The /ʊ/ vowel is spelled <u>, <oo>, <ou> in could and <or> in worsted (the fabric). The /3/ vowel is spelled <ur>, <er>, <ear>, <ir> and <or> in word and worsted (beaten). Notice the homograph worsted.
This is a contrast between a short and a long vowel, both central in tongue position, though /ʊ/ is rounded. It is not a problem for learners.
The density figure is 2.35%. The set makes 27 semantic contrasts giving a loading of 58.7%.
book burke booked burked booking burking books burkes bully Burley cook kirk cooks kirks Cookham Kirkham could curd full furl good gird goods girds hood heard hood herd hooded herded hooding herding hoods herds look lurk looked lurked looking lurking looks lurks pud purred pull pearl pulled pearled pulling pearling pulls pearls pulley pearly puss purse pussies purses pussy Percy put pert putsch perch putsches perches shook shirk stood stirred took Turk wood whirred wood word woods words wooded worded woody wordy woodier wordier woodiest wordiest wool whirl wools whirls worsted worsted
Credit: John Higgins