Why does using different kinematic equations yield different answers? Specifically in an example: say a mass is launched horizontally from a height of 20 m with an initial velocity of 10 m/s. What is the horizontal distance D, measured from the foot of the starting position, at which the mass hits the ground? If I use y_f = y_i + v_it + 1/2at² in the vertical component and solve for t with the vertical v_i = 0 and a = -10 m/s², I should get 2 s. Plugging in the 2 s into x = vt for the horizontal component should yield 20 m since the horizontal velocity remains constant.

However, why do I get inconsistent answers if I were to use an equation like v_f² = v_i² + 2a(x_f - x_i) horizontally? v_f² and v_i² both are 10 and a = 0. Plugging that in results in 100 = 100 + 2(0)(x_f - x_i). Solving for the distance then gives me 0/0 which is undefined.