Scientific Law
A scientific law is a concise statement, often mathematical, that describes a consistent pattern in nature without explaining why it occurs.
Definition
A scientific law is a rule that describes something that always happens the same way in nature; it tells you what happens, not why. It is a statement, often an equation, describing a relationship observed to hold consistently under specified conditions, and it is not stronger than a theory and does not turn into one. Laws are descriptive rather than explanatory, the explanation being supplied by theory, and a law may be derived from a theory as a limiting case. They are tested by checking whether measured quantities satisfy the stated relationship within experimental uncertainty, and a persistent violation redefines the law's domain rather than abolishing it.
Example
The law of conservation of mass says that dissolving $5\ \text{g}$ of salt in $100\ \text{g}$ of water gives $105\ \text{g}$ of salt water; nothing disappears. Newton's second law, $F = ma$, predicts that a cart's acceleration doubles if the net force along the ramp doubles, which a group can check by timing the cart at different angles. Conservation of mechanical energy predicts $v = \sqrt{2gh}$ for a frictionless cart, and a measured $2.6\ \text{m/s}$ against a predicted $2.8\ \text{m/s}$ does not refute the law; it shows the frictionless condition is not met, and accounting for energy lost to friction and rotation restores the balance.
Key Insight
A law is a very reliable description, and the reason behind it is the job of a theory. Laws have boundaries: conservation of mass holds for a salt-water lab, but for nuclear reactions mass and energy must be counted together. When a measurement seems to break a law, the first suspects are the assumptions and the instruments, because laws that have survived centuries of testing are usually the last thing to be wrong.