Physics
Motion, forces, energy, waves, light, electricity and magnetism, with labs that launch balls, run a roller coaster, build circuits and trace rays.
- 1MotionPosition, speed, velocity and acceleration, then falling and flying objects. The motion lab launches a ball on Earth, the Moon or Mars and works out the time, height and range as you watch.7 Units
- 2ForcesPushes, pulls, friction and gravity, and the three laws Newton used to connect force with motion. The motion lab swings a pendulum on Earth, the Moon or Mars and times it against the formula.6 Units
- 3EnergyWork, kinetic energy, potential energy, the conservation of energy and power, with the numbers worked out. The energy lab runs a roller coaster with energy bars and a friction slider, and lets you push or lift a load to see the work and power.6 Units
- 4Waves and soundWhat a wave is, how wavelength, frequency and speed fit together, how sound travels, how waves add up, why strings and pipes ring at set notes, and the electromagnetic spectrum. The wave lab draws ropes, springs, sound in a tube, ripple tanks and standing waves from the same formulas used in the lessons.6 Units
- 5LightShadows and straight-line light, mirrors, refraction and total internal reflection, lenses and the eye, colour and prisms, and the long effort to measure how fast light goes. The wave lab traces rays through glass, water and lenses with Snell’s law and the thin lens equation.6 Units
- 6ElectricityCharge, current, voltage and resistance, then Ohm's law, series and parallel circuits, power, the electricity bill and staying safe. The circuit lab lets you build circuits from batteries, bulbs, resistors, switches and fuses and works out every current as you go.7 Units
- 7MagnetismMagnets and their poles, field lines and compasses, Earth's own magnetic field, electromagnets, and the motors and generators built from them. The energy lab draws field lines you can probe with a compass, and a coil whose turns and current you can change.5 Units
Seven courses, from a thrown ball to a motor
Physics here starts with things you can see move and ends with the currents and fields inside a motor. The worked examples and most numbers come from OpenStax College Physics 2e, with constants from NIST and a few space figures from NASA. Every course does the arithmetic in full. Each unit ends with a summary and a quiz that explains its answers.
Motion before forces, forces before energy
Start with Motion: position, speed, acceleration, motion graphs, free fall, projectiles and air resistance, in seven units. Forces picks up the questions Motion leaves open, with Newton's three laws, friction, weight and the pendulum. Energy then goes further into work, kinetic and potential energy, conservation and power.
The other four can follow in any order. Waves and sound covers wavelength and frequency, sound, interference, standing waves in strings and pipes, and the electromagnetic spectrum. Light goes through shadows, mirrors, refraction, lenses and the eye, colour, and the long effort to measure the speed of light. Electricity runs from charge and Ohm's law to series and parallel circuits, the electricity bill and safety, and Magnetism from poles and field lines to Earth's field, electromagnets, motors and generators.
Sums worth being able to do
Explain why a car at 100 km/h carries four times the kinetic energy it has at 50 km/h. Work out that a ball thrown at 20 m/s and 45° stays up for 2.9 seconds and lands about 41 m away. Say why a 70 kg person weighs about 687 N on Earth and 113 N on the Moon with the same mass. Time a pendulum 99.4 cm long at 2 seconds a swing. Turn a 3 second gap between lightning and thunder into a distance of about 1 km.
Four labs that run the formulas
The motion lab launches a ball on Earth, the Moon or Mars and swings a pendulum that it times like a stopwatch. The energy lab runs a roller coaster with bars for potential energy, kinetic energy and heat, and in the magnetism course it draws field lines around magnets and a coil whose turns and current you set. In the circuit lab you close switches, drag bulbs into gaps and watch a fuse blow when three bulbs draw 1.5 A through a 1 A fuse. The wave lab draws ropes, springs, sound in a tube and a ripple tank, and traces rays through water, glass and lenses with Snell's law.
Common sense that physics overturns
A heavier object does not fall faster once air is out of the way, and the hammer and feather dropped on the Moon in 1971 show it. Shaking a rope faster does not make its wave travel faster; the rope sets the speed. A negative acceleration does not always mean slowing down. Holding a bag still does no work in the physics sense, however tired your arm gets. The magnetic pole near the geographic North Pole is, in magnet terms, a south pole. And a mirror only half your height shows your whole body, wherever you stand.
Questions about this subject
What is the difference between distance and displacement?
Distance measures the whole path travelled, while displacement compares the final position with the starting position and includes direction. Walking five metres east and three west gives eight metres of distance but only two metres east of displacement. A full lap returns you to the start, making displacement zero without making the distance zero. Motion
Can average velocity be zero while average speed is positive?
Yes: average velocity uses displacement, whereas average speed uses distance. A cyclist travelling six kilometres out and six back in one hour covers twelve kilometres but ends at the starting point. The average speed is twelve kilometres per hour and the average velocity is zero, because the net change of position is zero. Motion
Is a car accelerating when it turns at a constant speed?
Its velocity changes because direction is part of velocity, even when the speed stays fixed. Acceleration measures a change in velocity, not only an increase in the speedometer reading. A ball moving in a circle on a string likewise accelerates towards the centre throughout its motion despite maintaining a steady speed. Motion
Does negative acceleration always mean slowing down?
No: its sign describes direction relative to the positive direction you chose. If forward is positive, a car reversing faster has both negative velocity and negative acceleration while gaining speed. Braking during forward motion is another case with negative acceleration, but there it opposes the velocity, so the speed decreases instead of increasing. Motion
How do I calculate average speed for two parts of a trip?
Divide the total distance by the total time, rather than automatically averaging the two speeds. Thirty kilometres at sixty kilometres per hour takes half an hour, while thirty at thirty takes one hour. The combined sixty kilometres in one and a half hours gives forty kilometres per hour, because more time was spent at the slower speed. Motion