Ka'idar Motsi ta Farko, wacce aka fi sani da Newton's First Law of Motion ko kuma ka'idar Inertia, wata ka'ida ce ta asali a cikin makanikai na gargajiya wanda ke bayyana halayen abubuwa yayin hutawa ko cikin motsi iri ɗaya. Ma’anar ƙamus na Dokar Motsi ta Farko ita ce:Dokar da ke nuna cewa abin da ke hutawa yakan kasance yana hutawa, kuma abin da ke motsi yakan ci gaba da tafiya da sauri, sai dai idan an yi aiki da shi. da wani karfi na waje." Idan babu wani ƙarfi na waje da ya yi aiki a kan abu, ko dai zai kasance a tsaye (idan yana cikin hutawa da farko) ko kuma ya ci gaba da tafiya a madaidaiciyar layi tare da saurin gudu (idan farkon yana motsi) har abada. Ka'idar Motion ta farko ita ce tushen fahimtarmu game da rashin aiki, wanda shine juriyar abu ga canje-canje a yanayin motsinsa.