# Projectile motion problem (find angle theta)

1. Mar 2, 2013

### 5ymmetrica1

1. The problem statement, all variables and given/known data
A projectile is launched from the ground with an initial velocity of 140m/s at an angle of θ above the horizontal. (assume AR is negligible and the ground is level)
The time of flight is 18.7seconds and its range is 1.98*10^3
show that the launch angle θ is approximately 41 degrees

2. Relevant equations
I'm unsure which equation Im supposed to use for a problem like this

3. The attempt at a solution
voh = 140cosθ

∴ cosθ = voh/140

but Im not given enough information to find the initial horizontal velocity so this equation won't work. I also am sure that I have to use the time of flight and the range since its been given but I can't think how to rearrange things to make it work.
Can anyone point me in the right direction for what I'm supposed to be doing please?

2. Mar 2, 2013

### rude man

What must voh be if the projectile flew for 1.98e3 m and took 18.7sec. to do so?

3. Mar 2, 2013

### 5ymmetrica1

thanks for the reply rudeman!

So voh = 1.8e3m/ 18.7s = 105.88 m/s

then θ = cos-1 (105.88/140)

∴ θ = 40.86 degrees ≈ 41 degrees

4. Mar 2, 2013

Yay team!