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.
Drums or Keyboards
The synonym of imperceptible is unnoticed or invisible.
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