The frictional force remains constant, dependent on the friction, the kinetic friction coefficient, is always the same...
摩擦力不守恒,它取决于摩擦力,动能摩擦系数,一直是相同的。
2
If I look at this result — the sum of gravitational potential energy and kinetic energy is conserved - for gravitational force — then it is immediately obvious where we put the zero of kinetic energy.