state. Butter melting is solid to liquid. Boiling water and drying clothes are liquid to gas. Steam on a cold lid is gas to liquid. Water in an ice tray is liquid to solid. When heat goes into a substance it melts or becomes a gas. When heat comes out, it freezes or condenses. These changes are physical: the substance stays the same. Cooking an egg or baking bread makes new substances, so that is a chemical change, not a state change.
Water Cycle and Water Resources: State Changes and Saving Water – The water cycle is a loop of state changes powered by the Sun. Sun heat evaporates liquid water from seas and rivers into vapour (liquid to gas, heat in). High in the cold air the vapour condenses into drops and forms clouds (gas to liquid, heat out). Drops fall as rain, or as snow or hail when it is cold (solid). Melted snow and rain flow back to the sea. Only about 2.5 per cent of Earth's water is fresh, and most of that is ice, so we must save water.

5. Ch.4 Light

Rectilinear propagation · Reflection · Plane-mirror images · Refraction · Dispersion; colours

How Light Travels – Light comes from sources like the Sun or a lamp and travels in straight lines. We see an object when light from it enters our eyes. Transparent things let light through, translucent things let some through, and opaque things block it. Blocked light makes shadows; a wide source gives a dark umbra and a lighter penumbra. A pinhole camera makes an upside-down image. Light is very fast: about 300 000 km every second.
Reflection of Light: Plane and Spherical Mirrors – Light bounces off a shiny surface so that the angle of incidence equals the angle of reflection. A plane mirror gives a virtual, upright, same-size image as far behind as the object is in front. A concave mirror can give real or virtual images depending on where the object is; a convex mirror always gives a virtual, upright, smaller image. The mirror formula 1/v + 1/u = 1/f and magnification m = −v/u (with the New Cartesian sign convention) let you find the image without drawing.
Refraction of Light: Laws, Refractive Index and the Glass Slab – Light changes speed when it goes from one transparent medium to another, and so it bends at the boundary (unless it hits along the normal). Going into an optically denser medium it slows down and bends toward the normal; coming out it speeds up and bends away. For a pair of media, sin i / sin r stays constant (Snell's law); this constant is the refractive index, which also equals the ratio of the speeds of light, n = c/v. In a rectangular glass slab the emergent ray is parallel to the incident ray but shifted sideways (lateral displacement).
Refraction Through a Prism, Dispersion of Light and the Rainbow – A glass prism bends light twice, both times towards its base; the total turn is the angle of deviation. Each colour in white light bends by a slightly different amount (violet most, red least), so a prism fans white light out into a spectrum, VIBGYOR. An upside-down second prism joins the colours back into white. Raindrops act like tiny prisms plus a mirror, which gives us the rainbow.

6. Ch.5 Lenses

Lenses · Lenses in daily life · Convex-lens imaging rules · The eye and glasses · Interdisciplinary: make a telescope

Spherical Lenses: Images, Lens Formula and Power – A convex lens is thick in the middle and bends parallel light to meet at its focus; a concave lens is thin in the middle and spreads light out as if from its focus. With two simple rays (one parallel to the axis, one through the optical centre) you can find the image for any object position. A convex lens gives real, inverted images when the object is beyond F₁ and a virtual, erect, enlarged image inside F₁; a concave lens always gives a virtual, erect, diminished image. The lens formula 1/v − 1/u = 1/f, magnification m = v/u and power P = 1/f (in metres, unit dioptre) let you solve lens numericals.
The Human Eye: Parts, Accommodation and Defects of Vision – The eye is a camera made of living parts. The cornea and eye lens bend light to make a real, upside-down image on the retina. The ciliary muscles change the lens thickness so both far and near things look sharp (accommodation). When the eyeball is too long or too short, or the lens gets stiff, the image misses the retina; the right spectacle lens puts it back.
Optical Instruments: Microscopes and Telescopes – How big a thing looks depends on the angle it makes at the eye. A simple microscope (one convex lens) gives m = 1 + D/f (image at D) or D/f (image at infinity). A compound microscope uses a short-focus objective and an eyepiece: m = mₒ × mₑ ≈ (L/fₒ)(D/fₑ). An astronomical telescope uses a long-focus objective and short-focus eyepiece: m = fₒ/fₑ in normal adjustment, with tube length fₒ + fₑ. Reflecting telescopes use a concave mirror as objective.

