Black Friday: lifetime access is on a special offer in the app, until November 30. Black Friday: special offer on lifetime access, until November 30. Download
마찰력은 물체가 서로 미끄러지는 것을 방지합니다.Machalryeok-eun mulchega seoro mikkeureojineun geoseul bangjihamnida.Friction prevents objects from sliding against each other.