Basics of Physics

A push or a pull. You feel force when you press a wall or lift a bag. Measured in newtons (N). One newton is roughly the weight of a small apple held in your hand.

The heavier a thing is (mass, m) and the faster you want to speed it up (acceleration, a), the more force (F) you need. Push a shopping cart empty vs. full — same push, different result.

Force spread over an area. Stand on one foot and the pressure under it is higher than on two feet — same weight, smaller area, more stress. Measured in pascals (Pa); engineers use MPa (millions of pascals).

How much a material stretches compared to its original length. Pull a rubber band to twice its length and its strain is 1.0 (100%). It has no units — it's a ratio.

The capacity to do work — to move something, heat it, or bend it. It never disappears, only changes form: a falling ball turns height into motion into sound and heat. Measured in joules (J).

Temperature (°C) says how hot something is; heat is the energy that flows from hot to cold. A spark is very hot but carries little heat; a warm bath is cooler but holds a lot.

How hard a material is to stretch or bend. Steel and carbon fibre are stiff — they barely move under load; rubber is not. Stiffness is measured by the Young's modulus, E, in gigapascals (GPa). It is not the same as strength: a steel ruler is stiff but springs back, while a cream cracker is stiff but snaps.

The most stress a material can take before it breaks or bends for good. A thin fishing line can be strong; a thick stick of chalk is weak. Strength answers 'when does it fail?', while stiffness answers 'how much does it move first?' — two different questions about the same part.

How heavy a material is for its size — its mass packed into a volume. Lead is dense, foam is not. A carbon-fibre frame and a steel one can be the same shape, but the carbon one is far lighter because carbon is less dense. Measured in grams per cubic centimetre (g/cm³).