Physics for Kids

How the course is built

• It climbs. Unit 1 is guessing which of two things lands first and then finding out. Unit 12 is Newton's three laws and where even they stop working. Each rung assumes the one below it.
• Every unit is an experiment, written for two ages. The materials are the same, so a Grade 6 child and a Kindergartener can work at one table. The older track says what to do alone; the younger track says what the grown-up does.
• Everything is household stuff. An elastic band, a torch, a mirror, two cups and some string, a battery, a balloon, a bathroom scale. Nothing needs a laboratory.
• The formulas check themselves. Each one carries a numeric test that runs on every change, so a typo in a formula fails our tests instead of reaching a child.
• Physics facts are checked, not remembered. The gravity figures come from Wikidata with the record number kept. The speed of light is the exact SI value, because the metre is defined from it.
• The famous wrong answers are named out loud. Physics is the subject where confident misconceptions survive being corrected, so this course tackles them head on rather than hoping they do not come up.

The twelve units

Nine things nearly everybody gets wrong

These are not trick questions. They are the answers most adults would give, and every one of them is addressed directly somewhere in the course.

Everything you need, for the whole course

Twelve experiments, and this is the complete list. Nothing on it is laboratory equipment.

• A 1.5 volt battery
• A ball that bounces
• A calculator
• A chair to stand on, or just tall arms
• A drinking straw
• A few objects to weigh
• A glass of water
• A heavy book
• A long balloon
• A magnet
• A paperclip
• A pencil
• A pencil to make the holes
• A pile of things to test: coin, paperclip, foil, key, spoon, eraser
• A plank, board or stiff card for the ramp
• A ruler
• A ruler or a strong stick for the lever
• A shallow tray of water
• A sheet of paper
• A small mirror
• A small torch bulb or an LED
• A small toy car or a coin
• A stopwatch or a phone timer
• A straw
• A tape measure
• A tape measure or a metre stick
• A thermometer if you have one
• A thick elastic band
• A timer
• A torch
• A torch if you have one
• A towel, tin foil and a lid to insulate with
• A white wall or a sheet of white paper
• A wooden board, a tray or a large book to use as a ramp
• About 10 metres of string
• About 5 metres of string
• An elastic band
• Bathroom scales
• Chalk or two markers for the ends
• Coloured pencils
• Different surfaces: a towel, tin foil, paper, a tea towel
• Paper
• Sticky tape
• String
• Three identical cups or jars
• Two chairs or door handles to tie the string to
• Two objects of very different weight, such as a coin and a paperclip
• Two paper or plastic cups
• Two pencils, one to write with and one to use as the pivot
• Two pieces of insulated wire
• Warm water from the tap, not boiling

Every formula, in one place

All 12 formulas taught in the course. Each one carries a numeric self-test that runs whenever the course changes, so a typo fails our tests rather than reaching a child. Print this page for a reference sheet.

Gravity on six worlds

Checked against Wikidata rather than written from memory, with the record number kept in the code. Multiply your mass in kilograms by the second column to get your weight in newtons on each one. Your mass does not change on any of them.

Jupiter is the awkward one: it has no solid surface, so the figure is taken at the cloud tops and includes the planet's own spin. Other sources quote a higher number for a slightly different definition, which is a good early lesson in reading the small print.

What a magnet actually picks up

Attracted: iron, steel, nickel, cobalt.

Not attracted, even though several of these are metals: aluminium, copper, gold, silver, brass, plastic, wood, glass.

Ask your child to predict each one before testing. Most people, including most grown-ups, expect the aluminium foil and the copper coin to stick. They do not, and being wrong out loud is what makes it stay learned.

🔭 1 · What Is Physics?

Kindergarten to Grade 2

Physics is the study of how things move and why they do what they do. It is the science underneath all the others: chemistry runs on it, biology runs on chemistry, and the same handful of rules works on a dropped spoon and on a galaxy.

