Physics
posted by Lindsay on .
The launching speed of a certain projectile is 6.4 times the speed it has at its maximum height. Calculate the elevation angle at launching.
I have noo idea how to start this one. Please help!

The vertical component of velocity at max height is zero. The horizontal component remains Vh during flight, and that is the speed at max height. Let Voy be the initial vertical component of the launch speed.
(launch speed)^2 = Voy^2 + Vh^2
= (6.4 Vh)^2 = 40.96 Vh^2
Voy^2 = 39.96 Vh^2
Voy = 6.32 Vh
launch angle arctan(Voy/Vh) = arctan 6.32 = 81.0 degrees 
How did you come up with 39.96 Vh^2?

I squared 6.4 and subracted 1. See the formula
Voy^2 + Vh^2 = 40.96 Vh^2 
This is probably really obvious, but I'm still not undertanding why you have to subtract one.