Optimal Angle for a Fireman Shooting Water from a 20m High Roof

[Fusilli_Jerry89]

A fireman shoots water out of a hose at 12m/s. he is standing on a 20m high roof and is aiming for the top of another roof at the same height, but it is 21 m away. What angle should the fireman aim the hose above the horizontal in order to hit the other roof?

i set up a chart but have no idea what to do now:
x y
vi " '
vf " -'
a 0 -9.8
d 21 0
t

 

[Hootenanny]

You need to split up the trajectory into horizontal and vertical components, start by writing down the following;

• Initial Horizontal Velocity*
• Initial Vertical Velocity*
• Change in Vertical Displacement
• Change in Horizontal Displacement
• Acceleration in the vertical plane

Note that (*) will be trigonometric functions of the launch angle. Once you have done this, apply kinematic equations to each component independently.

 

[QuantumCrash]

In questions about projectiles, it is quite often, like in this case that you will need to find the time by resolving it vertically.