When solving the quadratic equation for the time of flight of a ball thrown vertically upward so that the displacement becomes zero again, two roots are obtained: t=0s and t=4s. Why is the root t=0s not taken as the answer?
(a) Because substituting t=0s makes the calculated displacement negative, so it cannot be accepted.
(b) It represents the instant of projection itself, not the later instant asked for.
(c) Because at t=0s the velocity would come out to be zero, contradicting the fact that the ball was thrown upward.
(d) Choosing t=0s would violate conservation of energy by making the ball start with no kinetic energy.