Physics

Problem

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 = 0\,\text{s}

and

t = 4\,\text{s}

. Why is the root

t = 0\,\text{s}

not taken as the answer? a) Because substituting

t=0\,\text{s}

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=0\,\text{s}

the velocity would come out to be zero, contradicting the fact that the ball was thrown upward. d) Choosing

t=0\,\text{s}

would violate conservation of energy by making the ball start with no kinetic energy.

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