Russia 7 класс Physics (basic)
Chapters: 5
1. Physics and its role in understanding the world
Physics as a natural science · Physical quantities and measurement · Scientific method
- Physics as a Natural Science: Phenomena and Links – Physics is the science that studies nature: how things move, heat, light, sound, electricity and magnets behave. Anything that happens in nature is a phenomenon. Physicists watch phenomena, find the rule behind them and write the rule with maths. Physics is connected to chemistry, biology, geography and technology, because their stories also use the same rules.
- 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.
- 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. First ideas about the structure of matter
Atoms and molecules · Motion of particles · Interaction of particles · States of matter
- The Particle Model of Matter – All matter is made of tiny particles that are always moving. In a solid they are close and only vibrate; in a liquid they are close but slide past each other; in a gas they are far apart and move fast in all directions. Heating gives particles more energy: either they move faster (temperature rises) or the forces between them are broken (the state changes while the temperature stays the same). Gas pressure comes from particles hitting the walls.
- Brownian Motion and Diffusion: Motion and Interaction of Particles – All matter is made of particles that never stop moving. Fast, tiny water particles hit a bigger grain from all sides and make it jiggle in a zigzag path: this is Brownian motion. The same restless motion makes different substances mix by themselves, which is diffusion. Heat makes particles faster, so both speed up. Between particles there are forces: they pull when a little apart and push when very close.
3. Motion and interaction of bodies
Mechanical motion and speed · Inertia and mass · Density · Elastic force and Hooke's law · Gravity and weight · Resultant force · Friction
- 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.
- 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.
- 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.
- Newton's Universal Law of Gravitation – Every mass in the universe pulls every other mass. The pull between two point masses m₁ and m₂ a distance r apart is F = G m₁ m₂ / r². It acts along the line joining them. The two bodies pull each other with equal and opposite forces. G = 6.67 × 10⁻¹¹ N m² kg⁻² is the same everywhere, so it is called the universal gravitational constant. When many masses pull one body, the forces add as vectors (superposition).
- 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.
4. Pressure of solids, liquids and gases
Pressure · Pascal's law · Liquid pressure · Atmospheric pressure · Archimedes' principle
- 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.
- 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.
5. Work, power, energy
Work and power · Simple machines · Mechanical energy
- 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.