Physical Laws
A handful of dependable rules that never break. Engineers lean on them to predict how a design will behave before it's ever built.
For small stretches, stress and strain are proportional: pull twice as hard, stretch twice as far. The constant E (modulus) is the material's stiffness. Springs obey this.
Energy is never created or destroyed, only converted. A stretched bow stores energy that becomes the arrow's motion. Useful for checking that a design 'adds up'.
Matter isn't lost in ordinary processes. The atoms that go into a reaction all come out — nothing vanishes. This lets you balance recipes for resin and hardener.
Every push has an equal push back. Press a table and it presses your hand just as hard. It's why a bolt clamps and why structures can hold still under load.
The second law of thermodynamics, in plain terms. Heat always spreads from warmer to cooler until they match. It's why parts cool in a mold and why insulation works.
An object in fluid is pushed up by the weight of fluid it displaces. It's why steel ships float and why a light composite hull sits high in the water.
A thing keeps doing what it is doing — sitting still, or moving in a straight line — unless a force acts on it. This is inertia. A ball on a table stays put; a puck on smooth ice glides on and on. Forces are what change motion, not what keep it going.
The first law of composites: blend a stiff fibre with a soft plastic and the stiffness of the mix is roughly the average of the two, weighted by how much of each you use. More fibre, stiffer part. The micromechanics calculator here is built on exactly this idea.
For a gas, pressure, volume and temperature are tied together: squeeze it into less volume and the pressure rises; heat it and the pressure rises too. It is why a hydrogen tank at 700 bar is such a feat of engineering, and why a sealed bottle bulges in the hot sun.