I am not sure if I got your second point. All what I am asking is since all kinematic equations are correct, all should give the same solution. Why isn't this applicable with Δy=[(vyi+vyf)/2]*Δt?
Exactly. According to what I have studied, all the kinematic equations are useable interchangeably as long as the givens allow. Why does this equation yield 0/0?
Givens:
Vyi=12.5 m/s
Vyf=-12.5 m/s (at the same horizontal level)
ay=-9.81 m/s^2
Δy= zero m (as the displacement on the y-axis, when the projectile reaches the same horizontal level, is zero m)
Δt=?
When I use
Δy=[(vyi+vyf)/2]*Δt
I get the time as undefined.
Δt= 2Δy/(vyi+vyf)
= 2*0 m/(12.5...