7. Ch.6 Mass and density

Mass · Density · Measuring density of liquids and solids · Uses of density

Mass and Weight – Mass is the amount of matter in an object. It stays the same everywhere and is measured in kilograms (kg) with a pan balance. Weight is the pull of gravity on that mass. It is a force, measured in newtons (N) with a spring scale, and it changes from world to world: W = m × g.
Density – Density tells how much mass is packed into each unit of volume: ρ = m ÷ V. Its SI unit is kg/m³; in the lab we often use g/cm³ (1 g/cm³ = 1000 kg/m³). Water has density 1 g/cm³. An object less dense than a liquid floats in it; a denser one sinks.

8. Ch.7 Force

Force · Elastic force · Gravity

Forces: Pushes and Pulls – A force is a push or a pull between two objects. It is measured in newtons (N). Contact forces need touching (push, friction, normal force, tension, buoyancy); non-contact forces act at a distance (gravity, magnetism, electric). A force is drawn as an arrow: start = point of application, direction, length = size. Forces can change an object's speed, direction or shape. If forces balance, a still object stays still (equilibrium). Forces always come in pairs: if A pushes B, B pushes A back equally (Newton's third law).
Elastic Force and the Spring Balance – When you stretch, squeeze or bend a spring, it pushes or pulls back. This pull-back is the elastic force. Inside the elastic limit, the stretch grows in step with the pulling force (double the load, double the stretch), and the spring returns to its old length when you let go. A spring balance uses this to measure force in newtons.
Gravity and Weight: W = m × g – Earth pulls every object towards its centre. This pull is the weight of the object, a force that always points straight down. It is found with W = m × g, where g is about 9.8 N for every kg on Earth. Every object acts as if its whole weight pulls at one point, the centre of gravity.

9. Ch.8 Motion and force

Newton's first law; inertia · Balance of two forces · Friction · Adding forces on one line

Force and Laws of Motion – A force is a push or a pull. Balanced forces (net force zero) do not change motion; an unbalanced force changes speed or direction. Friction opposes sliding. First law: a body keeps its state of rest or uniform motion unless an unbalanced force acts (inertia; heavier bodies have more inertia). Momentum p = mv. Second law: F = ma (rate of change of momentum), 1 N = 1 kg m/s². Third law: forces come in equal and opposite pairs acting on two different bodies. For a system with no outside force, internal forces cancel and total momentum is conserved.
Friction: The Force That Opposes Sliding – Friction is a contact force between two surfaces. It always acts against the motion, or against the way an object is trying to move. Surfaces have tiny bumps that catch on each other. Static friction stops a still object from moving; it grows to match your push, up to a limit. Once the object slides, sliding (kinetic) friction acts, and it is a little smaller. Rolling friction is much smaller still. Friction is bigger when the surfaces are rougher and when they are pressed together harder (more weight). It hardly depends on the area of contact. Friction helps us walk, write and brake, but it wastes energy as heat and wears things out. We increase it with treads and grip, and reduce it with oil, smooth surfaces, wheels and ball bearings.