What it teaches

• Physics asks how things move, how energy travels, and what everything is made of.
• The same rules work everywhere. A ball thrown here and a moon going round Jupiter obey the same maths.
• Physicists guess, then test the guess, then change the guess when the test disagrees. That last step is the whole job.
• A prediction that cannot be tested is not physics yet.
• Physics is measured, so it uses units: metres, seconds, kilograms, newtons, joules.
• Being wrong is normal and useful. Some of the biggest discoveries started with a result nobody expected.

Sample questions and answers

Worksheets in this course are generated fresh, with new numbers every time you press New Worksheet, and every answer is computed rather than stored.

💪 2 · Forces: Push and Pull

Kindergarten to Grade 3

A force is a push or a pull. That is the whole definition, and it is enough to explain why a door opens, why a magnet grabs a fridge, and why you do not float off the ground.

What it teaches

• Every force is either a push or a pull. Nothing else.
• Forces are measured in newtons, named after Isaac Newton.
• A force can start something moving, stop it, speed it up, slow it down, or change its direction.
• A force can also change something's shape, like squashing clay or stretching an elastic band.
• When the forces on something are balanced, its motion does not change. It stays still, or keeps going exactly as it was.
• When they are unbalanced, it changes its motion in the direction of the bigger force.
• Some forces need touching, like a push. Some work at a distance, like gravity and magnetism.

Sample questions and answers

Worksheets in this course are generated fresh, with new numbers every time you press New Worksheet, and every answer is computed rather than stored.

🏃 3 · Motion: Speed and Direction

Grades 2 to 6

Motion is just a change of position. To describe it properly you need two things: how fast, and which way. Physics has separate words for those, and using them precisely makes everything later easier.

What it teaches

• Speed is how much distance you cover in a certain time. Speed = distance divided by time.
• A car going 100 km in 2 hours has a speed of 50 km per hour.
• You can rearrange it: distance = speed x time, and time = distance divided by speed.
• Velocity is speed plus a direction. 50 km per hour north is a velocity. 50 km per hour on its own is a speed.
• Acceleration is how quickly the velocity itself changes. Speeding up, slowing down and turning are all acceleration.
• Average speed uses total distance over total time, which is not the same as the speed you were doing at any one moment.
• Motion is relative. Sitting still on a train you are not moving compared with your seat, and moving very fast compared with the ground.

Sample questions and answers

Worksheets in this course are generated fresh, with new numbers every time you press New Worksheet, and every answer is computed rather than stored.

🪐 4 · Gravity

Grades 1 to 6

Gravity is the pull every piece of matter has on every other piece. It is very weak, which is why you do not stick to your chair, and the Earth only wins because it is so enormous.

What it teaches

• Gravity pulls things together. The more mass something has, the harder it pulls.
• Heavy things do NOT fall faster. In a place with no air, a hammer and a feather land at exactly the same moment, and astronauts have actually tried it on the Moon.
• The reason a feather loses on Earth is air resistance pushing up on it, not its weight.
• Mass is how much stuff there is, measured in kilograms. It never changes.
• Weight is the pull of gravity on that mass, measured in newtons. It changes with where you are.
• On Earth gravity pulls at about 9.8 newtons for every kilogram. On the Moon it is about 1.6, which is roughly a sixth.
• So a 10 kg bag has a mass of 10 kg everywhere, but weighs about 98 newtons here and about 16 on the Moon.
• Gravity is what keeps the Moon going round the Earth and the Earth going round the Sun.

Sample questions and answers

Worksheets in this course are generated fresh, with new numbers every time you press New Worksheet, and every answer is computed rather than stored.

🧊 5 · Friction and Air Resistance

Grades 2 to 6

Friction is the force that shows up whenever two surfaces rub. It is the reason nothing on Earth keeps sliding forever, and it is why you can walk instead of skating everywhere.

What it teaches

• Friction acts against the direction something is moving. It always tries to slow things down.
• Rough surfaces make more friction than smooth ones. Sandpaper more than ice.
• Friction turns movement energy into heat. That is why rubbing your hands warms them.
• Air resistance is friction with air, and it is why a parachute works and why a feather drifts.
• Water resistance is the same thing in water, which is why boats are pointed at the front.
• Friction is often useful. Brakes, shoe grips and being able to hold a pencil all need it.
• Oil, wheels and polished surfaces reduce friction when you would rather not have it.

