Calculate Velocity to Reach Target: Sx=15000m, Sy=400m

[confusednic]

[SOLVED] Calculating Velocity

A shell is fired at an elevation of 60degrees from horizontal, it needs to reach a target
Sx=15000m, Sy = 400m. What is the initial velocity of the shell?


So far I have done this...
Ax=0 Ay=-g
Vx=V cos 60 Vy=-gt+V sin 30
Sx=V cos 60 t Sy=((-gt^2)/2)+sin 30


I've tried to work from here (using Sx to find t=15000/Vcos60, then substitute that into Sy formula) but I've been getting a negative number which can't be right.
Could someone show me the working or give me any help at all?

 

[lzkelley]

Your equations are correct (except for a missing V in Sy=.

Your methodology should work as-well, you must be getting a negative from taking the wrong square root, or something about the trig functions.

 

[confusednic]

Thanks Izkelley
So I've done this...
400= (-9.8/2)(15000/Vcos60)+Vsin30

Is that the correct?

 

[apelling]

The initial vertical velocity component is not Vsin30.

Sy=1/2at^2+ut. So the second term on the right hand side of your equation should be multiplied by t and you need to square t in the first term.

 

[lzkelley]

damn yeah, sorry.
the sin30's should be sin60 also (same angle opposite trig function) or cos30's (same trig function opposite angle --- sin30 = cos60, sin60=cos30 ).
and not only is Sy missing a V, its missing a t i.e.
Sy=((-gt^2)/2)+V*t*cos30