# Homework Help: Help with projectile motion

1. Oct 18, 2012

### nando94

Stuck on these hw problems.

1. A ball is launched 60 degrees above the x axis to a helicopter that is 110 feet away and 90 feet high. What is the initial velocity required to reach it?

I know the equation we have to use is

90 = viT + .5aT^2

and that 90 = vi(cos60)T

2. Another ball is launched with an initial velocity of 38 m/s and lands 29 meters away. Find the two angles that meet this.

would you use the same equations?

2. Oct 18, 2012

### superdave

Keep in mind that x and y are separate.
So you know that in time T the ball must travel 110 feet in the x direction and 90 feet in the y direction.

Your equations are close, but v_i in the first equation should only be the y component of v_i.
the second equation is for the x direction, so you should have 110 feet, not 90.

But otherwise, looks solid. Now, since you have 2 equations and 2 unknowns, you have to solve for one variable and then substitute it into the other equation. (Try solving the second equation for T and going from there)

Not sure about #2. Seems you have 3 unknowns (y, t and angle).