Question

a projectile is launched into the air with an initial velocoty of vi at a launched angle of 30° above the horizonta. The projectile lands on the ground two seconds later, calculate vi

Answer 1

You have to use vectors and the linear kinematic equations.

IMO the easiest way to solve this would actually be to use the position formula in the y-direction which gives us:

y=y(0)+v(y)t+1/2at^2, sub in known values and we get:

0=0+v(i) sin(30)(2)+1/2g(2)^2 This way we are saying that after 2 seconds the projectile is back on the ground making the problem quite simple since y and y(0) become 0. Also any acceleration in the y-direction will be due to gravity so:

-v(i)=2g which of course is just 19.6 m/s