Projectile Motion Help: Find Minimum Angle for Target Projection

[zebra1707]

Projectile motion - urgent help needed

Homework Statement

Target is projected at an an angle to the horizontal, initial speed 4 m/s, unknown angle and unknown distance to the target. Height of the target is 0.8m.

Need to find the minimum angle.

Homework Equations

v= u+at
v = delta s/delta t
sin theta ?

The Attempt at a Solution

0 = 4 + (-9.8)t
9.8t =4/9.8
t = 0.408 (0.41) s (i think that this is wrong as it may be the time for the "entire" flight of the projectile.

v = delta s/delta t
v delta t = delta s
4 x 0.41

Delta s = 1.63

sin-1 = 0.8/1.63

angle = 30 degrees

Can the gurus please check this because I think that I have the entire flight of the projectile - not the max height where it will hit the target.

Cheers and thanks.

 

[lswtech]

You should try setting up equations for both vertical and horizontal motions. Then you should have an unknown "d" for distance in the equations. Join x-y by cancelling the parameter "time t". Consider d as a real root and you are done!