10. Ch.9 Pressure

Pressure · Liquid pressure · Atmospheric pressure · Interdisciplinary: piston pump · Fluid pressure and speed

Pressure: Force on Each Square Metre – Pressure is the force acting on each unit of area: P = F ÷ A. Its SI unit is the pascal (1 Pa = 1 N/m²). The same force on a smaller area gives a bigger pressure, which is why pins, knives and nails are sharp, and why tractors and camels have wide feet or tyres. Liquids press in all directions, and their pressure grows with depth (P = ρgh). Gases press because their particles keep hitting the walls; squeezing a gas or heating it raises its pressure. The air around us presses with about 101 kPa. Blood pressure is the pressure of blood on the walls of the arteries.
Pressure in Fluids and Pascal's Law – A fluid (liquid or gas) pushes on every surface it touches. Pressure is this normal force per area, P = F/A. Because of gravity, the fluid above a point has weight, so pressure grows with depth: P = P₀ + ρgh. Points at the same depth in a still liquid have the same pressure, whatever the vessel's shape. Pascal's law says an extra pressure applied to an enclosed fluid reaches every point equally. The hydraulic lift and brakes use this: a small force on a small piston becomes a big force on a big piston, F = f × A/a.
Atmospheric Pressure – Air has weight. The whole column of air above a surface pushes down on it; this push per square metre is atmospheric pressure. At sea level it is about 101 kPa (101 300 Pa), the same as a 760 mm column of mercury or about 10 m of water. Higher up there is less air above you, so pressure falls. In still air, the upward push of pressure on a layer balances the layer's weight (hydrostatic balance). Air flows from high pressure to low pressure, which is wind.
Piston Pump: Air Pressure at Work – A piston pump lifts water with the help of air pressure. Pulling the piston up lowers the pressure below it, so the air pressure on the well water pushes water up through the inlet valve. Pushing the piston down closes that valve and moves the water above the piston. The next upstroke lifts it out of the spout. Air pressure alone can push water up only about 10 m.
Fluid Speed and Pressure – Where a liquid or gas moves faster, its pressure is lower. Where it moves slower, the pressure is higher. This is Bernoulli's principle. In a pipe that narrows, the fluid speeds up and the pressure drops. Over an aeroplane wing the air above moves faster, so pressure above is lower and the higher pressure below pushes the wing up (lift).

11. Ch.10 Buoyancy

Buoyant force · Archimedes' principle · Floating and sinking; applications · Interdisciplinary: mini hydrometer

Archimedes' Principle: Why Things Float or Sink – Any object in a liquid (or gas) is pushed up by a force called upthrust or buoyant force. Archimedes' principle says this upthrust equals the weight of the liquid the object pushes aside: F = ρ g V. An object floats if its density is less than the liquid's, sinks if it is more. A floating object sinks just deep enough that upthrust equals its weight.

12. Ch.11 Work and mechanical energy

Work · Power · Kinetic and potential energy · Mechanical energy transformation

Work, Energy and Power – Work is done when a force moves an object: W = F × s, measured in joules (J). Energy is the ability to do work. A moving body has kinetic energy ½mv²; a raised body has potential energy mgh. Energy is never made or destroyed, only changed from one form to another. Power is how fast work is done: P = W ÷ t, in watts. Simple machines like levers and pulleys let a small effort move a big load.

13. Ch.12 Simple machines

Levers · Interdisciplinary: steelyard balance · Pulleys · Mechanical efficiency

Levers: How a Small Push Lifts a Big Load – A lever is a stiff bar that turns about a fixed point called the fulcrum. It balances when load × load arm = effort × effort arm (the law of moments). A longer effort arm means less effort is needed. By where the fulcrum, load and effort sit, levers are Class 1, 2 or 3.
Pulleys: How Ropes Make Lifting Easier – A pulley is a grooved wheel that guides a rope. A fixed pulley only changes the direction of the pull. A movable pulley and a pulley block share the load between several rope strands, so effort = load ÷ number of strands, but you pull more rope. The wheel and axle and the inclined plane also trade a longer distance for a smaller force.
Mechanical Efficiency: How Much Work Is Really Useful? – No real machine turns all the work you put in into useful work. Some is wasted on friction and on moving the machine parts. Efficiency η = useful work ÷ total work × 100%. It is always less than 100%. Oiling and using lighter parts raise it.