Forces, Motion and Energy
Forces and Motion
A force is a push or pull measured in newtons. Balanced forces cause no change in motion; unbalanced forces cause acceleration. Weight = mass × gravitational field strength (W = mg), with g about 10 N/kg on Earth. Friction opposes motion and produces heat. Speed = distance ÷ time; acceleration = change in velocity ÷ time. On a distance–time graph, gradient is speed; on a velocity–time graph, gradient is acceleration and area under the line is distance travelled.
Energy
Energy is stored or transferred, never created or destroyed. Stores include kinetic, gravitational potential, elastic, chemical, thermal and nuclear. Transfers happen by heating, work done by forces, or waves. Efficiency = useful output ÷ total input × 100%. Power = energy transferred ÷ time.
Pressure and Moments
Pressure = force ÷ area, so sharp edges concentrate force. A moment = force × perpendicular distance from the pivot; for balance, clockwise moments equal anticlockwise moments.
Traps to Watch
- Mixing mass (kg) with weight (N).
- Reading speed from the y-axis of a distance–time graph rather than its gradient.
- Forgetting to convert units (cm to m, g to kg) before calculating.
Worked Example
A 2 kg book is lifted 3 m. Gravitational potential energy gained = mgh = 2 × 10 × 3 = 60 J. If it falls, that 60 J becomes kinetic energy just before impact.
Waves
Wave Basics
Waves transfer energy without transferring matter. Transverse waves vibrate perpendicular to travel (light, water ripples, seismic S-waves). Longitudinal waves vibrate parallel to travel (sound, seismic P-waves) and show compressions and rarefactions. Amplitude is maximum displacement; wavelength is distance between matching points; frequency is cycles per second (Hz); period = 1 ÷ frequency.
The Wave Equation
Wave speed = frequency × wavelength (v = fλ). For sound, speed depends on the medium; sound cannot travel through a vacuum. Light travels at about 3 × 10⁸ m/s in a vacuum and is part of the electromagnetic spectrum, which includes radio, microwave, infrared, visible, ultraviolet, X-rays and gamma rays, in order of increasing frequency.
Reflection and Refraction
Reflection follows the law: angle of incidence equals angle of reflection, measured from the normal. Refraction occurs when a wave changes speed at a boundary; entering a denser medium, light slows and bends towards the normal. Lenses focus light: converging lenses focus parallel rays; the more curved the lens, the shorter the focal length. Sound and light can both be reflected and absorbed.
Traps to Watch
- Measuring angles from the surface rather than the normal.
- Mixing frequency and period.
- Assuming sound travels faster in air than in solids (it does not).
Worked Example
A wave has frequency 50 Hz and wavelength 4 m. Speed = fλ = 50 × 4 = 200 m/s. If the frequency doubles to 100 Hz in the same medium, wavelength halves to 2 m because speed stays constant.
Electricity and Magnetism
Circuits
Current is the rate of flow of charge, measured in amperes; voltage (potential difference) is energy transferred per unit charge, in volts; resistance opposes current, in ohms. Ohm's law: V = IR. In series, current is the same everywhere and voltages add; in parallel, voltage is the same across branches and currents add. Resistance of a wire increases with length and decreases with thickness. A thermistor's resistance changes with temperature; an LDR's falls as light intensity rises.
Electrical Energy and Power
Power = voltage × current (P = VI). Energy transferred = power × time. Mains electricity is dangerous; safety features include fuses, earth wires and circuit breakers. A fuse melts if current is too high.
Magnetism and Electromagnetism
Magnets have north and south poles; like poles repel. A current-carrying wire produces a magnetic field; a coil with a current acts as an electromagnet whose strength rises with current and turns. A changing magnetic field can induce a voltage (electromagnetic induction), the basis of generators; motors use the force on a current-carrying wire in a magnetic field.
Traps to Watch
- Adding currents in series or voltages in parallel.
- Confusing power (W) with energy (J).
- Forgetting that induced voltage requires change, not just presence, of a magnetic field.
Worked Example
A 12 V supply drives 3 A through a resistor. R = V/I = 12/3 = 4 Ω. Power = VI = 12 × 3 = 36 W, so in 10 s the energy transferred is 36 × 10 = 360 J.
Earth, Environment, Ecology and Resource Use
Earth and Atmosphere
The Earth has a layered structure: crust, mantle and core. The atmosphere is mostly nitrogen (about 78%) and oxygen (about 21%), with small amounts of argon, carbon dioxide and water vapour. The carbon cycle moves carbon between the atmosphere, living things, oceans and rocks through photosynthesis, respiration, combustion and decay. Burning fossil fuels raises carbon dioxide, enhancing the greenhouse effect.
Ecosystems
An ecosystem includes living organisms and their physical environment. A food chain shows energy flow; arrows point in the direction of energy transfer. Producers capture light by photosynthesis; consumers eat; decomposers break down dead matter. Energy is lost at each trophic level through respiration, movement, heat and egestion, so food chains are short. Pyramids of biomass can be used to show quantities. Competition and predation control population size; abiotic factors include light, temperature and water.
Resources and Sustainability
Renewable resources are replaced naturally (wind, solar, timber); non-renewable resources are finite (coal, oil, gas). Recycling, reducing waste and using renewable energy reduce environmental impact. Water treatment and sewage treatment protect health and ecosystems. Biodiversity is valuable for stability and medicine.
Traps to Watch
- Drawing arrows backwards in food chains.
- Claiming energy is recycled in ecosystems (nutrients are recycled, energy is not).
- Confusing the greenhouse effect (natural and necessary) with enhanced greenhouse effect (human-driven).
Rule of Thumb
For any food chain, ask: what is the source of energy, and where does it go? Only about 10% passes to the next level.