Hooke's Law: F = -kx, where k is the spring constant (N/m, dimension [MT^{-2}]) and x is displacement from natural length.

Key Properties:

• Restoring force: always directed toward natural length
• Linear: force proportional to displacement (within elastic limit)
• Energy stored: U = (1/2)kx^2
• Massless spring: tension same throughout

Combinations:

• Series: 1/k_eq = 1/k_1 + 1/k_2 (softer — more extension for same force)
• Parallel: k_eq = k_1 + k_2 (stiffer — less extension for same force)
• Cut into n parts: each part has constant nk

Equilibrium extension: For mass m hanging from spring: kx_0 = mg, x_0 = mg/k.