Physics Intermediate

Motion is a change in position over time. To describe motion you need a reference point — something you measure position relative to.

Speed tells you how fast an object moves. The formula is:

speed = distance ÷ time
s = d / t

• If a cyclist travels 60 km in 2 hours, their speed is 60 ÷ 2 = 30 km/h.
• Speed only tells you how fast; velocity also tells you the direction.
• Average speed = total distance ÷ total time.
• Rearranging the formula: d = s × t and t = d / s.

Example: A runner covers 400 m in 50 s.
  speed = 400 ÷ 50 = 8 m/s

Forces always act in pairs. The net force is the overall force after all individual forces are added together (taking direction into account).

• Balanced forces: net force = 0. The object stays still or keeps moving at constant speed.
• Unbalanced forces: net force ≠ 0. The object accelerates (speeds up, slows down, or changes direction).
• Friction is a force that opposes motion between surfaces.
• Air resistance (drag) is friction between an object and the air.

Book resting on a table:
  Weight (↓) = Normal force (↑) → balanced → book stays still

Car accelerating:
  Engine force > friction → unbalanced → car speeds up

Mass is the amount of matter in an object. It is measured in kilograms (kg) and does not change wherever you go.

Weight is the force of gravity on an object. It is measured in newtons (N) and does change depending on the strength of gravity.

weight = mass × gravitational field strength
W = m × g

On Earth: g ≈ 10 N/kg (or 9.8 N/kg, often rounded to 10)
Example: mass = 5 kg  →  weight = 5 × 10 = 50 N

• On the Moon, g ≈ 1.6 N/kg, so a 5 kg object weighs only 8 N there.
• Your mass is the same on Earth and on the Moon; your weight is different.

Energy is the ability to do work. It cannot be created or destroyed — it can only be transferred or transformed from one form to another (the law of conservation of energy).

• Kinetic energy (KE): energy of motion. A moving car, a flying ball.
• Gravitational potential energy (GPE): stored energy due to height. A book on a shelf, water behind a dam.
• Thermal (heat) energy: energy due to the motion of particles.
• Sound energy: energy carried by vibrating waves.
• Light energy: energy carried by electromagnetic waves.
• Electrical energy: energy carried by moving electric charges.

Transformation examples:
  Torch: chemical energy → electrical energy → light + heat
  Falling rock: GPE → KE as it falls
  Solar panel: light energy → electrical energy

Thermal energy (heat) moves from hotter regions to cooler ones in three ways:

• Conduction: heat passes through a material by direct particle-to-particle contact. Metals conduct heat well; wood and plastic are poor conductors (insulators). Touching a hot metal pan hurts because of conduction.
• Convection: heat moves through fluids (liquids and gases) by the movement of the fluid itself. Warm fluid rises because it is less dense; cool fluid sinks. This creates a convection current. Boiling water and room heating by radiators use convection.
• Radiation: heat travels as electromagnetic waves (infrared) and needs no medium — it works through a vacuum. The Sun heats Earth by radiation across empty space.

Conduction → through solids (particle to particle)
Convection → through fluids (moving currents)
Radiation  → through empty space (electromagnetic waves)

A wave transfers energy from one place to another without permanently moving matter. Two key wave properties are:

• Frequency: the number of waves passing a point per second, measured in hertz (Hz). Higher frequency → higher pitch in sound, higher colour energy in light.
• Wavelength: the length of one complete wave. Longer wavelength means lower frequency.

Sound waves are longitudinal — particles vibrate in the same direction the wave travels. They need a medium (solid, liquid, or gas) to travel. Sound travels at about 340 m/s in air.

Light waves are transverse electromagnetic waves. They travel at 300,000 km/s in a vacuum and do not need a medium. Visible light is only a small part of the full electromagnetic spectrum.

Higher frequency → shorter wavelength → higher pitch/energy
Lower frequency  → longer wavelength  → lower pitch/energy

Electric current is the flow of electric charge (electrons) around a circuit. It is measured in amperes (A).

A simple series circuit has one path for current to flow: battery → wire → component → back to battery. If any component is removed or the wire is broken, the circuit opens and current stops.

• Conductors allow current to flow easily — copper, silver, most metals.
• Insulators do not allow current to flow — rubber, plastic, wood, glass.
• Voltage (measured in volts, V) is the "push" that drives current around the circuit.
• Adding more batteries increases the voltage and makes bulbs shine brighter.

Series circuit: Battery — Bulb 1 — Bulb 2 — back to Battery
(one path: if Bulb 1 breaks, Bulb 2 also goes out)

Static electricity is a build-up of electric charge on the surface of an object. It occurs when two objects rub together and electrons transfer from one to the other.

• Rubbing a balloon on your hair transfers electrons — the balloon becomes negatively charged and your hair becomes positively charged.
• Opposite charges attract; like charges repel (just as with magnets).
• Static electricity can cause sparks when the charge suddenly jumps across a gap — lightning is a giant static discharge between a cloud and the ground.
• The shock you sometimes get touching a door handle after walking on carpet is static discharge.

Balloon rubbed on hair:
  Balloon gains electrons → negative (−)
  Hair loses electrons   → positive (+)
  → balloon sticks to hair (opposite charges attract)

Magnetism is a force produced by moving electric charges. All magnets have a north and south pole; like poles repel and opposite poles attract.

An electromagnet is made by passing electric current through a coil of wire (often wound around an iron core). When the current flows, a magnetic field is produced; when the current is switched off, the magnetism stops.

• More turns of wire → stronger electromagnet.
• More current → stronger electromagnet.
• Adding an iron core greatly increases the strength.
• Electromagnets are used in electric motors, doorbells, MRI scanners, and recycling yards (to pick up scrap metal).

Wire coil + current → magnetic field
Switch off current  → magnetic field disappears

Density is how much mass is packed into a given volume:

density = mass ÷ volume
ρ = m / V

• Units: g/cm³ or kg/m³.
• Water has a density of 1 g/cm³. Objects less dense than 1 g/cm³ float in water; objects denser than 1 g/cm³ sink.
• Example: A block of wood has mass 40 g and volume 80 cm³. Density = 40 ÷ 80 = 0.5 g/cm³ — it floats.
• Ice has a density of about 0.9 g/cm³, which is why icebergs float (with most of the ice under water).

Example: mass = 120 g, volume = 60 cm³
  density = 120 ÷ 60 = 2 g/cm³  → denser than water → sinks

Matter can change from one state to another when thermal energy is added or removed. The temperature stays constant during a change of state (the energy goes into breaking or forming bonds between particles, not into raising temperature).

• Melting: solid → liquid (e.g. ice melting at 0 °C).
• Freezing: liquid → solid (e.g. water freezing at 0 °C).
• Evaporation: liquid → gas (can happen below boiling point at the surface).
• Boiling: liquid → gas throughout the liquid (e.g. water boiling at 100 °C).
• Condensation: gas → liquid (e.g. water vapour on a cold window).
• Sublimation: solid → gas directly (e.g. dry ice / solid CO₂).

SOLID ←(freezing)— LIQUID ←(condensation)— GAS
SOLID —(melting)→  LIQUID —(evaporation)→  GAS