EN: Friction is a force that opposes motion between surfaces (or through air/water). - Friction converts mechanical energy into less useful forms, mainly thermal energy (and sometimes sound). - This is why motion usually slows down without additional energy input.

中文：摩擦力是阻碍相对运动的力（包括表面摩擦和空气/水阻力）。 - 摩擦会把机械能转化为较难利用的热能（有时还有声能）。 - 因此没有持续能量输入时，运动往往会逐渐减慢。