Sample questions and answers

Worksheets in this course are generated fresh, with new numbers every time you press New Worksheet, and every answer is computed rather than stored.

⚙️ 6 · Simple Machines

Grades 3 to 8

A simple machine does not create energy. It changes a big force over a short distance into a small force over a long one, which is a trade rather than a saving, and it is one of the oldest ideas in engineering.

What it teaches

• Work in physics has an exact meaning: work = force x distance. It is measured in joules.
• A machine lets you use a smaller force, but you have to apply it over a greater distance. The work comes out the same.
• A lever turns on a pivot called a fulcrum. Move the fulcrum and you change the trade.
• A ramp lets you raise something with less force by pushing it further. A screw is a ramp wrapped round a pole, and a wedge is a ramp you drive into things.
• A pulley changes the direction you pull. More pulleys share the load and make it easier still.
• A wheel and axle turns a small turn at the edge into a strong twist at the middle, or the other way round.
• Power is how fast you do the work: power = work divided by time, measured in watts.
• No machine gives you energy for free. Anything claiming to is wrong, and people have been claiming it for centuries.

Sample questions and answers

Worksheets in this course are generated fresh, with new numbers every time you press New Worksheet, and every answer is computed rather than stored.

⚡ 7 · Energy

Grades 3 to 8

Energy is the thing that lets something happen. It comes in several forms, it moves between them constantly, and the total never changes. That last part is one of the deepest rules in all of science.

What it teaches

• Energy is measured in joules, the same unit as work.
• The forms worth knowing are movement, heat, light, sound, electrical, chemical and stored energy.
• Energy is never created or destroyed. It only changes form. That is called conservation of energy.
• So energy is never really used up. When people say wasted energy they nearly always mean it turned into heat.
• Stored energy is energy waiting to happen: a stretched elastic band, a book on a shelf, a charged battery.
• Movement energy depends on speed twice over. Double the speed and you get four times the energy, which is why fast crashes are so much worse than slow ones.
• A chain of changes is easy to trace: food is chemical, your muscles make it movement, friction makes it heat.
• The Sun is where almost all of Earth's energy came from originally, including the energy in your dinner.

Sample questions and answers

Worksheets in this course are generated fresh, with new numbers every time you press New Worksheet, and every answer is computed rather than stored.

🌡️ 8 · Heat and Temperature

Grades 3 to 8

Heat and temperature sound like the same thing and are not. Temperature is how hot something is. Heat is energy on the move, and it only ever moves one way.

What it teaches

• Temperature measures how hot something is. Heat is the energy that flows because of a temperature difference.
• A bath at 40 degrees holds far more heat energy than a cup of tea at 90, because there is so much more of it.
• Heat always flows from hotter to colder. Never the other way round on its own.
• So cold never gets in. Heat gets out. Almost every sentence about cold getting in is backwards.
• Heat travels three ways: conduction through solids, convection in liquids and gases, and radiation across empty space.
• Metals conduct heat well, which is why a metal spoon in soup gets hot and a wooden one does not.
• That is also why metal feels colder than wood at the same temperature. It is pulling heat out of your hand faster.
• The coldest anything can get is about minus 273 degrees Celsius, called absolute zero. Nothing can go below it.

Sample questions and answers

Worksheets in this course are generated fresh, with new numbers every time you press New Worksheet, and every answer is computed rather than stored.

💡 9 · Light

Grades 3 to 8

Light is how almost everything you know about the world got into your head. It travels in straight lines, it bounces, it bends, and it is the fastest thing there is.

What it teaches

• Light travels in straight lines until something bounces it or bends it.
• You see an object because light bounces off it and into your eye. Your eyes do not send anything out.
• A shadow is where an object has blocked the straight path of the light.
• Reflection is light bouncing. A mirror is smooth enough to keep the picture intact.
• Refraction is light bending as it moves from one material into another. It is why a straw looks broken in a glass of water.
• White light is all the colours mixed. A prism or a raindrop splits it into a spectrum, which is what a rainbow is.
• An object looks red because it reflects red light and soaks up the rest.
• Light travels at about 300000 kilometres a second, so sunlight takes about 8 minutes and 20 seconds to reach us.

