SURAT (arabi -zudi, tezi yake az mafhumhoi asosii mekhanika buda, harakati nuqta (jism)-ro tavsif medihad. Hengomi muntazam harakat kardani nuqta Surati on ba nisbati darozii rohi tayshuda s bar fosilai vaqt t barobar ast,
yane: = s/t . Umuman dv =ds/dt ast.