Year 9 Physics Notes

Physical Quantities and Measurement Techniques

• Ruler: Measures length (1 mm - 1 meter), SI unit is meters.

• Volume:

  • Regular object: height x length x width.

  • Irregular object: Water displacement in a measuring cylinder.

• Time: Measured using clocks/watches, SI unit is seconds.

• Accuracy: Increase by repeating measurements and averaging.

• Micrometre Screw Gauge: Measures small widths/thicknesses, resolution of 0.01 mm.

  • Main scale (sleeve/barrel)

  • Thimble scale (rotating)

Motion

• Distance: Total length travelled, SI unit is meters.

• Speed: Rate of change in distance, SI unit is m/s. $v = d/t$

• Velocity: Speed with direction, SI unit is m/s. $v = s/t$

• Scalar Quantity: Magnitude only (e.g., distance, time, speed, mass).

• Vector Quantity: Magnitude and direction (e.g., displacement, velocity, acceleration, force).

• Displacement-time graph: Gradient = velocity.

• Velocity-time graph: Gradient = acceleration, area under graph = displacement.

• Free Falling: Motion under gravity only (constant acceleration). Acceleration due to gravity on Earth $(g = 9.8 \frac{m}{s^2})$. Terminal velocity is reached when air resistance equals gravity.

Mass, Weight & Density

• Mass: Amount of matter in an object, SI unit is kg (scalar).

• Weight: Force of gravity on an object, SI unit is Newtons. $W = mg$

• Density: Mass per unit volume, SI unit is $\frac{kg}{m^3}$. $\rho = \frac{m}{V}$

• Floating/Sinking: Less dense objects float, more dense objects sink.

Forces

• Force: Push or pull on an object. SI unit: Newton. Vector quantity.

• Inertia: Tendency to maintain state of motion.

• Newton’s 1st Law: Object at rest stays at rest; object in motion stays in motion with constant velocity (unless acted upon by external force).

• Newton’s 2nd Law: $F = ma$

• Circular Motion: Object at steady speed in circular orbit is always accelerating.

• Centripetal Force: Force towards center of circle.

• Newton’s 3rd Law: Every action has an equal and opposite reaction.

• Hooke’s Law: Springs extend in proportion to loads (within proportional limit).

• Elastic Limit: Point beyond which spring does not return to original shape.

• Friction: Force opposing motion, causing slowdown and heat.

• Turning Effect: Moments of a force (Newton meters), clockwise or anticlockwise.

• Equilibrium: Clockwise moment = anticlockwise moment, sum of forces = 0.

Energy, Work and Power

• Gravitational Potential Energy: Energy due to height. $GPE = mgh$

• Elastic Potential Energy: Energy stored in elastic materials. $EPE = \frac{1}{2} kx^2$

• Kinetic Energy: Energy of a moving object. $KE = \frac{1}{2} mv^2$

• Law of Conservation of Energy: Energy cannot be created or destroyed, only transformed.

• Work: Force x distance. SI unit: Joule. Scalar. $W = Fd$

• Energy: Capacity to do work. SI unit: Joules. Scalar.

  • Renewable: Inexhaustible (e.g., solar, hydroelectric, wind).

  • Non-Renewable: Exhaustible (e.g., fossil fuels).

• Power: Rate at which work is done. SI unit: Watt (J/s). Scalar. $P = \frac{W}{t}$

Pressure

• Pressure: Force per unit area. $P = \frac{F}{A}$. SI unit: Pascal (N/m²).

Thermal Physics

• Matter Changes: Solid to liquid (melting), liquid to solid (freezing), liquid to gas (boiling), gas to liquid (condensing).

• Gas Molecules: Move randomly and quickly.

• Temperature: Related to average kinetic energy of molecules.

• Brownian Motion: Erratic motion of small particles due to collisions with gas/liquid molecules.

• Temperature Conversion: $K = \degree C + 273$

• Thermal Expansion: Substances expand when heated (gas > liquid > solid).

• Heat: Form of energy, SI unit is Joules.

• Specific Heat Capacity: Heat required to raise temperature of 1kg by 1°C (or 1K). $c = \frac{Q}{m\Delta T}$

• Evaporation: Escape of energetic particles from liquid surface, causing cooling.

  • Accelerated by: increasing temperature/ surface area, reducing humidity, blowing air across surface.

• Thermal Radiation:

  • Infrared waves (electromagnetic spectrum), travels through vacuum.

  • Emitter: Sends out thermal radiation, cools quickly.

  • Reflector: Reflects thermal radiation, bad absorber.

  • Absorber: Absorbs thermal radiation, heats quickly.

  • Factors: Temperature, color (black = more radiation), surface area.

Waves

• Speed of Wave: $v = f\lambda$

• Transverse Wave: Particles vibrate perpendicular to motion (peaks and valleys).

• Longitudinal Wave: Particles vibrate along the line of motion (compressions and expansions).

• Amplitude: Maximum displacement from original position (meters).

• Wavelength: Distance between two points in phase (meters).

• Period: Time for one complete cycle (seconds).

• Frequency: Number of complete cycles per second (Hertz). $f = \frac{1}{T}$

• Electromagnetic Waves: Transverse waves, no medium needed, speed = $3 \times 10^8 \frac{m}{s}$.

• Electromagnetic Spectrum: Radio, microwave, infrared, visible, ultraviolet, X-ray, gamma (increasing frequency, decreasing wavelength).

Electricity

• Thermistors: Resistance decreases with temperature.

• Light-Dependent Resistors (LDRs): Resistance decreases with light intensity.

• Fuses: Break circuit when current is too high.

• Relays: Open/close circuit under certain conditions.

• Diodes: Allow current in one direction only.

• Light-Emitting Diodes (LEDs): Emit light when current passes through.

Astrophysics

• Earth's Rotation: 24 hours, tilted axis (23.5°), causes day and night.

• Earth's Orbit: 365 days, elliptical, causes seasons.

• Moon: Natural satellite, reflects sunlight, orbits Earth in 28 days, phases.

• Solar System: Sun, planets, minor planets, moons, etc.

  • Planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

  • Inner rocky planets and outer gas giants.

• Formation: 4.6 billion years ago from gas and dust cloud collapse.