Ka'idar Motion ta biyu ta Newton wata ka'ida ce ta kimiyyar lissafi wacce ta bayyana cewa saurin abu yana daidai da karfin da ke aiki da shi kai tsaye kuma ya yi daidai da girmansa. Ma’ana, gwargwadon girman karfin da ake yi wa abu, gwargwadon saurinsa zai kasance, kuma gwargwadon girman abin, gwargwadon karfinsa zai ragu. Ta hanyar lissafi, ana iya bayyana doka a matsayin F = ma, inda F shine ƙarfin da ake amfani da abu, m shine yawansa, kuma shine hanzarin da yake samu a sakamakon karfi.