one of them is stiff.
The synonym for collapsed is "fallen" or "crumbled."
The word thermodynamics does not have a synonym or an antonym.
One synonym is Opaque and antonym is transparent.
A synonym for "rupture" is "break" or "fracture."
Drums or Keyboards
rigid
Neither rigid nor initially are synonyms for worse.
flexible, invariable, rigid
determined, fixed, rigid, set, firm
severe, rigid, bitter, rough
If you mean hard as in the opposite of soft, then rigid, firm, adamatine. If you mean the opposite of easy, then difficult
You could use fixed, rigid, solid, stiff, or tough as a synonym for unyielding.
antipathy, anger, cruelty, spite, rancor, rigor
hard, strong, sturdy, tough, rugged, determined, resolute, unyielding, unbending, firm, rigid, unwavering
Some synonyms for the verb stiffened are made rigid, firmed, fixed, hardened, petrified, reinforced, or tensed.
straight, upright, tough, disciplinary, harsh
adamant, adamantine, brassbound, die-hard, grim, implacable, incompliant, inexorable, inflexible, intransigent, iron, obdurate, relentless, remorseless, rigid, unbendable, unbending, uncompliant, uncompromising, unrelenting, unyielding