The number of protons plus the number of neutrons in an atom is equal to its

Why is alchemy not considered to be a science ?

An observer that stands still as the electron moves by him, will observe

Alead ball is dropped into a lake from a diving board 5m above the water . it hits the water with a certain velocity and then sink with constant velocity in bottom . it reaches the bottom 5 second after it is dropped if g= 10m/s2 . find depth of the lake and average velocity of the ball ?