A force of 20N is required to continuously apply on a person with a mass of 60kg over an unknown surface to keep them moving. What is the coefficient of kinetic friction of this surface?

when it gains or loses electrons

photocells convert light energy to electricity

see earlier answer

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.