It’s All About Trade‑offs
RememberbuildingOllie’sdoghouse?Thescrewswerethesamelength,butthethreadsweredifferent.Thescrewswithmorethreadswereeasiertoturn.But,youhadtoturnthemmore often.
That’sthethingaboutsimplemachines.Therearealwaystrade‑offs.Themorethreadsascrewhas,thelessforceyouuse.But,youusethatforceoveragreaterdistance.“Whatdistance?”youask.Yourbodydoesn’tmove,butyourhand does.
Thinkaboutthecircleyourhandmakeswhenyouturnthescrewdriver.Ascrewwith30threadsmeans30turnsofthescrewdriver.Stretchoutthoseturns.Addthemup.You’veappliedtheturningforceoveradistanceofseveral feet.
Doyouwanttocutdownonthatdistance?Okay,but beprepared.Youwillneedtousemoreforcewitheachturn.It’slikemovingastackofbooksfromonedesktoanother.Youcanmovethestackbytakingacouplebooksatatime.Youuselittleforce,butyouhavetomakemoretrips.Oryoucanmovethewholestackatonce.Youusealotmoreforce,butit’sonlyone trip.
It’sthesamethingwiththescrew.Ascrewwithfewerthreadshasdrawbacks.Itrequiresmoreeffort.Plus,itprovideslessfrictiontoholdthescrewin place.
A jaranditslidhavethreadsthatfit together.
HoldonTight
Screwscomeinmanydifferentshapesandsizes.Howdoyoutakeoffthelidfromajarofpickles?Youunscrewit!Ajarlidisascrew.Unscrewthelidandtakealookattherimofthejar.Youcanfeelthethreadsthatspiralaround.Nowlookinsidethelid.Ithasthreads,too.Theyfitbetweenthethreadsoftherim.Thethreadsmatchupandgriptoformatightsealonthe jar.
Screwskeepmorethanlidstight.Whatisthestemofalightbulb?Correct!It’sascrew.Thethreadsofthestemturnbetweenthethreadscutintothesocket.Frictionbetweenthesethreadskeepsthebulbinplace.Agoodthing,too!Lightbulbsintheceilinghangupsidedown.Iftheyjustslippedintoplaceinsteadbeingscrewedintoplace,theymightfalloutonto your head!
Thethreadsonalightbulbstemscrewintothesocketofa light.
Adrill bitisshapedlikeascrew!Canyouseethe thread?
DigIn
Whenyoudriveascrewintowood,it’seasierifanarrowholeisalreadythere.That’swhereadrillhelps.Andguesswhat?Adrillbitisa screw.
Amotorspinsthedrillbitquickly.Thetipofthebitispointedsothatiteasilydigsintothewood.Inafewseconds,youhaveanarrowhole.Youcanmakeitbiggerwhenyoudrivein the screw.
Somelargerdrillsmakelargeholesintheground.Thedrillsdigthroughsoilandrock.Theymakewellsforwateroroil.Somesmallerdrillsmakeholestosetfencepostsandtelephone poles.