Teach physics and engineering with things children can lift, pour, burn under supervision and break: a lever with a movable fulcrum, a ramp, a column of water, a fire with and without air, and a frame that wobbles until a diagonal brace is added. Each model should show one rule, and each rule should then be used on a real job so that the rule is tied to a result.
The physical-science branch of applied science: force multiplication, water, fire and structures, taught by models first.
Rules A Child Can Feel In Their Hands
The parent article lists four areas. Each has a model that costs nothing:
- Force multiplication. A plank on a log, a stone as load. Move the log toward the stone and the push needed falls while the distance pushed rises. See Simple Machines, Lever Systems, Inclined Planes and Pulley Systems.
- Water. A pot with a hole low on the side and another higher up. Water squirts farther from the lower hole because it carries the weight of more water above it.
- Fire. Fire needs fuel, heat and air. Take away one and it stops. See Combustion Principles.
- Structures. Build a square frame of sticks with loose joints and it folds; add one diagonal and it holds. Triangles do not change shape unless a side bends or breaks.
Worked Example: Lever Advantage
A 3 m lever has the fulcrum 0.5 m from the load, so the load arm is 0.5 m and the effort arm is 3 minus 0.5, which is 2.5 m. Mechanical advantage is effort arm divided by load arm: 2.5 divided by 0.5 is 5. A 100 kg stone weighs about 100 times 9.81, which is 981 newtons. Dividing by 5 gives 196 newtons of effort, about the weight of 20 kg. This ignores friction and the weight of the lever, and assumes the lever is strong enough not to snap. The distance trade follows: the effort end moves 5 times as far as the load end.
Worked Example: Water Pressure
Pressure from a column of water is density times gravity times height. For fresh water, 1,000 kg per cubic metre times 9.81 metres per second squared times 1 m gives 9,810 pascals, about 9.8 kilopascals, for every metre of depth. A tank 3 m above a tap gives 3 times 9,810, which is 29,430 pascals. Depth, not pipe width, sets the pressure; this explains why a leak low on a barrel is the worst one.
Method: From Model To Job
Step 1. Show the effect with a model and let the child predict the result first.
Step 2. Measure it with the arithmetic from Tier 2.
Step 3. Put it to use on a real job: lifting a stone, filling a tank, building a gate that does not sag. For right-angle layouts see 3-4-5 Triangle.
Common Mistakes
- Treating a force-saving machine as free work. A lever or pulley saves force by spending distance.
- Confusing mass and weight. Kilograms measure mass; newtons measure force. A load of 100 kg is about 981 newtons, not 100.
- Skipping the diagonal. An unbraced frame stands until the first push.
Fire, heavy loads and water
Children handle fire, heavy levers and block-and-tackle only with an adult beside them. A lifted stone can crush a hand or foot if the lever slips, and a rope under load can snap back. Never stand under a suspended load. Water in a bucket, barrel or pond is a drowning risk for small children even when shallow, so water lessons keep young children under direct watch.
What’s Next
- Learn next: Teaching Methods Without Schools
- Combine with: Task Assignment by Age for what children may operate on site