The range is the horizontal distance from the cannon when the pumpkin hits the ground. this distance is given by the product of the horizontal velocity (which is constant) and the amount of time the pumpkin is in the air (which is determined by the vertical component of the initial velocity, as you just discovered). set the initial speed to 14 m/s, and fire the pumpkin several times while varying the angle between the cannon and the horizontal. for which angle is the range a maximum (with the initial speed held constant)?
The answer is 45 degrees. I am not doing a field experiment for you that involves a cannon and a day's work, for 5 points.
the answer is b