마찰력이 · 마찰력을
frictional force, friction
Example
얼음 위에서는 마찰력이 작아서 미끄러지기 쉬워요.
On ice there's little friction, so it's easy to slip.
Word parts
마 rub+찰 scrape+력 force
Literally 'rubbing force'. -력 = force, power: 마찰력, 중력, 노력.
Good to know
마찰 alone also means conflict between people or countries, as in 무역 마찰 'trade friction'.