ไธๅฎใฎๅ \(F\) ใๆ้ \(\Delta t\) ใ ใๅ ใใใจใ็ฉไฝใฎ้ๅ้ใฏใกใใใฉ \(F\,\Delta t\) ใ ใๅขใใพใใใใใๅ็ฉ = ้ๅ้ใฎๅคๅใจใใ้ขไฟใงใใ
ใคใกใผใธใฏใไธๅฎใฎๅใงๆผใ็ถใใฆ้ใใไธใใฃใฆใใๅฐ่ปใใๆผใๆ้ใ้ทใใปใฉใๆ็ต็ใช้ใใๅคงใใใ
(1) ๅ ใใๅ็ฉใฎๅคงใใ \(I = F\,\Delta t\)ใ(2) ๅ็ฉใซใใ้ๅ้ใฎๅคๅใ(3) ้ๆญข็ถๆ ใใๅงใพใฃใใจใใฎ้ใใใในใฆ \(I = mv' - mv\) ใจใใๅใ้ขไฟใใๅฐใใพใใ
ๅ็ฉใฎๅไฝ Nยทs ใจ้ๅ้ใฎๅไฝ kgยทm/s ใฏๅใๆฌกๅ ใงใ๏ผ\(1\,\mathrm{N\cdot s} = 1\,\mathrm{kg\cdot m/s}\)๏ผใใ ใใใๅ็ฉใๆฑใใๅคใใใใฎใพใพใ้ๅ้ใฎๅคๅใใซใชใใพใใ