A student obtains the quadratic equation 5t2−20t=0 while calculating the time of flight for a ball thrown vertically upward with initial velocity 20m/s (upward positive, g=10m/s2). The roots are t=0s and t=4s.
(i) Explain the physical significance of each root.
(ii) State why no additional root is expected in this situation.
(iii) If the student had taken the downward direction as positive, would the numerical values of the roots change? Give reasons for your answer.