Engineering

Stress is the internal force carried per unit area, measured in pascals (N/m²), while strain is the resulting deformation expressed as the change in length divided by the original length, so it is a dimensionless ratio. In short, stress is the cause and strain is the effect.

Young’s modulus is the ratio of stress to strain in the elastic region, so it measures stiffness — how much a material resists being stretched or compressed. A high value like steel’s (~200 GPa) means the material barely deforms under load, while rubber has a very low modulus.

The factor of safety is the ratio of a material’s strength (or a structure’s failure load) to the actual load it is expected to carry. A factor above 1 provides a margin for uncertainties in loads, materials and analysis; typical values range from about 1.5 to 4 depending on the application and consequences of failure.

Power equals torque multiplied by angular speed, so for the same power a shaft can deliver high torque at low speed or low torque at high speed. Using SI units, power (watts) = torque (N·m) × angular velocity (rad/s), which is why gearboxes trade speed for torque.