Answer
39 m
Work Step by Step
First, find the initial speed, i.e., the muzzle speed at which the dart gun fires. The upward flight takes two seconds. At the top of the path, the vertical velocity is zero, so we find the launch velocity from Eq. 2–11a, and the usual acceleration. The gun fires at 19.6 m/s.
Second, we use this initial velocity, and an angle of 45$^o$, in the level horizontal range formula. This yields the maximum range for the gun.
