The standard form in modern Mandarin is
摩擦 (mócā).
磨擦 is listed in CC-CEDICT as a variant — older texts and some dialect usage prefer it because
磨 ('to grind') feels closer in meaning to the action. In physics textbooks, scientific writing, and exams, always use
摩擦. Don't confuse with
磨蹭 (mócèng, 'to dawdle') which is a different word entirely.