Sample questions and answers

Worksheets in this course are generated fresh, with new numbers every time you press New Worksheet, and every answer is computed rather than stored.

🔊 10 · Sound and Waves

Grades 3 to 8

Sound is a wave of squeezes and stretches travelling through stuff. That means it needs stuff to travel through, which has one famous consequence: space is silent.

What it teaches

• Sound starts with something vibrating and travels as a wave through a material.
• It needs a medium. There is no sound in space at all, because there is nothing to carry it.
• Sound travels about 343 metres a second in air at room temperature.
• It travels faster in water, and faster still in solids, because the particles are closer together.
• Light is roughly a million times faster than sound, which is why you see lightning first and can count the gap.
• Pitch comes from frequency: more vibrations a second means a higher note.
• Loudness comes from amplitude: a bigger vibration means a louder sound.
• An echo is sound reflecting off a hard surface and coming back to you.

Sample questions and answers

Worksheets in this course are generated fresh, with new numbers every time you press New Worksheet, and every answer is computed rather than stored.

🔌 11 · Electricity and Magnetism

Grades 4 to 9

Electricity and magnetism look like two subjects and are really one. A moving electric charge makes a magnetic field, and a moving magnet makes electricity, which is how almost all the world's power is generated.

What it teaches

• An electric current is a flow of charge, and it only flows round a complete loop called a circuit.
• Break the loop anywhere, with a switch or a gap, and everything in it stops at once.
• Current is measured in amps, the push behind it in volts, and how much a component resists it in ohms.
• Ohm's law ties them together: voltage = current x resistance.
• Conductors let current through, like copper and most metals. Insulators do not, like plastic, rubber and glass.
• Current is not used up going round a circuit. The same amount comes back as went out. What gets used is the energy it carries.
• Magnets attract only some metals: iron, steel, nickel and cobalt. They do not attract aluminium, copper, gold or silver.
• Every magnet has a north and a south pole. Opposite poles attract and like poles push apart.
• Electricity from a socket is dangerous and is never part of an experiment. Batteries of 1.5 volts are the ones to learn with.

Sample questions and answers

Worksheets in this course are generated fresh, with new numbers every time you press New Worksheet, and every answer is computed rather than stored.

🍎 12 · Newton's Laws and Beyond

Grades 5 to 12

In 1687 Isaac Newton wrote down three rules that between them describe the motion of everything you are ever likely to see. They held up for more than two hundred years before anybody found their edges.

What it teaches

• First law: an object keeps doing exactly what it was doing, still or moving, unless a force acts on it.
• That is why a moving thing does not need a constant push. It needs one to start, and friction is why things seem to need one on Earth.
• The tendency to keep going is called inertia, and more mass means more inertia.
• Second law: force = mass x acceleration. Push harder and the change is bigger. Push the same on something heavier and the change is smaller.
• Third law: for every force there is an equal and opposite force.
• The pair in the third law acts on two DIFFERENT objects, which is exactly why they do not cancel each other out.
• You walk by pushing the ground backwards, so the ground pushes you forwards. A rocket works the same way with its exhaust.
• Newton's laws are not the last word. Near the speed of light you need Einstein's relativity, and for atoms you need quantum physics. Newton is still right for everything in your house.

Sample questions and answers

Worksheets in this course are generated fresh, with new numbers every time you press New Worksheet, and every answer is computed rather than stored.

Working safely

Nine of the twelve experiments carry their own safety line, and those lines are checked automatically so one cannot go missing when the course is edited. The three that do not need one really do not: weighing yourself, bouncing a ball and rolling a toy car down a ramp. Padding those with warnings would only teach a child to skim past the warnings that matter.

The electricity rule is absolute: batteries only, never a wall socket, and never put anything into one. If a wire or a battery feels warm, disconnect it straight away.

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